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Are you prescribing an appropriate MPO?

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.