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Ear & Hearing:

Hearing Aid Accessories for Adults: The Remote FM Microphone

Boothroyd, Arthur

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Objective: The primary goal was to determine the benefits and limitations of a remote FM microphone as a hearing aid accessory. A secondary goal was to determine the predictors of aided and FM-assisted speech perception by adults with hearing loss, in quiet and in noise, using methods derived from Articulation Index theory.

Design: Twelve adults with mild to severe hearing loss, aged 52 to 85, were fit with behind-the-ear FM hearing aids and used them for a minimum of 2 wk. Phoneme recognition was measured before and after the trial period at several speech levels under three conditions: aided in quiet, aided in spectrally matched noise, and FM-assisted in noise. A single session of counseling, instruction and demonstration was provided before the trial period. Perceived benefit was assessed by questionnaire at the end of the trial period. Measured and perceived benefit were examined in relation to age and pure-tone thresholds.

Results: FM-assisted phoneme recognition in noise equaled aided phoneme recognition in quiet. Both were very well predicted by the average pure-tone threshold at 2 and 4 kHz, under a model that assumes Articulation Index (AI) falls with increasing high-frequency threshold at the rate of about 1 percentage point per dB. Aided phoneme recognition in noise was quite well predicted by the same average threshold, under a model that assumes a signal-to-noise ratio of 0 dB reduces Articulation Index to one third of that in quiet. Average perceived benefit was highest for one talker, at a distance, in quiet or in noise. It was lowest for multiple talkers (where the system would be expected to be ineffective) and one close talker in quiet (where the system should be unnecessary). Older subjects and subjects with poorer aided recognition in noise tended to express lower perceived benefit. Many subjects reported that the system was ineffective in reducing background noise. This finding was attributed to use of an “equal gain” criterion in adjusting relative gains via the hearing aid and FM microphones. The single pretrial session of counseling, instruction and demonstration was clearly inadequate for many of the subjects. None expressed an intention to acquire an FM system.

Conclusions: The expected benefits of a remote FM microphone in reducing the negative effects of distance and noise, for a single talker, can be demonstrated under both laboratory and field conditions. The effects of hearing loss, noise and FM assistance, on aided phoneme recognition, are well predicted by methods derived from Articulation Index theory. Considerable counseling, instruction and coaching will be needed, however, to ensure optimal use of this technology. In addition, the relative gains via FM and hearing aid microphones must be adjusted with care.

Remote wireless microphones have a long history of application in the education of children with hearing loss (Ross, 1992). Their use in that context has three goals: a) eliminating the negative effects of noise and reverberation on speech perception, b) increasing speech output levels for children with severe and profound hearing loss and c) maintaining constant speech input regardless of distance between talker and listener. These goals are attained by placement of the microphone within a few inches of the talker’s mouth. As a result the speech level and signal-to-noise ratio are typically 15 to 20 dB higher than at the listener’s location. They also remain constant as the talker moves around. Wireless transmission of the resulting signal is not essential but is obviously more convenient than a wired connection (Boothroyd, 1992).

In recent years miniature FM receivers have been introduced that can be attached to, or built into, behind-the-ear hearing aids. This development has enhanced the possibility of the application of remote, wireless microphones as hearing aid accessories by adults with hearing loss. There are, however, several potential limitations in this application. Obvious examples are intrusiveness and the fact that enhanced reception only applies to the speech of the person wearing the microphone. Less obvious are the loss of control of the microphone by the hearing aid user, and the difficulties of balancing gains via the remote microphone and the hearing aid’s own microphone (American Speech-Language-Hearing Association, 2002). Whether the benefits might outweigh the limitations, and for what type of hearing aid user, is uncertain.

The present study was designed to assess the benefits and limitations of a remote wireless microphone, as a hearing aid accessory for adults, in both laboratory and field tests. Speech perception was assessed as a function of input level, under controlled listening conditions, before and after a trial period with a behind-the-ear FM system. Perceived benefits were assessed at the end of the trial period by questionnaire. Specific research questions were as follows.

1. Are laboratory measures consistent with the expected effects of signal level and signal-to-noise ratio on aided speech perception in quiet and noise, and on FM-assisted speech perception in noise?

2. To what extent do unaided pure-tone thresholds and age account for individual differences in speech perception measures?

3. To what extent can the effects of hearing loss and noise on speech perception be modeled using Articulation Index theory?

4. How does listening condition affect perceived benefit during everyday use?

5. To what extent can individual differences of perceived benefit be accounted for by unaided pure-tone threshold, laboratory measures of speech perception, and age?

6. What are the benefits and limitations of such a system as identified by users during everyday use?

© 2004 Lippincott Williams & Wilkins, Inc.