Starkey Research & Clinical Blog

What considerations should be made when fitting open-canal directional microphone hearing aids?

 

Article of interest:

Speech Perception in Noise Using Directional Microphones in Open-Canal Hearing Aids

Klemp, E.J. and Dhar, S. (2008)

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.

One of the most common complaints of hearing aid users is difficulty understanding speech in noisy places. Improvements in hearing aid noise reduction may have helped alleviate this problem somewhat, but the most effective solution has been the use of directional microphones (Bentler, 2005).

Behind-the-ear (BTE) hearing aids equipped with directional microphones have been available since the early 1970′s. By the mid 1990′s, the availability of directional microphones in custom hearing aids gave audiologists the opportunity to offer this benefit to patients who preferred in-the-ear (ITE) styles. In recent years, because of their discreet, unobtrusive appearance and comfortable, lightweight fit, open-canal BTEs have become increasingly popular. Most open-canal instruments on the market today have either automatic or manually accessible directional microphone programs.

Research with traditional ITE and BTE instruments has shown that the larger the air vent diameter, the more low-frequency sounds are attenuated (Lybarger, 1985). Therefore, enlarged venting has long been a successful method of reducing perceived occlusion for hearing aid users.  Because the design of open fit hearing aids allows for significantly increased venting, the inherent low-frequency attenuation makes them an excellent option for patients with normal hearing in the low to mid-frequency range and reduces the likelihood of occlusion.

However effective increasing vent diameter may be for reducing occlusion, it has also been demonstrated that the advantage from a directional microphone system is inversely related to vent diameter in traditional hearing aids (Ricketts, 2000). In other words, as the vent diameter increases, the directional effect decreases. The potential reduction in directional benefit, coupled with the possibility of noise entering the open ear canal raises questions about the performance of open-fit hearing aids in noisy situations. The purpose of Kemp and Dhar’s 2008 study was to compare directional hearing aid performance in noise to omnidirectional and unaided conditions.

Sixteen adult subjects with sloping high-frequency hearing losses were tested using the Hearing in Noise Test (HINT) (Nilsson et. al., 1994). The HINT sentences were presented at 65dB in the presence of three channels of competing speech-weighted noise.  Because the authors intended to evaluate hearing aid performance with active noise reduction, the noise was presented with a 12-second lead-in to allow hearing aid signal-processing to become fully activated before the sentences began.

Subjects were fitted with open-fit hearing aids and were evaluated in five counter-balanced conditions:

  • Unaided
  • Omnidirectional mode, no digital noise reduction (OMNI)
  • Omnidirectional mode with digital noise reduction (DNR)
  • Directional mode, no digital noise reduction (DIR)
  • Directional mode with digital noise reduction (BOTH)

The analysis of HINT thresholds (in terms of performance and benefit) in these conditions yielded a number of interesting findings, including:

  1. Directionality alone and combined with digital noise reduction improved thresholds as compared to omnidirectional (by 3.32dB) or unaided (by 2.26 dB) conditions.
  2. Digital noise reduction alone did not improve thresholds.
  3. Omnidirectional conditions (with or without noise reduction) yielded poorer thresholds than unaided conditions.

Though the authors pointed out that the directional benefit found with open-fit BTEs in this study is smaller compared to previous findings with traditional occluded fittings (Nordrum et al, 2006), there was still significant improvement with the use of directional microphones over unaided and omnidirectional conditions. However, they did not find significant improvement with the use of digital noise reduction only and in some DNR-only trials performance was worse than with omnidirectional amplification alone. These findings support the recommendation and use of directional microphones in open-fit hearing aids. Furthermore, they underscore the importance of combining digital noise reduction processing with directionality rather than relying on noise reduction alone to improve speech perception in noise.

Perhaps the most interesting finding, however, is the decrement in performance that the authors found in the omnidirectional conditions. As audiologists, our goal is to help patients function better in everyday situations, so we obviously want to avoid recommendations that could result in increased difficulty. Most open-fit BTEs available today have either automatic or manually adjustable directional programs, so it is possible for patients to be in omnidirectional modes in noisy places unless they are counseled thoroughly on the appropriate use of their programs.

Typical candidates for open-fit hearing aids have normal hearing in the low to middle frequency range. The hearing aids are not providing low-frequency amplification and are therefore not providing a directional advantage in the low-frequency range. High-frequency directionality has been enhanced in recent hearing instruments by reduction in microphone port spacing (Fabry, 2006). This, along with other signal processing advances to extend high-frequency response are likely to result in even better performance in noise with open-fit BTE instruments.

As is often the case in our profession, a patient’s ultimate success and satisfaction with their open-fit hearing aids may depend on adequate counseling on the use of omnidirectional and directional programs. Even patients who prefer to use automatic programs might benefit from having an additional, manually-accessible directional program, for use in situations when the automatic program does not adequately reduce competing noise.  Either way, patients need to understand directionality and how their hearing aids are likely to respond to noise backgrounds in everyday conditions so that they can position themselves appropriately and adjust their aids to the proper setting.

References

Bentler, R. (2005) Effectiveness of directional microphones and noise reduction schemes in hearing aids: a systematic review of the evidence. Journal of the American Academy of Audiology 16: 473-484.

Fabry, D. (2006) Facts vs. myths: the “skinny” on slim-tube open fittings: separating truth from fiction in open fittings. Hearing Review, May. http://www.hearingreview.com /issues/articles/ 2006-05_04.asp

Klemp, E.J., & Dhar, S. (2008). Speech perception in noise using directional microphones in open-canal hearing aids. Journal of the American Academy of Audiology, 19, 571-578.

Lybarger, S. (1985) Earmolds. In: Katz, J. ed. Handbook of Clinical Audiology. 3rd edition. Baltimore: Williams and Wilkins, 885-910.

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(2): 1085-1099.

Nordrum, S., Erler, S., Garstecki, D., Dhar, S. (2006) Comparison of performance on the hearing in noise test using directional microphones and digital noise reduction algorithms. American Journal of Audiology15: 81-91.

Ricketts, T. (2000) Directivity quantification in hearing aids: fitting and measurement effects. Ear and Hearing 21: 45-58.