Yearly Archives: 2011

Differences Between Directional Benefit in the Lab and Real-World

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Relationship Between Laboratory Measures of Directional Advantage and Everyday Success with Directional Microphone Hearing Aids

Cord, M., Surr, R., Walden, B. & Dyrlund, O. (2004). Relationship between laboratory measures of directional advantage and everyday success with directional microphone hearing aids.Journal of the American Academy of Audiology 15, 353-364.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

People with hearing loss require a better signal-to-noise ratio (SNR) than individuals with normal hearing (Dubno et al, 1984; Gelfand et al, 1988; Bronkhorst and Plomp, 1990).  Among many technological improvements, a directional microphone is arguably the only effective hearing aid feature for improving SNR and subsequently, improving speech understanding in noise. A wide range of studies support the benefit of directionality for speech perception in competing noise (Agnew & Block, 1997; Nilsson et al, 1994; Ricketts and Henry, 2002; Valente, 1995) Directional benefit is defined as the difference in speech recognition ability between omnidirectional and directional microphone modes. In laboratory conditions, directional benefit averages around 7-8dB but varies considerably and has ranged from 2-3dB up to 14-16dB (Valente et al, 1995; Agnew & Block, 1997).

An individual’s perception of directional benefit varies considerably among hearing aid users. Cord et al (2002) interviewed individuals who wore hearing aids with switchable directional microphones and 23% reported that they did not use the directional feature. Many respondents said they had initially tried the directional mode but did not notice adequate improvement in their ability to understand speech and therefore stopped using the directional mode. This discrepancy between measured and perceived benefit has prompted exploration of the variables that affect performance with directional hearing aids. Under laboratory conditions, Ricketts and Mueller (2000) examined the effect of audiometric configuration, degree of high frequency hearing loss and aided omnidirectional performance on directional benefit, but found no significant interactions among any of these variables.

The current study by Cord and her colleagues examined the relationship between measured directional advantage in the laboratory and success with directional microphones in everyday life. The authors studied a number of demographic and audiological variables, including audiometric configuration, unaided SRT, hours of daily hearing aid use and length of experience with current hearing aids, in an effort to determine their value for predicting everyday success with directional microphones.

Twenty hearing-impaired individuals were selected to participate in one of two subject groups. The “successful” group consisted of individuals who reported regular use of omnidirectional and directional microphone modes. The “unsuccessful” group of individuals reported not using their directional mode and using their omnidirectional mode all the time. Analysis of audiological and demographic information showed that the only significant differences in audiometric threshold between the successful and unsuccessful group were at 6-8 kHz, otherwise the two groups had very similar audiometric configurations, on average. There were no significant differences between the two groups for age, unaided SRT, unaided word recognition scores, hours of daily use or length of experience with hearing aids.

Subjects were fitted with a variety of styles – some BTE and some custom – but all had manually accessible omnidirectional and directional settings. The Hearing in Noise Test (HINT; Nilsson et al, 1994) was administered to subjects with their hearing aids in directional and omnidirectional modes. Sentence stimuli were presented in front of the subject and correlated competing noise was presented through three speakers: directly behind the subject and on each side. Following the HINT participants completed the Listening Situations Survey (LSS), a questionnaire developed specifically for this study. The LSS was designed to assess how likely participants were to encounter disruptive background noise in everyday situations, to determine if unsuccessful and successful directional microphone users were equally likely to encounter noisy situations in everyday life.  The survey consisted of four questions:

1) On average, how often are you in listening situations in which bothersome background noise is present?

2) How often are you in social situations in which at least 3 other people are present?

3) How often are you in meetings (e.g. community, religious, work, classroom, etc.)?

4) How often are you talking with someone in a restaurant or dining hall setting?

The HINT results suggest average directional benefit of 3.2dB for successful users and 2.1dB for unsuccessful users. Although directional benefit was slightly greater for the successful users, the difference between the groups was not statistically significant.  There was a broad range of directional benefit for both groups: from -0.8 to 6.0dB for successful users and from -3.4 to 10.5dB for the unsuccessful users. Interestingly, three of the ten successful users obtained little or no directional benefit, whereas seven of the ten unsuccessful users obtained positive directional benefit.

Analysis of the LSS results showed that successful users of directional microphones were somewhat more likely than unsuccessful users to encounter listening situations with bothersome background noise and to encounter social situations with more than three other people present. However, statistical analysis showed no significant differences between the two groups for any items on the LSS survey, indicating that users who perceived directional benefit and used their directional microphones were not significantly more likely to encounter noisy situations in everyday life.

These observations led the authors to conclude that directional benefit as measured in the laboratory did not predict success with directional microphones in everyday life. Some participants with positive directional advantage scores were unsuccessful directional microphone users and conversely, some successful users showed little or no directional advantage. There are a number of potential explanations for their findings. First, despite the LSS results, it is possible that unsuccessful users did not encounter real-life listening situations in which directional microphones would be likely to help. Directional microphone benefit is dependent on specific characteristics of the listening environment (Cord et al, 2002; Surr et al, 2002; Walden et al, 2004), and is most likely to help when the speech source is in front of and relatively close to the listener, with spatial separation between the speech and noise sources. Individuals who rarely encounter this specific listening situation would have limited opportunity to evaluate directional microphones and may therefore perceive only limited benefit from them.

Unsuccessful directional microphone users may have also had unrealistically high expectations about directional benefits. Directionality can be a subtle but effective way of improving speech understanding in noise. Reduction of sound from the back and sides helps the listener focus attention on the speaker and ignore competing noise. Directional benefit is based on the concept of face-to-face communication, if users expect their hearing aids to reduce all background noise from all angles they are likely to be disappointed. Similarly, if they expect the aids to completely eliminate background noise, rather than slightly reduce it, they will be unimpressed. It is helpful for hearing aid users, especially those new to directional microphones, to be counseled about realistic expectations as well as proper positioning in noisy environments. If listeners know what to expect and are able to position themselves for maximum directional effect they are more likely to perceive benefit from their hearing aids in noisy conditions.

To date, it has been difficult to correlate directional benefit under laboratory conditions with perceived directional benefit. It is clear that directionality offers performance benefits in noise, but directional benefit measured in a sound booth does not seem to predict everyday success with directional microphones. There are many factors that are likely affect real-life performance with directional microphone hearing aids, including audiometric variables, the frequency response and gain equalization of the directional mode, the venting of the hearing aid and the contribution of visual cues to speech understanding (Ricketts, 2000a; 2000b). Further investigation is still needed to elucidate the impact of these variables on the everyday experiences of hearing aid users.

As is true for all hearing aid features, directional microphones must be prescribed appropriately and hearing aid users should be counseled about realistic expectations and appropriate circumstances in which they are beneficial. Although most modern hearing instruments have the ability to adjust automatically to changing environments, manually accessed directional modes offer hearing aid wearers increased flexibility and may increase use by allowing the individual to make decisions regarding their improved comfort and performance in noisy places. Routine reinforcement of techniques for proper directional microphone use are encouraged. Hearing aid users should be encouraged to experiment with their directional programs to determine where and when they are most helpful. For the patient, proper identification of and positioning in noisy environments is essential step toward meeting their specific listening needs and preferences.

References

Agnew, J. & Block, M. (1997). HINT thresholds for a dual-microphone BTE. Hearing Review 4, 26-30.

Bronkhorst, A. & Plomp, R. (1990). A clinical test for the assessment of binaural speech perception in noise. Audiology 29, 275-285.

Cord, M.T., Surr, R.K., Walden, B.E. & Olson, L. (2002). Performance of directional microphone hearing aids in everyday life. Journal of the American Academy of Audiology 13, 295-307.

Cord, M., Surr, R., Walden, B. & Dyrlund, O. (2004). Relationship between laboratory measures of directional advantage and everyday success with directional microphone hearing aids. Journal of the American Academy of Audiology 15, 353-364.

Dubno, J.R., Dirks, D.D. & Morgan, D.E. (1984).  Effects of age and mild hearing loss on speech recognition in noise. Journal of the Acoustical Society of America 76, 87-96.

Gelfand, S.A., Ross, L. & Miller, S. (1988). Sentence reception in noise from one versus two sources: effects of aging and hearing loss. Journal of the Acoustical Society of America 83, 248-256.

Kochkin, S. (1993). MarkeTrak III identifies key factors in determining customer satisfaction. Hearing Journal 46, 39-44.

Nilsson, M., Soli, S.D. & Sullivan, J.A. (1994). Development of the Hearing in Noise Test for the measurement of speech reception thresholds in quiet and in noise. Journal of the Acoustical Society of America 95, 1085-1099.

Ricketts, T. (2000a). Directivity quantification in hearing aids: fitting and measurement effects. Ear and Hearing 21, 44-58.

Ricketts, T. (2000b). Impact of noise source configuration on directional hearing aid benefit and performance. Ear and Hearing 21, 194-205.

Ricketts, T. (2001). Directional hearing aids. Trends in Amplification 5, 139-175.

Ricketts, T.  & Henry, P. (2002). Evaluation of an adaptive, directional microphone hearing aid. International Journal of Audiology 41, 100-112.

Ricketts, T. & Henry, P. (2003). Low-frequency gain compensation in directional hearing aids. American Journal of Audiology 11, 1-13.

Ricketts, T. & Mueller, H.G. (2000). Predicting directional hearing aid benefit for individual listeners. Journal the American Academy of Audiology 11, 561-569.

Surr, R.K., Walden, B.E. Cord, M.T. & Olson, L. (2002). Influence of environmental factors on hearing aid microphone preference. Journal of the American Academy of Audiology 13, 308-322.

Valente, M., Fabry, D.A. & Potts, L.G. (1995). Recognition of speech in noise with hearing aids using dual microphones. Journal of the American Academy of Audiology 6, 440-449.

Walden, B.E., Surr, R.K., Cord, M.T. & Dyrlund, O. (2004). Predicting microphone preference in everyday living. Journal of the American Academy of Audiology 15, 365-396.

Are you prescribing an appropriate MPO?

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Effect of MPO and Noise Reduction on Speech Recognition in Noise

Kuk, F., Peeters, H., Korhonen, P. & Lau, C. (2010). Effect of MPO and noise reduction on speech recognition in noise. Journal of the American Academy of Audiology, submitted November 2010.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the original authors.

A component of clinical best practice would suggest that clinicians determine a patient’s uncomfortable listening levels in order to prescribe the output limiting characteristics of a hearing aid (Hawkins et al., 1987). The optimal maximum power output (MPO) should be based on two goals: preventing loudness discomfort and avoiding distorted sound quality at high input levels. The upper limit of a prescribed MPO must allow comfortable listening; less consideration is given to the consequences that under prescribing MPO might have on hearing aid and patient performance.

There are two primary concerns related to the acceptable lower MPO limit: saturation and insufficient loudness. Saturation occurs when the input level of a stimulus plus gains applied by the hearing aid exceed the MPO, causing distortion and temporal smearing (Dillon & Storey, 1998). This results in a degradation of speech cues and a perceived lack of clarity, particularly in the presence of competing noise. Similarly, insufficient loudness reduces the availability of speech cues. There are numerous reports of subjective degradation of sound when MPO is set lower than prescribed levels, particularly in linear hearing instruments (Kuk et al., 2008; Storey et al., 1998; Preminger, et al., 2001). There is not yet consensus on whether low MPO levels also cause objective degradation in performance.

The purpose of the study described here was to determine if sub-optimal MPO could affect speech intelligibility in the presence of noise, even in a multi-channel, nonlinear hearing aid. Furthermore, the authors examined if gain reductions from a noise reduction algorithm could mitigate the detrimental effects of the lower MPO. The authors reasoned that a reduction in output at higher input levels, via compression and noise reduction, could reduce saturation and temporal distortion.

Eleven adults with flat, severe hearing losses participated in the reviewed study. Subjects were fitted bilaterally with 15-channel, wide dynamic range compression, behind-the-ear hearing aids. Microphones were set to omnidirectional and other than noise reduction, no special features were activated during the study. Subjects responded to stimuli from the Hearing in Noise Test (HINT, Nilsson et al., 1994) presented at a 0-degree azimuth angle in the presence of continuous speech-shaped noise. The HINT stimuli yielded reception thresholds for speech (RTS) scores for each test condition.

Test conditions included two MPO prescriptions: the default MPO level (Pascoe, 1989) and 10dB below that level. The lower setting was chosen based on previous work that reported an approximately 18dB acceptable MPO range for listeners with severe hearing loss  (Storey et al., 1998). MPOs set at 10dB below default would therefore be likely to approach the low end of the acceptable range, resulting in perceptual consequences. Speech-shaped noise was presented at two levels: 68dB and 75dB. Testing was done with and without digital noise reduction (DNR).

Analysis of the HINT RTS scores yielded significant main effects of MPO and DNR, as well as significant interactions between MPO and DNR, and DNR and noise level. There was no significant difference between the two noise level conditions. Subjects performed better with the default MPO setting versus the reduced MPO setting. The interaction between the MPO and DNR showed that subjects’ performance in the low-MPO condition was less degraded when DNR was activated. These findings support the authors’ hypotheses that reduced MPO can adversely affect speech discrimination and that noise reduction processing can at least partially mitigate these adverse effects.

Prescriptive formulae have proven to be reasonably good predictors of acceptable MPO levels (Storey et al., 1988; Preminger et al., 2001). In contrast, there is some question as to the value of clinical UCL testing prior to fitting, especially when validation with loudness measures is performed after the fitting (Mackersie, 2006). Improper instruction for the UCL task may yield inappropriately low UCL estimates, resulting in compromised performance and sound quality. The authors of the current paper recommend following prescriptive targets for MPO and conducting verification measures after the fitting, such as real-ear saturation response (RESR) and subjective loudness judgments.

Another scenario, and an ultimately avoidable one, involves individuals who have been fitted with inappropriate instruments for their loss, usually because of cosmetic concerns. It is unfortunately not so unusual for some individuals with severe hearing losses to be fitted with RIC or CIC instruments because of their desirable cosmetic characteristics. Smaller receivers will likely have MPOs that are too low for hearing aid users with severe hearing loss. Many hearing-aid users may not realize they are giving anything up when they select a CIC or RIC and may view these styles as equally appropriate options for their loss. The hearing aid selection process must therefore be guided by the clinician; clients should be educated about the benefits and limitations of various hearing aid options and counseled about the adverse effects of under-fitting their loss with a more cosmetically appealing option.

The results of the current study are important because they illuminate an issue related to hearing aid output that might not always be taken into clinical consideration. MPO settings are usually thought of as a way to prevent loudness discomfort, so the concern is to avoid setting the MPO too high. Kuk and his colleagues have shown that an MPO that is too low could also have adverse effects and have provided valuable information to help clinicians select appropriate MPO settings. Additionally, their findings show objective benefits and support the use of noise reduction strategies, particularly for individuals with reduced dynamic range due to severe hearing loss or tolerance issues. Of course their findings may not be generalizable to all multi-channel compression instruments, with the wide variety of compression characteristics that are available, but they present important considerations that should be examined in further detail with other instruments.

References

ANSI (1997). ANSI S3.5-1997. American National Standards methods for the calculation of the speech intelligibility index. American National Standards Institute, New York.

Dillon, H. & Storey, L. (1998). The National Acoustic Laboratories’ procedure for selecting the saturation sound pressure level of hearing aids: theoretical derivation. Ear and Hearing 19(4), 255-266.

Hawkins, D., Walden, B., Montgomery, A. & Prosek, R. (1987). Description and validation of an LDL procedure designed to select SSPL90. Ear and Hearing 8, 162-169.

Kuk , F., Korhonen, P., Baekgaard, L. & Jessen, A. (2008). MPO: A forgotten parameter in hearing aid fitting. Hearing Review 15(6), 34-40.

Kuk et al., (2010). Effect of MPO and noise reduction on speech recognition in noise. Journal of the American Academy of Audiology, submitted November 2010, fast track article.

Kuk, F. & Paludan-Muller, C. (2006). Noise management algorithm may improve speech intelligibility in noise. Hearing Journal 59(4), 62-65.

Mackersie, C. (2006). Hearing aid maximum output and loudness discomfort: are unaided loudness measures needed? Journal of the American Academy of Audiology 18 (6), 504-514.

Nilsson, M., Soli, S. & Sullivan, J. (1994). Development of the Hearing in Noise Test for the measurement of speech reception thresholds in quiet and in noise. Journal of the Acoustical Society of America 95(2), 1085-1099.

Pascoe, D. (1989). Clinical measurements of the auditory dynamic range and their relation to formulae for hearing aid gain. In J. Jensen (Ed.), Hearing Aid Fitting: Theoretical and Practical Views. Proceedings of the 13th Danavox Symposium. Copenhagen: Danavox, pp. 129-152.

Preminger, J., Neuman, A. & Cunningham, D. (2001). The selection and validation of output sound pressure level in multichannel hearing aids. Ear and Hearing 22(6), 487-500.

Storey, L., Dillon, H., Yeend, I. & Wigney, D. (1998). The National Acoustic Laboratories, procedure for selecting the saturation sound pressure level of hearing aids: experimental validation. Ear and Hearing 19(4), 267-279.

Addressing patient complaints when fine-tuning a hearing aid

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Jenstad, L.M., Van Tasell, D.J. & Ewert, C. (2003). Hearing aid troubleshooting based on patient’s descriptions. Journal of the American Academy of Audiology 14 (7).

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

As part of any clinically robust protocol, a hearing aid fitting will be objectively verified with real-ear measures and validated with a speech-in-noise test. Fine tuning and follow-up adjustments are an equally important part of the fitting process. This stage of the routine fitting process does not follow standardized procedures and is almost always directed by a patient’s complaints or descriptions of real-world experience with the hearing aids. This can be a challenging dynamic for the clinician. Patients may have difficulty putting their auditory experience into words and different people may describe similar sound quality issues in different ways.  Additionally, there may be several ways to address any given complaint and a given programming adjustment may not have the same effect on different hearing aids.

Hearing aid manufacturers often include a fine-tuning guide or automated fitting assistant within their software to help the clinician make appropriate adjustments for common patient complaints. There are limitations to the effectiveness of these fine tuning guides in that they are inherently specific to a limited range of products and the suggested adjustments are subject to the expertise and resources of that manufacturer.  The manner in which a sound quality complaint is described may differ between manufacturers and the recommended adjustments in response to the complaint may differ as well.

There have been a number of efforts to develop a single hearing aid troubleshooting guide that could be used across devices and manufacturers (Moore et. al., 1998; Gabrielsson et al., 1979, 1988, 1990; Lundberg et al., 1992; Ovegard et al., 1997). The first and perhaps most challenging step toward this goal has been to determine the most common descriptors that patients use for sound quality complaints. Moore (1998) and his colleagues developed a procedure in which responses on three ratings scales (e.g., “loud versus quiet”, “tinny versus boomy”) were used to make adjustments to gain and compression settings. However, their procedure did not allow for the bevy of descriptors that patients create; limiting potential utility for everyday clinical settings.  Gabrielsson colleagues, in a series of Swedish studies, developed a set of reliable terms to describe sound quality. These descriptors have since been translated and used in English language research (Bentler et al., 1993).

As hearing instruments become more complicated with numerous adjustable parameters, and given the wide range of experience and expertise of individuals fitting hearing instruments today, an independent fine tuning guide is an appealing concept. Lorienne Jenstad and her colleagues proposed an “expert system” for troubleshooting hearing aid complaints.  The authors explained that expert systems “emulate the decision making abilities of human experts” (Tharpe et al., 1993).  To develop the system, two primary questions were asked:

1) What terms do hearing impaired listeners use to describe their reactions to specific hearing aid fitting problems?

2) What is the expert consensus on how these patient complaints can be addressed by hearing aid adjustment?

There were two phases to the project. To address question one, the authors surveyed clinicians for their reports on how patients describe sound quality with regard to specific fitting problems. To address question two, the most frequently reported descriptors from the clinicians’ responses were submitted to a panel of experts to determine how they would address the complaints.

The authors sent surveys to 1934 American Academy of Audiology members and received 311 qualifying responses. The surveys listed 18 open-ended questions designed to elicit descriptive terms that patients would likely use for hearing aid fitting problems. For example, the question “If the fitting has too much low-frequency gain…” yielded responses such as “hollow”, “plugged” and “echo”.  The questions probed common problems related to gain, maximum output, compression, physical fit, distortion and feedback.  The survey responses yielded a list of the 40 most frequent descriptors of hearing aid fitting problems, ranked according to the number of occurrences.

The list of descriptors was used to develop a questionnaire to probe potential solutions for each problem.  Each descriptor was put in the context of, “How would you change the fitting if your patient reports that ___?”, and 23 possible fitting solutions were listed.  These questionnaires were completed by a panel of experts with a minimum of five years of clinical experience. Respondents could offer more than one solution to a problem and the solutions were weighted based on the order in which they were offered. There was strong agreement among experts, suggesting that their responses could be used reliably to provide troubleshooting solutions based on sound quality descriptions. The expert responses also agreed with the initial survey that was sent to the group of 1934 audiologists, supporting the validity of these response sets.

The expert responses resulted in a fine-tuning guide in the form of tables or simplified flow charts. The charts list individual descriptors with potential solutions listed below in the order in which they should be attempted.  For example, below the descriptor “My ear feels plugged”, the first solution is to “increase vent” and the second is to “decrease low frequency gain”. The idea is that the clinician would first try to increase the vent diameter and if that didn’t solve the problem, they would move on to the second option, decreasing low frequency gain. If an attempted solution creates another sound quality problem, the table can be utilized to address that problem in the same way.

The authors correctly point out that there are limitations to this tool and that proposed solutions will not necessarily have the same results with all hearing aids. For instance, depending on the compressor characteristics, raising a kneepoint might increase OR decrease the gain at input levels below the kneepoint. It is up to the clinician to be familiar with a given hearing aid and its adjustable parameters to arrive at the appropriate course of action.

Beyond manipulation of the hearing aid itself, the optimal solution for a particular patient complaint might not be the first recommendation in any tuning guide. For instance, for the fitting problem labeled “Hearing aid is whistling”, the fourth solution listed in the table is “check for cerumen”.  This solution appeared fourth in the ranking based on the frequency of responses from the experts on the panel. However, any competent clinician who encounters a patient with hearing aid feedback should check for cerumen first before considering programming modifications.

The expert system proposed by Jenstad and her colleagues represents a thoroughly examined, reliable step toward development of a universal troubleshooting guide for clinicians. Their paper was published in 2003, so some items should be updated to suit modern hearing aids. For example, current feedback management strategies result in fewer and less challenging feedback problems.  Solutions for feedback complaints might now include, “calibrate feedback management system” versus gain or vent adjustments. Similarly, most hearing aids now have solutions for listening in noise that extend beyond the simple inclusion of directional microphones, so “directional microphone” might not be an appropriately descriptive solution to address complaints about hearing in noise, as the patient is probably already using a directional microphone.

Overall, the expert system proposed by Jenstad and colleagues is a helpful clinical tool; especially if positioned as a guide to help patients find the appropriate terms to describe their perceptions. However, as the authors point out, it is not meant to replace prescriptive methods, measures of verification and validation, or the expertise of the audiologist. The responsibility is with the clinician to be informed about current technology and its implications for real world hearing aid performance and to communicate with their patients in enough detail to understand their patients’ comments and address them appropriately.

References

Bentler, R.A., Nieburh, D.P., Getta, J.P. & Anderson, C.V. ( 1993). Longitudinal study of hearing aid effectiveness II: subjective measures. Journal of Speech and Hearing Research 36, 820-831.

Jenstad, L.M., Van Tasell, D.J. & Ewert, C. (2003). Hearing aid troubleshooting based on patient’s descriptions. Journal of the American Academy of Audiology 14 (7).

Moore, B.C.J., Alcantara, J.I. & Glasberg, B.R. (1998). Development and evaluation of a procedure for fitting multi-channel compression hearing aids. British Journal of Audiology 32, 177-195.

Gabrielsson A. ( 1979). Dimension analyses of perceived sound quality of sound-reproducing systems. Scandinavian Journal of Psychology 20, 159-169.

Gabrielsson, A., Hagerman, B., Bech-Kristensen, T. & Lundberg, G. (1990). Perceived sound quality of reproductions with different frequency responses and sound levels. Journal of the Acoustical Society of America 88, 1359-1366.

Gabrielsson, A. Schenkman, B.N. & Hagerman, B. (1988). The effects of different frequency responses on sound quality judgments and speech intelligibility. Journal of Speech and Hearing Research 31, 166-177.

Lundberg, G., Ovegard, A., Hagerman, B., Gabrielsson, A. & Brandstom, U. (1992). Perceived sound quality in a hearing aid with vented and closed earmold equalized in frequency response. Scandinavian Audiology 21, 87-92.

Ovegard, A., Lundberg, G., Hagerman, B., Gabrielsson, A., Bengtsson, M. & Brandstrom, U. (1997). Sound quality judgments during acclimatization of hearing aids. Scandinavian Audiology 26, 43-51.

Schweitzer, C., Mortz, M. & Vaughan, N. (1999). Perhaps not by prescription – but by perception. High Performance Hearing Solutions 3, 58-62.

Tharpe, A.M., Biswas, G. & Hall, J.W. (1993). Development of an expert system for pediatric auditory brainstem response interpretation. Journal of the American Academy of Audiology 4, 163-171.

Recommendations for fitting patients with cochlear dead regions

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Cochlear Dead Regions in Typical Hearing Aid Candidates:

Prevalence and Implications for Use of High-Frequency Speech Cues

Cox, R.M., Alexander, G.C., Johnson, J. & Rivera, I. (2011).  Cochlear dead regions in typical hearing aid candidates: Prevalence and implications for use of high-frequency speech cues. Ear & Hearing 32 (3), 339-348.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

Audibility is a well-known predictor of speech recognition ability (Humes, 2007) and audibility of high-frequency information is of particular importance for consonant identification.  Therefore, audibility of high-frequency speech cues is appropriately regarded as an important element of successful hearing aid fittings (Killion & Tillman, 1982; Skinner & Miller, 1983). In contrast to this expectation, some studies have reported that high-frequency gain might have limited or even negative impact on speech recognition abilities of some individuals (Murray & Byrne, 1986; Ching et al., 1998; Hogan & Turner, 1998). These researchers observed that when high-frequency hearing loss exceeded 55-60dB, some listeners were unable to benefit from increased high-frequency audibility.  A potential explanation for this variability was provided by Brian Moore (2001), who suggested that an inability to benefit from amplification in a particular frequency region could be due to cochlear “dead regions” or regions where there is a loss of inner hair cell functioning.

Moore suggested that hearing aid fittings could potentially be improved if clinicians were able to identify patients with cochlear dead regions (DRs). Working under the assumption that diagnosis DRs may contraindicate high-frequency amplification. He and his colleagues developed the TEN test as a method of determining the presence of cochlear dead regions (Moore et al., 2000, 2004). The advent of the TEN test provided a standardized measurement protocol for DRs, but there is still wide variability in the reported prevalence of DRs. Estimates range from as 29% (Preminger et a., 2005) to as high as 84% (Hornsby & Dundas, 2009), with other studies reporting DR prevalence somewhere in the middle of that range. Several potential factors are likely to contribute to this variability, including degree of hearing loss, audiometric configuration and test technique.

In addition to the variability in reported prevalence of DRs, there is also variability in the reports of how DRs affect the ability to benefit from high-frequency speech cues (Vickers et al., 2001; Baer et al., 2002; Mackersie et al., 2004). It remains unclear as to whether high-frequency amplification recommendations should be modified to reflect the presence of DRs.  Most research is in agreement that as hearing thresholds increase, the likelihood of DRs also increases.  Hearing aid users with severe to profound hearing losses are likely to have at least one DR. Because a large proportion of hearing aid users have moderate to severe hearing losses, Dr. Cox and her colleagues wanted to determine the prevalence of DRs in this population. In addition, they examined the effect of DRs on the use of high-frequency speech cues by individuals with moderate to severe loss.

Their study addressed two primary questions:

1) What is the prevalence of dead regions (DRs) among listeners with hearing thresholds in the 60-90dB range?

2) For individuals with hearing loss in the 60-90dB range, do those with DRs differ from those without DRs in their ability to use high-frequency speech cues?

One hundred and seventy adults with bilateral, flat or sloping sensorineural hearing loss were tested. All subjects had thresholds of 60 to 90dB in the better ear for at least part of the range from 1-3kHz and thresholds no better than 25dB for frequencies below 1kHz. Subjects ranged in age from 38 to 96 years, and 59% of the subjects had experience with hearing aids.

First, subjects were evaluated for the presence of DRs with the TEN test. Then, speech recognition was measured using high-frequency emphasis (HFE) and high-frequency emphasis, low-pass filtered (HFE-LP) stimuli from the QSIN test (Killion et al. 2004). HFE items on this test are amplified up to 32dB above 2.5kHz, whereas the HFE-LP items have much less gain in this range. Comparison of subjects’ responses to these two types of stimuli allowed the investigators to assess changes in speech intelligibility with additional high frequency cues. Presentation levels for the QSIN were chosen by using a loudness scale and bracketing procedure to arrive at a level that the subject considered “loud but okay”. Finally, audibility differences for the two QSIN conditions were estimated using the Speech Intelligibility Index based on ANSI 3.5-1997 (ANSI, 1997).

The TEN test results revealed that 31% of the participants had DRs at one or more test frequencies. Of the 307 ears tested, 23% were found to have a DR for one or more frequencies. Among those who tested positive for DRs, about 1/3 had DRs in both ears and 2/3 had DRs in one ear or the other in equal proportion. Mean audiometric thresholds were essentially identical for the two groups below 1kHz, but above 1kHz thresholds were significantly poorer for the group with DRs than for the group without DRs.  DRs were most prevalent at frequencies above 1.5kHz. There were no age or gender differences.

On the QSIN test, the mean HFE-LP scores were significantly poorer than the mean HFE scores for both groups.  There was also a significant difference in performance based on whether or not the participants had DRs. Perhaps more interestingly, there was a significant interaction between the DR group and test stimuli conditions, in that the additional high-frequency information in the HFE stimuli resulted in slightly greater performance gains for the group without DRs than it did for the group with DRs.  Furthermore, subjects with one or more isolated DRs were more able to benefit from the high frequency cues in the HFE lists than were those subjects with multiple, contiguous DRs. Although there were a few isolated individuals who demonstrated lower scores for the HFE stimuli, the differences were not significant and could have been explained by measurement error. Therefore, the authors conclude that the additional high frequency information in the HFE stimuli was not likely to have had a detrimental effect on performance for these individuals.

As had also been reported in previous studies, subject groups with DRs had poorer mean audiometric thresholds than the groups without DRs, so it was possible that audibility played a role in QSIN performance. Analysis of the audibility of QSIN stimuli for the two groups revealed that high frequency cues in the HFE lists were indeed more audible for the group without DRs. In accounting for this audibility effect, the presence of DRs still had a small but significant effect on performance.

The results of this study suggest that listeners with cochlear DRs still benefit from high frequency speech cues, albeit slightly less than those without dead regions.  The performance improvements were small and the authors caution that it is premature to draw firm conclusions about the clinical implications of this study.  Despite the need for further examination, the results of the current study certainly do not support any reduction in prescribed gain for hearing aid candidates with moderate to severe hearing losses.  The authors acknowledge, however, that because the findings of this and other studies are based on group data, it is possible that specific individuals may be negatively affected by amplification within dead regions. Based on the research to date, this seems more likely to occur in individuals with profound hearing loss who may have multiple, contiguous DRs.

More study is needed to determine the most effective clinical approach to managing cochlear dead regions in hearing aid candidates. Future research should be done with hearing aid users, including for example, the effects of noise on everyday hearing aid performance for individuals with DRs. A study by Mackersie et. al. (2004) showed that subjects with DRs suffered more negatives effects of noise than did the subjects without DRs. If there is a convergence of evidence to this effect, then recommendations about the use of high frequency gain, directionality and noise reduction could be determined as they relate to DRs. For now, Dr. Cox and her colleagues recommend that until there are clear criteria to identify individuals for whom high frequency gain could have deleterious effects, clinicians should continue using best-practice protocols and provide high frequency gain according to current prescriptive methods.

References

ANSI ( 1997). American National Standard Methods for Calculation of the Speech Intelligibility Index (Vol. ANSI S3.5-1997). New York: American National Standards Institute.

Ching,T., Dillon, H. & Byrne, D. (1998). Speech recognition of hearing-impaired listeners: Predictions from audibility and the limited role of high-frequency amplification. Journal of the Acoustical Society of America 103, 1128-1140.

Cox, R.M., Alexander, G.C., Johnson, J. & Rivera, I. (2011).  Cochlear dead regions in typical hearing aid candidates: Prevalence and implications for use of high-frequency speech cues. Ear & Hearing 32 (3), 339-348.

Hogan, C.A. & Turner, C.W. (1998). High-frequency audibility: Benefits for hearing-impaired listeners. Journal of the Acoustical Society of America 104, 432-441.

Humes, L.E. (2007). The contributions of audibility and cognitive factors to the benefit provided by amplified speech to older adults. Journal of the American Academy of Audiology 18, 590-603.

Killion, M. C. & Tillman, T.W. (1982). Evaluation of high-fidelity hearing aids. Journal of Speech and Hearing Research 25, 15-25.

Moore, B.C.J. (2001). Dead regions in the cochlear: Diagnosis, perceptual consequences and implications for the fitting of hearing aids. Trends in Amplification 5, 1-34.

Moore, B.C.J., Huss, M., Vickers, D.A.,  et al. (2000). A test for the diagnosis of dead regions in the cochlea. British Journal of Audiology 34, 2-5-224.

Moore, B.C.J., Glasberg, B.R., Stone, M.A. (2004). New version of the TEN test with calibrations in dB HL. Ear and Hearing 25, 478-487.

Murray, N. & Byrne, D. (1986). Performance of hearing-impaired and normal hearing listeners with various high-frequency cut-offs in hearing aids. Australian Journal of Audiology 8, 21-28.

Skinner, M.W. & Miller, J.D. (1983). Amplification bandwidth and intelligibility of speech in quiet and noise for listeners with sensorineural hearing loss.  Audiology 22, 253-279.

A preferred speech stimulus for testing hearing aids

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Development and Analysis of an International Speech Test Signal (ISTS)

Holube, I., Fredelake, S., Vlaming, M. & Kollmeier, B. (2010). Development and analysis of an international speech test signal (ISTS). International Journal of Audiology, 49, 891-903.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

Current hearing aid functional verification measures are described in the standards IEC 60118 and ANSI 3.22 and use stationary signals, including sine wave frequency sweeps and unmodulated noise signals. Test stimuli are presented to the hearing instrument and frequency specific gain and output is measured in a coupler or ear simulator.  Current standardized measurement methods require the instrument to be set at maximum or a reference test setting and adaptive parameters such as noise reduction and feedback management are turned off.

These procedures provide helpful information for quality assurance and determining fitting ranges for specific hearing aid models. However, because they were designed for linear, time-invariant hearing instruments, they have limitations for today’s nonlinear, adaptive instruments and cannot provide meaningful information about real-life performance in the presence of dynamically changing acoustic environments.

Speech is the most important stimulus encountered by hearing aid users and nonlinear hearing aids with adaptive characteristics process speech differently than they do stationary signals like sine waves and unmodulated noise. Therefore, it seems preferable for standardized test procedures to use stimuli that are as close as possible to natural speech.  Indeed, there are some hearing aid test protocols that use samples of natural speech or live speech. But natural speech stimuli will have different spectra, fundamental frequencies, and temporal characteristics depending on the speaker, the source material and the language. For hearing aid verification measures to be comparable to each other it is necessary to have standardized stimuli that can be used internationally.

Alternative test stimuli have been proposed based on the long-term average speech spectrum (Byrne et al., 1994) or temporal envelope fluctuations (Fastl, 1987). The International Collegium for Rehabilitative Audiology (ICRA) developed a set of stimuli (Dreschler, 2001) that reflect the long-term average speech spectrum and have speech-like modulations that differ across frequency bands.  ICRA stimuli have advantages over modulated noise and sine wave stimuli in that they share some similar characteristics with speech, but they lack speech-like comodulation characteristics (e.g., fundamental frequency). Furthermore, ICRA stimuli are often classified by signal processing algorithms as “noise” rather than “speech”, so they are less than optimal for measuring how hearing aids process speech.

The European Hearing Instrument Manufacturers Association (EHIMA) is developing a new measurement procedure for nonlinear, adaptive hearing instruments and an important part of their initiative is development of a standardized test signal or International Speech Test Signal (ISTS).  The development and analysis of the ISTS was described in a paper by Holube, et al. (2010).

There were fifteen articulated requirements for the ISTS, based on available test signals and knowledge of natural speech, the most clinically salient of which are:

  • The ISTS should resemble normal speech but should be non-intelligible.
  • The ISTS should be based on six major languages, representing a wide range of phonological structures and fundamental frequency variations.
  • The ISTS should be based on female speech and should deviate from the international long-term average speech spectrum (ILTASS) for females by no more than 1dB.
  • The ISTS should have a bandwidth of 100 to 16,000Hz and an overall RMS level of 65dB.
  • The dynamic range should be speech-like and comparable to published values for speech (Cox et al., 1988; Byrne et al., 1994).
  • The ISTS should contain voiced and voiceless components. Voiced components should have a fundamental frequency characteristic of female speech.
  • The ISTS should have short-term spectral variations similar to speech (e.g., formant transitions).
  • The ISTS should have modulation characteristics similar to speech (Plomp, 1984).
  • The ISTS should contain short pauses similar to natural running speech.
  • The ISTS stimulus should have a 60 second duration, from which other durations can be derived.
  • The stimulus should allow for accurate and reproducible measurements regardless of signal duration.

Twenty-one female speakers of six different languages (American English, Arabic, Mandarin, French, German and Spanish) were recorded while reading a story, the text and translations of which came from the Handbook of the International Phonetic Association (IPA).  One recording from each language was selected based on a number of criteria including voice quality, naturalness and median fundamental frequency. The recordings were filtered to meet the ILTASS characteristics described by Byrne et al. (1994) and were then split into 500ms segments that roughly corresponded to individual syllables. These syllable-length segments were attached in pseudo-random order to generate sections of 10 or 15 milliseconds. Each of the resulting sections could be combined to generate different durations of the ISTS stimulus and no single language was used more than once in any 6-segment section.  Speech interval and pause durations were analyzed to ensure that ISTS characteristics would closely resemble natural speech patterns.

For analysis purposes, a 60-second ISTS stimulus was created by concatenation of 10- and 15-second sections.  This ISTS stimulus was measured and compared to natural speech and ICRA-5 stimuli based on several criteria:

  • Long-term average speech spectrum (LTASS)
  • Short term spectrum
  • Fundamental frequency
  • Proportion of voiceless segments
  • Band-specific modulation spectra
  • Comodulation characteristics
  • Pause and speech duration
  • Dynamic range (spectral power level distribution)

On all of the analysis criteria, the ISTS stimulus resembled natural speech stimuli as well or better than ICRA-5 stimuli. Notable improvements for the ISTS over the ICRA-5 stimulus were its comodulation characteristics and dynamic range of 20-30dB, as well as pauses and combinations of voiced and voiceless segments that more closely resembled the distributions in natural speech.  Overall, the ISTS was deemed an appropriate speech-like stimulus proposal for the new standard measurement protocol.

Following the detailed analysis, the ISTS stimulus was used to measure four different hearing instruments, which were programmed to fit a flat, sensorineural hearing loss of 60dBHL.  Each instrument was nonlinear with adaptive noise reduction, compression and feedback management characteristics. The first-fit algorithms from each manufacturer were used, with all microphones fixed to an omnidirectional mode.  Instead of yielding gain and output measurements across frequency for one input level, the results showed percentile dependent gain (99th, 65th and 30th) across frequency as referenced to the long-term average speech spectrum.  The percentile dependent gain values provided information about nonlinearity, in that the softer components of speech were represented by the 30th percentile, moderate and loud speech components were represented by the 65th and 99th percentiles, respectively.  Relations between these three percentiles represented the differences in gain for soft, moderate and loud sounds.

The measurement technique described by Holube and colleagues, using the ISTS stimulus, offers significant advantages over current measurement protocols with standard sine wave or noise stimuli. First and perhaps most importantly, it allows hearing instruments to be programmed to real-life settings with adaptive signal processing features active. It measures how hearing aids process a stimulus that very closely resembles natural speech, so clinical verification measures may provide more meaningful information about everyday performance. By showing changes in percentile gain values across frequency, it also allows compression effects to be directly visible and may be used to evaluate noise reduction algorithms as well. The authors also note that the acoustic resemblance of ISTS to speech with its lack of linguistic information may have additional applications for diagnostic testing, telecommunications or communication acoustics.

The ISTS is currently available in some probe microphone equipment and will likely be introduced in most commercially available equipment over the next few years. Its introduction brings a standardized speech stimulus, for the testing of hearing aids, to the clinic. An important component of clinical best practice is the measurement of a hearing aid’s response characteristics. This is most easily accomplished through insitu probe microphone measurement in combination with a speech test stimulus such as the ISTS.

References

American National Standards Institute (ANSI ). ANSI S3.22-2003. Specification of hearing aid characteristics. New York: Acoustical Society of America.

Byrne, D., Dillon, H., Tran, K., Arlinger, S. & Wibraham, K. (1994). An international comparison of long0term average speech spectra. Journal of the Acoustical Society of America, 96(4), 2108-2120.

Cox, R.M., Matesich, J.S. & Moore, J.N. (1988). Distribution of short-term rms levels in conversational speech. Journal of the Acoustical Society of America, 84(3), 1100-1104.

Dreschler, W.A., Verschuure, H., Ludvigsen, C. & Westerman, S. (2001). ICRA noises: Artificial noise signals with speech-like spectral and temporal properties for hearing aid assessment. Audiology, 40, 148-157.

Fastl, H. (1987). Ein Storgerausch fur die Sprachaudiometrie. Audiologische Akustik, 26, 2-13.

Holube, I., Fredelake, S., Vlaming, M. & Kollmeier, B. (2010). Development and analysis of an international speech test signal (ISTS). International Journal of Audiology, 49, 891-903.

International Electrotechnical Commission, 1994, IEC 60118-0. Hearing Aids: Measurement of electroacoustical characteristics, Bureau of the International Electrotechnical Commission, Geneva, Switzerland.

IPA, 1999. Handbook of the International Phonetic Association. Cambridge University Press.

Plomp, R. (1984). Perception of speech as a modulated signal. In M.P.R. van den Broeche, A. Cohen (eds), Proceedings of the 10th International Congress of Phonetic Sciences, Utrecht, Dordrecht: Foris Publications, 29-40.

 

 

 

Will placing a receiver in the canal increase occlusion?

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The influence of receiver size on magnitude of acoustic and perceived measures of occlusion.

Vasil-Dilaj, K.A., & Cienkowski, K.M. (2010). The influence of receiver size on magnitude of acoustic and perceived measures of occlusion. American Journal of Audiology 20, 61-68.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

The occlusion effect, an increase in bone conducted sound when the ear canal is occluded, is a consideration for many hearing aid fittings.  The hearing aid shell or earmold restricts the release of low-frequencies from the ear canal (Revit, 1992), resulting in an increase in low-frequency sound pressure level at the eardrum, sometimes up to 25dB (Goldstein & Hayes, 1965; Mueller & Bright, 1996; Westermann, 1987).  Hearing aid users suffering from occlusion will complain of an “echo” or “hollow” quality to their voices and hearing their own chewing can be particularly annoying. Indeed, perceived occlusion is reported to be a common reason for dissatisfaction with hearing aids (Kochkin, 2000).

Occlusion from a hearing aid shell or earmold is usually managed by increasing vent diameter or decreasing the length of the vent in order to decrease the acoustic mass of the vent (Dillon, 2001; Kiessling, et al, 2005). One potential risk of increasing vent diameter is increased risk of feedback, but this problem has been alleviated by improvements in feedback cancellation. Better feedback management has also resulted in more widespread use of open fit, receiver-in-canal (RIC) instruments which have proven effective in reducing measured and perceived occlusion (Dillon, 2001; Kiessling et al., 2005; Kiessling et al., 2003; Vasil & Cienkowski, 2006).

Though open fit BTE hearing instruments are designed to be acoustically transparent, some open fittings still result in perceived occlusion.  Interestingly, perceived occlusion is not always strongly or even significantly correlated with measured acoustic occlusion (Kiessling et al., 2005; Kuk et al., 2005; Kampe & Wynne, 1996), so it is apparent that other factors do contribute to the perception of occlusion.  The size of the receiver and/or eartip, as well as the size of the ear canal, affect the amount of air flow in and out of the ear canal and it seems likely that these factors could affect the amount of acoustic and perceived occlusion.

Thirty adults, 17 men and 13 women, participated in the study. All had normal hearing, unremarkable otoscopic examinations and normal tympanograms. Two measures of ear canal volume were obtained: volume estimates from the tympanometry screener and estimates determined from earmold impressions that were sent to a local hearing aid manufacturer.  Participants were fitted binaurally with RIC hearing instruments.  Instead of domes used clinically with RIC instruments flexible receiver sleeves designed specifically for research purposes were used.  Use of the special receiver sleeves allowed the researchers to increase the overall circumference of the receiver systematically so that six receiver size conditions could be evaluated:  no receiver, receiver only (with a circumference of 0.149 in.), 0.170 in., 0.190 in., 0.210in. and 0.230 in.

Real-ear unoccluded and occluded measures were obtained with subjects vocalizing the vowel /i/. Subjects monitored the level of their vocalizations via a sound level meter. Real ear occlusion effect (REOE) was determined by subtracting the SPL levels for the unoccluded response from the occluded response (REOR-REUR = REOE).  Subjective measures were obtained by asking subjects to rate their perception of occlusion on a five point scale ranging from “no occlusion” to “complete occlusion”. To avoid bias in the occlusion ratings, participants were not allowed to view the hearing aids or receiver sleeves until after testing was completed.

Results indicated that measured acoustic occlusion was very low for all conditions, especially below 500Hz, where it was below 2dB for most of the receiver conditions. For frequencies above 500Hz, REOE increased as receiver size increased. The no receiver and receiver only conditions had the least amount of measured occlusion and the largest receiver sizes had the most. There was no significant interaction between receiver size and frequency.

Perceived occlusion also increased as receiver size increased and though it was mild for most participants in most of the conditions, for the largest receiver condition, some participants rated occlusion as severe. Perceived occlusion was not significantly correlated with measured acoustic occlusion for low frequencies, and the two measures were only weakly correlated for frequencies between 700-1500Hz.

There was no significant relationship between either measure of ear canal volume and perceived or acoustic measures of occlusion. However, adequate ear canal volume to accommodate all receiver sizes was an inclusion criterion for the study, so the authors suggest that smaller ear canal volume could still be a factor in perceived or acoustic occlusion and may warrant further study.

The results of the current investigation show that occlusion was minimal for most of the receiver sizes. These findings are in agreement with previous studies of vented hollow molds, completely open IROS shells (Vasil & Cienkowski, 2006), large 2.4mm vents and silicone ear tips (Kiessling et al, 2005). REOEs for the two largest receivers matched results for a hollow mold with 1mm vent (Kuk et al, 2009) and the REOEs for the two smallest receivers matched results for hollow molds with 2mm and 3mm vents (Kuk et al, 2009).  The authors also point out that there was minimal insertion loss for all conditions. Insertion loss from closed earmolds can amount to 20dBHL (Sweetow, 1991) and can contribute to a perception of occlusion or poor voice quality.  The relative lack of insertion loss is yet another potential advantage of open and RIC fittings.

Perception of occlusion did increase with the size of the receiver, but overall differences were small. This is in agreement with prior research suggesting that reduction of air flow out of the ear canal results in more low-frequency energy in the ear canal (Revit, 1992), which can cause an increase in occlusion (Dillon, 2001). The authors point out that although subjects were not able to see the receivers prior to insertion, they were probably aware of the size and weight differences and could have been influenced by the perception of a larger object in the ear as opposed to actual occlusion. This may also be the case for hearing aid users, perhaps particularly so for individuals with smaller or tortuous ear canals.

The occlusion effect can be challenging, especially when anatomical or other constraints result in the use of minimal venting for individuals with good low-frequency hearing. The results reported here suggest that acoustic occlusion with RIC instruments is slight and may not always be related to perceived occlusion. Therefore, a client’s perception of “hollow” voice quality, “echoey” sound quality or a plugged sensation may be the most reliable indication of occlusion and the most important determinant of eartip size or venting characteristics. The administration of an occlusion rating scale or other self-evaluation techniques may also prove helpful in evaluating occlusion and its impact on overall hearing aid satisfaction.

References

Dillon, H. (2001). Hearing aids. New York, NY: Thieme.

Goldstein, D.P.,  & Hayes, C.S. (1965). The occlusion effect in bone conduction hearing.  Journal of Speech and Hearing Research 8, 137-148.

Kampe, S.D., & Wynne, M.K. ( 1996). The influence of venting on the occlusion effect. The Hearing Journal 49(4), 59-66.

Kiessling, J., Brenner, B., Jespersen, C.T., Groth, J., & Jensen, O.D. (2005). Occlusion effect of earmolds with different venting systems. Journal of the American Academy of Audiology, 16, 237-249.

Kiessling. J., Margolf-Hackl, S., Geller, S., & Olsen, S.O. (2003). Researchers report on a field test of a non-occluding hearing instrument. The Hearing Journal , 56(9), 36-41.

Kochkin, S. (2000). MarkeTrak V: Why my hearing aids are in the drawer: The consumer’s perspective. The Hearing Journal 53 (2), 34-42.

Kuk, F.K. , Keenan, D., & Lau, C.C. (2005). Vent configurations on subjective and objective occlusion effect. Journal of the American Academy of Audiology 16, 747-762.

Mueller, H.G., & Bright, K.E. (1996). The occlusion effect during probe microphone measurements. Seminars in Hearing 17 (1), 21-32.

Revit, L. (1992). Two techniques for dealing with the occlusion effect. Hearing Instruments 43 (12), 16-18.

Sweetow, R. W. (1991). The truth behind “non-occluding” earmolds. Hearing Instruments 42 (1), 25.

Vasil, K.A., & Cienkowski, K.M. (2006). Subjective and objective measures of the occlusion effect for open-fit hearing aids. Journal of the Academy of Rehabilitative Audiology 39, 69-82.

Vasil-Dilaj, K.A., & Cienkowski, K.M. (2010). The influence of receiver size on magnitude of acoustic and perceived measures of occlusion. American Journal of Audiology 20, 61-68.

Westermann, V.H. (1987). The occlusion effect. Hearing Instruments, 38 (6), 43.

Does Expansion Decrease Low Level Speech Understanding?

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Effects of Expansion on Consonant Recognition and Consonant Audibility

Brennan, M., & Souza, P. (2009). Effects of expansion on consonant recognition and consonant audibility. Journal of the American Academy of Audiology 20, 119-127.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors. 

The primary goal of a hearing aid fitting is to improve audibility and availability of speech sounds while maintaining comfort and loudness tolerance.  A linear hearing aid fitting may provide audibility for average speech sounds but may result in discomfort or for loud sounds and inaudibility for soft sounds. The use of wide-dynamic range compression (WDRC) has addressed these issues, helping maximize the useful dynamic range of hearing for individuals who require amplification for quiet and moderate sounds, yet have limited tolerance for loud sounds.

One potential issue with WDRC has been the increased audibility of very soft environmental sounds, which may be unwelcome for individuals who have adjusted to long-term hearing loss and are not used to perceiving these sounds. An additional problem for individuals with good residual hearing at some frequencies is that they will hear circuit noise from the hearing aid itself. Both of these issues can be unpleasant for the listener, possibly resulting in rejection or limited use of the hearing aids.

Expansion makes hearing aids quieter at low input levels. This is done in almost all modern hearing aids in order to reduce annoying environmental or circuit noise.  There is concern, however, that if too aggressive, expansion can have a detrimental effect on speech intelligibility (Plyler et al., 2005). It has been proposed that reduced speech recognition ability with expansion is due to reduced audibility of speech cues (Walker et al, 1984; Plyler et al, 2007).

Previous examinations of expansion have measured its effect on audibility of room noise (Plyler et al, 2005) or the long-term average speech spectrum or LTASS (Zakis and Wise, 2007) but did not directly measure the effect of expansion on audibility of the speech signals.  The current authors sought more specific insight into the effect of expansion on speech recognition by studying the relationship between expansion and consonant audibility.

Though there may be other related parameters warranting examination, the primary variables of interest relating to expansion are the ratio and the kneepoint.  In theory, a high expansion kneepoint should have a negative effect on speech recognition, because gain for stimuli below the kneepoint is reduced, resulting in decreased audibility.  Speech presented above the expansion kneepoint should be less affected by the expansion.

Therefore, the hypotheses for Brennan and Souza’s study were as follows:

1. A high expansion kneepoint will significantly reduce consonant-vowel (CV) recognition.

2. A high expansion kneepoint will significantly reduce CV audibility.

3. The effect of expansion on speech recognition and audibility will be reduced for increased speech input levels.

4. There will be a significant positive correlation between CV recognition and audibility for each condition.

Thirteen hearing-impaired individuals participated in the experiment. Nine were experienced hearing aid users; the remaining four did not use hearing aids. Subjects were fitted monaurally with a multi-channel, digital, behind-the-ear hearing aid.  Venting was 3mm for most subjects, but was reduced to 1mm for two subjects and plugged for one subject. 

The hearing aids were set to DSL 4.1 targets and had three separate programs:

1. Multichannel WDRC with an expansion kneepoint of 50dB SPL (high kneepoint condition)

2. Multichannel WDRC with an expansion kneepoint of 30dB SPL (low kneepoint condition)

3. Linear amplification with output compression limiting (control condition)

Expansion ratio was constant at 0.7:1, which represents a typcial expansion ratio currently available in hearing aids.

Eight CV nonsense syllables, four voice and four unvoiced, from the Nonsense Syllable Test (Dubno and Dirks, 1982) were presented to subjects at 50, 60 and 71dB SPL. Recordings of aided stimuli were measured at the tympanic membrane for each subject (Souza and Tremblay, 2006) and signal audibility was determined using the Aided Audibility Index -AAI (Stelmachowicz et al, 1994) using modifications for hearing-impaired subjects as described by Souza and Turner (1998).

Three of Brennan and Souza’s hypotheses were confirmed:  high expansion kneepoints significantly reduced signal audibility for speech at all presentation levels and consonant-vowel recognition scores were significantly lower for the high kneepoint condition, especially at presentation levels of 50dB and 60dB SPL. Subsequent regression analyses revealed that CV syllable recognition scores were significantly associated with audibility. The authors’ presumption that the effect of expansion on audibility and speech recognition would decrease with increasing speech presentation levels was not confirmed.  Instead, expansion had negative effects on CV recognition and audibility at all presentation levels.  This was in contrast with previous work reporting that expansion did not affect speech recognition above certain levels (Walker et al, 1984; Bray and Ghent, 2001; Plyler et al, 2005a, 2007), but this discrepancy may be explained by differences in presentation level, speech materials, expansion ratio or time constants or other hearing aid settings.

Despite some variability in results across studies, it is clear that high expansion kneepoints result in decreased speech recognition scores, presumably due in part to decreased audibility. Other potential explanations involve degradation of temporal cues and disruption of the intensity relationships between consonants and vowels, which provide important cues for consonant recognition (Walker et al, 1984; Hedrick and Younger, 2007).

Expansion is a feature of modern hearing aids that is often misunderstood; because it is characterized in terms of a ratio and kneepoint, it may be easily confused with compression.  Essentially the opposite of compression, expansion results in less amplification for softer sounds than louder sounds.  The expansion kneepoint is often the same as the compression kneepoint, indicating that expansion occurs below the kneepoint level and compression occurs above it. Alternatively, the input/output function of a circuit might show a region of linearity between the expansion and compression kneepoints.

Regardless of its various characteristics, expansion may help reduce the perception of unwanted environmental and hearing aid circuit noise, resulting in improved subjective hearing aid performance.  However, because audibility and speech recognition are two primary goals of amplification, it is essential to ensure that expansion does not result in decreased objective performance.  Brennan and Souza suggest that the use of active noise reduction for lower level stimuli may provide the benefits of expansion without the negative effect on speech recognition, but more research on this topic is warranted. Because expansion is commonly used in current hearing instruments, it is important for audiologists to understand the principles of compression and expansion so that appropriate settings can be selected to maximize audibility and comfort for individual hearing aid users.  And as always, counseling is essential for preparing both new and experienced hearing aid users for adjustment to new hearing aid technology and the perception of normal environmental sounds.

References

Brennan, M. & Souza, P. (2009). Effects of expansion on consonant recognition and consonant audibility. Journal of the American Academy of Audiology 20: 119-127.

Dubno, J.R. & Dirks, D.D. (1982). Evaluation of hearing-impaired listeners using a Nonsense-Syllable Test: I test reliability. Journal of Speech, Language and Hearing Research 25: 135-141.

Hedrick, M.S. & Younger, M.S. (2007). Perceptual weighting of stop consonant cues by normal and impaired listeners in reverberation versus noise. Journal of Speech, Language and Hearing Research 50: 254-269.

Plyler, P., Hill, A., & Trine, T. D. (2005a). The effects of expansion on the objective and subjective performance of hearing instrument users. Journal of the American Academy of Audiology, 16, 101-113.

Plyler, P.N., Lowery, K.J., Hamby, H.M. & Trine, T.D. (2007). The objective and subjective evaluation of multichannel expansion in wide dynamic range compression hearing instruments. Journal of Speech, Language and Hearing Research 50: 15-24.

Souza, P.E. & Tremblay, K.L. (2006). New perspectives on assessing amplification effects. Trends in Amplification 10: 119-143.

Souza, P.E. & Turner, C.W. (1998). Multichannel compression, temporal cues and audibility. Journal of Speech, Language and Hearing Research 41: 315-326.

Stelmachowicz, P.G., Lewis, D.E., Kalberer, L. & Creutz, T. (1994). Situational Hearing Aid Response Profile User’s Manual (SHARP, v 6.0). Omaha: Boys Town National Research Hospital.

Walker, G., Byrne, D., & Dillon, H. (1984). The effects of multichannel compression/expansion amplification on the intelligibility of nonsense syllables in noise. Journal of the Acoustical Society of America 76: 746-757.

Zakis, J.A. & Wise, C. (2007). The acoustic and perceptual effects of two noise suppression algorithms. Journal of the Acoustical Society of America 121: 433-441. 

Understanding the best listening configurations for telephone use when wearing hearing aids

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Understanding the best listening configurations for telephone use when wearing hearing aids

Picou, E.M. & Ricketts, T.A. (2010) Comparison of wireless and acoustic hearing aid based telephone listening strategies. Ear and Hearing 31(6), 1-12.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

Telephone use is an important consideration for hearing aid users. It is often challenging to arrive at the appropriate coupling method to the ear and related hearing aid settings. Many people with hearing loss have difficulty hearing on the telephone and concerns about telephone use may result in reluctance to purchase new hearing aids or to use aids that have already been purchased (Kochkin, 2000).  Indeed, in a survey of hearing aid satisfaction, one in five respondents reported dissatisfaction when using the telephone with a hearing aid (Kochkin, 2005).

There are a number of factors that affect a hearing aid user’s ability to hear on the phone, including lack of visual cues, reduced bandwidth, background noise and difficulty coupling the phone to the hearing aid. The lack of visual cues has been addressed recently with videoconferencing applications, but these are not commonly used, especially among older individuals. The reduced bandwidth (approximately 300 to 3,300 Hz) is characteristic of sound transmission over the phone, so there is little an individual can do improve the availability of high frequency speech cues over the phone. Background noise and coupling issues can be addressed in a number of ways, depending on the individual and the circumstances.

There are two ways a hearing aid can be coupled directly to the telephone; acoustically and with an inductive telecoil  or with acoustic settings that focus on the telephone’s limited frequency range. A drawback to the acoustic setting is that the hearing aid microphone is active which may result in feedback (Latzel et. al., 2001; Palmer, 2001; Chung, 2004).  Despite recent improvements in feedback control, this remains a problem, especially for those with severe hearing loss whose hearing aids require more gain.  Additionally, the microphone picks up environmental noise that competes with the telephone signal, decreasing the signal to noise ratio.  Telecoils can be a solution for feedback and poor signal to noise ratios, but they are subject to interference from fluorescent lights, computer equipment and power lines.  Furthermore, it can be difficult to determine the proper positioning of the phone for optimal sound quality, as the telephone receiver must be placed as close to the telecoil as possible (Tannahill, 1983; Compton, 1994; Yanz & Preves, 2003).

A recent option for telephone is through the use of intermediate wireless accessories, these route sound from the phone to the hearing aids via a combination of Bluetooth and a direct-to-hearing aid wireless technology. These devices address the problems with acoustic or telecoil coupling, and have the possibility of providing some additional benefit if the telephone signal is bilaterally routed (Green, 1976; Moore, 1998; Hall et al, 1984; Quaranta and Cervellera, 1974).  Many hearing aid manufacturers offer wireless devices, but it is unclear whether their use results in significantly improved speech recognition over the phone. Even with wireless routing of the phone signal, there may still be detrimental effects of background noise, especially for individuals with open-canal hearing aids (Dillon, 1985; 1991).

The purpose of Picou and Ricketts’ study was to examine speech recognition performance with monaural and binaural wireless phone transmission, as well as a monaural acoustic condition, in the presence of two levels of background noise. They also evaluated performance with occluding versus non-occluding domes.

Twenty individuals with sloping, high-frequency, sensorineural hearing loss participated in the study. Subjects were fitted with binaural, receiver-in-canal hearing instruments with a wireless transmitter accessory. Half of the subjects were tested with open, non-occluding domes and half were tested with closed, occluding domes.

A total of seven hearing aid and telephone configurations were tested in two background noise levels (55dBA and 65dBA). Subjects responded to sentences from the Connected Speech Test (CST, Cox et al., 1987).  Speech stimuli were band pass filtered from 300 to 3400Hz to simulate telephone transmission and presented at 65dB.  Competing speech babble was presented through four loudspeakers positioned around the listener at a distance of 1 meter. All test conditions – hearing aid condition, dome type, noise level – were counterbalanced to avoid effects of learning and fatigue.

This study illuminates some important considerations in telephone use and supports the use of wireless telephone accessories, especially with bilateral routing.  The participants subjects performed best with external hearing aid microphones turned off, but the authors acknowledge that for safety and monitoring of environmental sounds, it may be advisable to leave microphones active at an attenuated level. The authors suggest that further investigation is warranted to determine optimal levels of microphone attenuation to allow for successful speech recognition over the phone, while preserving environmental awareness.

Performance with occluding domes was better than open domes for wireless telephone signal routing in noise. Occluding domes reduce the environmental noise entering the ear canal, providing an improvement in signal to noise ratio. In the acoustic phone condition, open domes performed better than occluding domes. Subjects tended to position the phone directly over the ear canal which likely improved signal to noise ratio by blocking background noise and isolating the speech transmitted from the phone.

Specific observations were made for participants wearing open-canal hearing aids. Specifically, users with open domes should be instructed to hold the phone directly over the ear canal for optimal speech recognition. Programming adjustments may be necessary to increase availability of low and mid-frequency speech cues and improve signal to noise ratio.  Conversely, users with occluding domes should be advised of the potential limitations of direct acoustic coupling to the phone and should be instructed to hold the phone receiver as close to the microphone as possible. Alternatively, patients with occluding domes may be better off using a telecoil, if available, for situations in which they cannot use a wireless device.

Interestingly, the no significant improvement in speech recognition resulted from plugging the non-test ear or muting the hearing aid on the non-test ear.  This is consistent with previous research on masking level differences for tones (Green, 1976; Moore 1998) as well as a previous study of speech recognition over the phone, which found no improvement for normal-hearing listeners when the non-phone ear was plugged.  This is inconsistent, however, with reported preferences of hearing aid users.  Despite the lack of improvement in the current study, the authors acknowledged that muting the hearing aid on the non-phone ear may reduce listening effort, which is therefore preferred by the listener.

For users of wireless accessories, the results of this study clearly indicate that binaural routing is ideal. But for hearing aid users who do not have wireless devices, the optimal hearing aid settings and coupling method may depend on several factors. The extent of venting or openness should be considered when choosing an acoustic phone coupling; individuals with minimal venting may not hear well unless they are able to hold the telephone over the hearing aid microphone, while patients with open fittings may experience more challenges with background noise interference than the more occluded wearer.

Regardless of whether a client uses an intermediate wireless device for binaural telephone streaming, monaural acoustic listening or telecoil coupling, the attenuation level of the hearing aid microphones is also a consideration. For binaural wireless routing or streaming it is advisable to keep both hearing aid microphones active but attenuated, to preserve awareness of environmental sounds. For monaural acoustic/telecoil combinations the microphone level on the opposite ear can be attenuated slightly to allow environmental awareness but reduce distraction from surrounding noise. As noted earlier, further study is warranted to determine optimal microphone attenuation levels.

References

Chung, K. (2004). Challenges and recent developments in hearing aids. Part II. Feedback and occlusion effect reduction strategies, laser shell manufacturing processes and other signal processing technologies. Trends in Amplification 8, 125-164.

Compton, C. (1994). Providing effective telecoil performance with in-the-ear hearing instruments. Hearing Journal 47, 23-26.

Cox, R.M., Alexander, G.C. & Gilmore, C.A. (1987). Development of the connected speech test (CST). Ear and Hearing, 8 (supplement): 119S-126S.

Dillon, H. (1985). Earmolds and high frequency response modification. Hearing Instruments 36, 8-12.

Dillon, H. (1991). Allowing for real ear venting effects when selecting the coupler gain of hearing aids. Ear and Hearing 12(6), 406-416.

Green, D.M. (1976). An Introduction to Hearing. Hillsdale, NJ: Lawrence Erlbaum Associates.

Hall, J.W., Tyler, R.S., Fernandes, M.A. (1984). Factors influencing the masking level difference in cochlear hearing-impaired and normal-hearing listeners. Journal of Speech and Hearing Research 27, 145-154.

Hawkins, D.B. (1984). Comparisons of speech recognition in noise by mildly-to-moderately hearing-impaired children using hearing aids and FM systems. Journal of Speech and Hearing Disorders 49, 409-418.

Kochkin, S. (2000). MarkeTrak V: “Why my hearing aids are in the drawer”: The consumers’ perspective. Hearing Journal 53, 34-42.

Kochkin, S. (2005). MarkeTrak VII: Customer satisfaction with hearing aids in the digital age. Hearing Journal 58, 30-39.

Latzel, M., Gebhart, T.M. & Kiessling, J. (2001). Benefit of a digital feedback suppression system for acoustical telephone communication. Scandanavian Audiology Supplementum 52, 69-72.

Moore, B.C.J. (1998). Cochlear Hearing Loss. London: Whurr Publishers.

Palmer, C.V. (2001). Ring, ring! Is anybody there? Telephone solutions for hearing aid users. Hearing Journal 54, 10.

Picou, E.M. & Ricketts, T.A. (2010) Comparison of wireless and acoustic hearing aid based telephone listening strategies. Ear and Hearing 31(6), 1-12.

Quaranta, A. & Cervellera, G. (1974). Masking level difference in normal and pathological ears. Audiology 13, 428-431.

Tannahill, J.C. (1983). Performance characteristics for hearing aid microphone versus telephone and telephone/telecoil reception modes. Journal of Speech and Hearing Research 26, 195-201.

Yanz, J.L. & Preves, D. (2003). Telecoils: Principles, pitfalls, fixes and the future. Seminars in Hearing 24, 29-41.

 

 

Some benefits of increasing the number of compression channels

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Multichannel compression hearing aids: Effect of number of channels on speech discrimination in noise.

Yund, W.E. & Buckles, K.M. (1995).  Multichannel compression hearing aids: Effect of number of channels on speech discrimination in noise. Journal of the Acoustical Society of America 97(2), 1206-1223.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors. 

Yund and Buckles 1995 study of multichannel compression hearing aids offers valuable insight for the recommendation and fitting of current hearing aids.  A wide array of hearing aids are available, at different technology levels, and the number of compression channels is often a distinguishing feature. Typically, clinicians may recommend higher quality or more sophisticated processing for clients who lead active lives and participate in activities in challenging listening environments.  The effect of multiple compression channels on hearing aid performance in noise is therefore an important consideration in determining the most appropriate instrument for an individual client.

There are theoretical reasons to expect multiple channels of compression to improve or reduce speech recognition ability in the presence of noise.  Potential benefits of multichannel compression include plausible reduction of competing noise and preservation of audible speech information, as well as better fit to audiometric thresholds. Because the amplification within a given channel is based on the signal-to-noise ratio within that channel, it follows that speech information could be lost due to inadequate application of gain in instruments with fewer channels. Most previous studies of multichannel compression hearing aids compared their performance to that of linear hearing aids, but a study by Barfod (1978) showed improvement in performance as the number of channels increased from 2 to 4. At the time (1995) the reviewed paper was published, there were few studies to support performance increases with an increasing number of compression channels.

Yund and Buckles also acknowledged potential detrimental effects of multichannel compression, including reduction of temporal and spectral contrasts, resulting in increased phoneme confusion (Bustamante & Braida, 1987; Plomp, 1988, 1989; Villchur, 1989). The authors point out that theoretically, any deleterious effects of multichannel compression should increase with increasing number of channels, because systems with more channels would be better able to detect and respond to intensity variations across frequency. It follows, then, that at some point any benefit in multiple channel compression should plateau and possibly begin to decrease with increasing channels. Some have argued, however, that a reduction in amplitude contrasts might be counterbalanced somewhat by recruitment effects, which could result in larger perceptual contrasts (Moore, 1991).

Sixteen subjects with sensorineural hearing loss of varying degrees and etiologies participated in the experiment. The authors included participants with a broad range of hearing losses in an effort to examine any interactions between compression characteristics and hearing loss configuration or severity.  Test materials were based on the Nonsense Syllable Test (NST) by Resnick et al. (1975). The original test used nonsense syllables presented in a carrier phrase, but the present study used newly recorded stimuli presented in isolation. Test stimuli were CV or VC monosyllables that varied according to voicing (voiced or voiceless), consonant position (initial or final) and vowel context (/a/, /i/, or /u/).  All stimuli were recorded by a male speaker and a female speaker.

The nonsense syllables were combined with speech-shaped noise (French & Steinberg, 1947) prior to any linear or multichannel compression processing. The intensity of the noise was always 70dB, and speech was added to noise at 85, 80, 75, 70 and 65dB SPL to yield signal-to-noise ratios ranging from +15dB to -5dB.  Stimuli for each talker and each signal-to-noise ratio were presented in seven different processing conditions:  unprocessed, linear amplification (20 dB, flat), 4-channel compression, 6-channel compression, 8-channel compression, 12-channel compression and 16-channel compression. 

Not surprisingly, the results showed a strong effect of signal-to-noise ratio, with performance deteriorating as signal intensity decreased.  In quiet conditions, discrimination was better for the male voice than for the female voice, but this  reversed in the presence of noise, resulting in an advantage for discrimination of the female voice. The average spectrum for the female voice had relatively more energy in the high frequencies than the male voice and had peaks occurring at higher frequencies. The authors pointed out that because speech-spectrum noise has less energy in high frequencies, it may be less effective at masking the female voice, thus causing a reduced performance decrement for the female voice in the presence of noise.

There was a significant interaction between number of channels and voice; increasing the number of channels produced a greater improvement in discrimination for the male than it did for the female voice.  As noted above, speech-weighted noise has more energy in low frequencies, which could be more effective at masking the male voice. Because gain reductions in a multi-channel hearing aid are based on the level within a given channel, an instrument with fewer channels would be expected to lose more low-frequency speech information in its attempts to reduce gains applied to high-level, low-frequency noise. Instruments with a higher number of processing channels would therefore be expected to have less of a detrimental effect on low-frequency speech information, which is exactly what the results of the current study suggest.

There was a significant effect overall for the number of processing channels. Speech discrimination clearly improved as the number of channels increased from 4 to 8 channels and remained consistent above 8 channels.  There was no significant interaction between number of channels and signal-to-noise ratio; increases in the number of channels did not yield further improvement or decrement for performance at poor signal-to-noise ratios. 

The authors conducted a detailed analysis of consonant discrimination, which yielded information about the transmission of specific stimulus features and how they changed with increasing channels. In general, voiceless consonants were discriminated better than voiced, CV monosyllables were discriminated better than VC, and consonants in the context of /a/ were discriminated better than those in the context of /i/ or /u/. None of these features varied significantly as the number of channels increased. There was, however, a differential effect of number of channels on the perception of place and manner of articulation. Increasing the number of channels yielded more improvement for middle consonants than front or back consonants, and improved fricative perception than nasals and glides, and voiceless stops more than voiced stops. The most striking characteristic to benefit from increased channels was duration, which is a particularly important cue for differentiating fricatives.

The authors analyzed the frequency responses of the multi-channel instruments in their study and found that overall they were remarkably similar with one notable exception. Average amplification at 4 kHz and above increased as the number of processing channels increased from 4 to 8 to 16 channels. The improved high frequency response for instruments with more channels of processing, resulting in better  transmission of high-frequency speech cues is likely to help account for the noted improvements in consonant discrimination.

One aim of the current study was to determine any negative effects of increasing channels of compression. They found no negative effects, at least up to 16 channels, which was the maximum number used in the study. The authors mentioned a few contemporary studies that found increasingly negative effects with increasing number of channels for normal and hearing-impaired subjects, but only when high compression ratios were used (greater than 3:1).  It appears that for the compression characteristics used in the current study, any potential negative effect of increasing compression channels was negated by increases in the availability of speech information or possibly, as Moore suggested, recruitment effects.

Clinicians usually make recommendations for hearing aid style based on audiometric configuration, manual dexterity and anatomical variables, but the choice of technology level is often based on a patient’s lifestyle. There may be many reasons to recommend premium instruments for patients with active lifestyles, including more effective directional microphones, better automatic processing and more precise programming adjustments. The current study supports the importance of multiple channels of processing for better performance in noise, which is one of the major considerations for hearing aid users who participate in activities in challenging listening environments. Specifically, their study showed benefits for instruments with up to 8 channels of processing, a delineation that differentiates many entry-level hearing aids from their more sophisticated counterparts.

Yund and Buckles used multichannel compression instruments that were simpler than those available today.  Advances in digital processing have led to instruments that vary with regard to speed of processing, compression characteristics, adaptive directionality, noise reduction and other parameters.  Although an updated investigation of their hypotheses with current hearing aid technology could provide interesting insights into their findings, their study still supports the potential benefit of increased channels of processing for individuals with a wide range of hearing losses.

References

Barfod, J. (1978).  Multichannel compression hearing aids: Experiments and considerations on clinical applicability. In Sensorineural Hearing Impairment and Hearing Aids, ed. C. Ludvigsen and J. Barfod. Scandanavian Audiology Supplementum 6, 315-340.

Bustamante, D.K. & Braida, L.D. (1987). Principal-component amplitude compression for the hearing impaired. Journal of the Acoustical Society of America 82, 1227-1242.

French, N.R.  & Steinberg, J.C. ( 1947). Factors governing the intelligibility of speech sounds. Journal of the Acoustical Society of America 19, 90-119.

Moore, B.C.J. (1991). Characterization and simulation of impaired hearing: Implications for hearing aid design. Ear and Hearing 12, 154S-161S.

Plomp, R. (1988). The negative effect of amplitude compression in multichannel hearing aids in the light of the modulation-transfer function. Journal of the Acoustical Society of America 83, 2322-2327.

Plomp, R. (1989). “Reply to ‘Comments on ‘The negative effect of amplitude compression in multichannel hearing aids in the light of the modulation-transfer function.  [Journal of the Acoustical Society of America 83, 2322-2327.]’ [Journal of the Acoustical Society of America 86, 425-427].” Journal of the Acoustical Society of America 86, 428.

Resnick, S.B., Dubno, J.R., Hoffnung, S. & Levitt, H. (1975). Phoneme errors on a nonsense syllable test. Journal of the Acoustical Society of America 58, 114(A).

Villchur, E. (1989). “Comments on ‘The negative effect of amplitude compression in multichannel hearing aids in the light of the modulation-transfer function. ‘ [Journal of the Acoustical Society of America 83, 2322-2327.]” Journal of the Acoustical Society of America 86, 425-427.

Yund, W.E. & Buckles, K.M. (1995).  Multichannel compression hearing aids: Effect of number of channels on speech discrimination in noise. Journal of the Acoustical Society of America 97(2), 1206-1223.

 

 

 

The DSL 5.0a is a successful fitting formula for adults

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Fit to Targets, Preferred Listening Levels, and Self-Reported Outcomes for the DSL v5.0a Hearing Aid Prescription for Adults

Polonenko, M.J., Scollie, S.D., Moodie, S., Seewald, R.C., Laurnagaray, D., Shantz, J. & Richards, A. (2010) Fit to targets, preferred listening levels and self-reported outcomes for the DSL v5.0a hearing aid prescription for adults. International Journal of Audiology 49, 550-560.

This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.

The importance of perceived benefit for successful hearing aid fittings is well established. According to two MarkeTrak studies by Sergei Kochkin (2005, 2007), perceived benefit was the number one factor contributing to hearing aid user satisfaction.  Similarly, the lack of benefit was the most commonly cited reason for hearing aid returns.  Perceived benefit from hearing aids may be determined by a number of factors, but the appropriateness of the individually fitted gain is one of the main contributors (Cox & Alexander, 1994).

The Desired Sensation Level (DSL) prescriptive method was originally developed for children and prescribes targets that are generally very close to children’s preferred listening levels. However, DSL v4.1 targets have been found to prescribe gain that is 9 to 11 dB greater than adult preferred listening levels (Scollie et al., 2005).  Therefore, DSL v5.0a was developed with lower perceived loudness levels, ones that more closely approximate the needs of adult hearing aid users.

The success of a hearing aid prescription can be measured in terms of clinical efficacy, or how closely the hearing aid settings achieve a desired clinical result or test outcome. One such measure is the Preferred Listening Level (PLL). The PLL is defined as “the sound pressure level at the eardrum that the person chooses or prefers for listening to hearing aid amplified speech”(Cox & Alexander, 1994) and represents a compromise between comfort, intelligibility, background noise and distortion (Cox, 1982).  One method of measuring the PPL is by instructing listeners to adjust the volume setting of their hearing instruments to the level that sounds best to them, as they listen to speech presented at a conversational level.

A related but different way to determine the success of a hearing aid fitting strategy is measure effectiveness, or how well hearing aid settings help the user function in real-world situations.  One commonly used measure of hearing aid effectiveness is the Client Oriented Scale of Improvement or COSI (Dillon et al., 1997).  On the COSI questionnaire, the hearing aid user lists up to 5 typical listening situations in which he struggles to hear or would like to hear better.  Following a period of acclimatization, they rate the degree of perceived change in these situations as well as their final ability to function in each situation.

Although the DSL v5.0a prescriptive method was specifically developed for adults with acquired hearing loss, there have been relatively few studies evaluating it. Therefore the current authors sought to determine the electroacoustic feasibility, clinical efficacy, and effectiveness with adult hearing aid users. They had three primary goals:

1.  To measure final fit versus targets in a clinical environment

2.  To evaluate the preferred listening levels (PLLs) of adults versus the DSL v5.0a targets

3.  To measure the effectiveness of the DSL v5.0a prescription as reported on the COSI

Thirty subjects with predominantly sensorineural hearing loss participated in the study. Nineteen were new hearing aid users and eleven were experienced hearing aid users. Twenty-four were fitted binaurally, six were monaural users. Subjects were fitted in private clinics and the audiologists were specifically instructed to program and adjust the instruments to meet the patients’ needs, rather than to meet prescriptive targets.

Hearing aid fittings were matched to DSL 5.0 prescribed targets and verified with simulated real ear measurements, to ensure consistency between test sites and to promote replicable measures. Hearing aids were set to their primary programs and were measured in 2cc couplers, after individual Real Ear to Coupler Differences (RECD) were measured.  Following electroacoustic measures, the aids were fitted to the patients’ ears and adjustments were made based on patients’ subjective satisfaction. These procedures were not carried out according to any protocol established by the authors; the audiologists conducted fine tuning adjustments as needed for each individual. After an approximately 30-day period, subjects returned to the clinics for fine tuning.  After a total acclimatization period of 90 days, preferred listening levels (PLLs) and COSI outcome evaluations were conducted.

Electroacoustic analyses revealed that the clinical fittings were significantly correlated with the DSL v5.0a targets.  Sixty-eight percent of initial fittings were within 2.9 to 4.2 dB of target and 95% were within 5.8 to 8.4 dB of target across frequencies. These results contrast with previous research using NAL-R and NAL-NL1 targets, in which initial fittings differed from targets by 10-15dB. (Sammeth, 1993; Aazh and Moore, 2007).

Preferred listening levels (PLLs) were compared to targets and initial fittings and differed by only about 2dB.  The DSL v5.0a targets were on average 2.6dB lower than PLLs and 1.95dB lower than initial fittings.  Furthermore, DSL v5.0a targets were significantly correlated with PLLs at all frequencies and the targets and PLLs did not differ significantly as a function of degree of hearing loss.  The authors noted a trend for higher PLLs than targets at 250Hz, indicating that some users preferred more low-frequency output than prescribed.

COSI ratings of real-world performance were obtained at the 90-day appointment. The top five situations in which subjects hoped to hear better were similar to those chosen by subjects in the COSI normative study (Dillon et al, 1999):

1.  conversation with a group in noise

2.  conversation with a group in quiet

3.  conversation with one or two partners in noise

4.  listening to the television or radio

5.  conversation with one or two partners in quiet

Subjects were asked to rate the degree of change in their hearing with amplification as well as the final hearing ability (or hearing aid performance) in these situations. Results indicated that they judged their hearing to be “better” or “much better” for 83% of the fittings, which compares well to the normative results obtained by Dillon et al. (1999) of 80%. For final hearing ability, 93% of the current respondents reported hearing 75% of the time (a COSI rating of 4 or better) as compared to 90% of the normative study participants.

The purpose of the current study was to determine if DSLv5.0a prescriptive targets, developed for adults, provided electroacoustically appropriate fittings and subjectively favorable real-world results.  Indeed, clinician-adjusted fittings were within 10 dB of prescriptive targets for 92% of the subjects.  Targets also closely approximated preferred listening levels, which is particularly important because prior studies showed DSL v4.1 targets were generally higher than adults’ preferred levels.  COSI measurements indicated positive ratings for benefit and communication performance which were similar or slightly better than those obtained for the normative population.

An incidental finding of the current study was that instruments with more than six channels of processing may meet prescriptive targets more accurately than those with only six channels.  This was not specifically studied in the current paper, but the authors provided a matrix of number of channels versus errors in matching to target, showing that instruments with more than six channels yielded fewer and smaller errors than those with only six channels of processing. This result is probably consistent with clinical observations, in which sophisticated hearing aid circuits with more channels of processing often provide better fittings than instruments with fewer channels.  The importance of this factor may depend on the client’s hearing loss.  Gently sloping audiometric configurations may generally require fewer channels to meet targets.

The current results show that in a group of adults preferred listening levels and positive real-world outcomes were achieved with programs matched to DSL v5.0a targets, at least in quiet situations. In noisy listening situations, participants may have accessed alternate memories with directionality and noise reduction, causing amplification characteristics to differ from DSL settings.  Even if this is the case, the current study shows that the DSL v5.0a prescriptive measure for adults yields a close approximation to patient preferred settings for a wide range of hearing losses.

References

Aazh, H. &Moore, B.C.J. (2007). The value of routine real ear measurement of the gain of digital hearing aids. Journal of the American Academy of Audiology 18, 653-664.

Cox, R.M. (1982). Functional correlates of electroacoustic performance data. In: G.A. Studebaker & F.H. Bess (eds.) The Vanderbilt Hearing Aid Report. Parkton, MD: York Press, pp. 78-84.

Cox, R.M. & Alexander, G.C. (1994). Prediction of hearing aid benefit: the role of preferred listening levels. Ear and Hearing 15(1), 22-29.

Dillon, H., James, A. & Ginis, J. (1997). Client Oriented Scale of Improvement (COSI) and its relationship to several other measures of benefit and satisfaction provided by hearing aids. Journal of the American Academy of Audiology 8, 27-43.

Dillon, H., Birtles, G. & Lovegrove, R. (1999). Measuring the outcomes of a National Rehabilitation Program: normative data for the Client Oriented Scale of Improvement (COSI) and the Hearing Aid User’s Questionnaire (HAUQ). Journal of the American Academy of Audiology 10, 67-79.

Kochkin, S. (2005). MarkeTrak VII: Customer satisfaction with hearing instruments in the digital age. Hearing Journal 58(9), 30-43.

Kochkin, S. (2008). MarkeTrak VII:  Obstacles to adult non-user adoption of hearing aids. Hearing Journal 60(4), 24-51.

Polonenko, M.J., Scollie, S.D., Moodie, S., Seewald, R.C., Laurnagaray, D., Shantz, J. & Richards, A. (2010) Fit to targets, preferred listening levels and self-reported outcomes for the DSL v5.0a hearing aid prescription for adults. International Journal of Audiology 49, 550-560.

Sammeth, C., Peek, B., Bratt, G., Bess, F. & Amberg, S. (1993). Ability to achieve gain/frequency response and SSPL-90 under three prescription formulas with in-the-ear hearing aids. Journal of the American Academy of Audiology 4, 33-41.

Scollie, S., Seewald, R., Cornelisse, L., Moodie, S., Bagatto, M., et al. (2005). The Desired Sensation Level Multistage Input/Output Algorithm. Trends in Amplification 4(9), 159-197.