Hearing aid accessories for adults: the remote FM microphone

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.

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.

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.

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.