Second Biennial Hearing Aid Research and Development Conference

September 22-24, 1997
National Institutes of Health
Bethesda, Maryland

Classroom Experiments on FM Amplification

The primary goal was to collect empirical data on the improvement of speech perception, by persons with severe and profound hearing-impairment, when an FM link is added to a personal hearing aid. Additional goals were: a) to measure the effect of noise on performance and on the FM benefit, b) to compare a body-worn and a behind-the-ear FM amplification system, and c) to compare an ìequal gainî with an ìequal outputî criterion for adjusting relative gains via the FM and local microphones. Subjects were 13 hearing-impaired teenagers with thresholds ranging from 87 to 113 dBHL. Consonant-vowel-consonant words were produced live at 12 inches from the FM microphone and 10 feet from the hearing aids in a classroom in a school for the deaf. Recognition of phonemes was measured: a) with and without the FM microphone, b) with and without multitalker babble (s/n = 5 dB at the studentsí aids and 20 dB at the FM microphone, c) using a body-worn and a behind-the-ear receiver/hearing aid. Adding the FM link was equivalent, on average, to doubling the Articulation Index, independent of degree of hearing loss. The percentage point improvement in phoneme recognition score, however, increased with decreasing hearing loss. Adding noise was equivalent, on average, to reducing articulation index by 50%. There was evidence of a small but significant advantage for the body-worn system, especially when listening via the FM microphone in quiet. This last finding is attributed to the higher saturation sound pressure level available from the body-worn system. When the sensitivity of the FM microphone was reduced by 15 dB (to match an ìequal outputî criterion recommended in a recent publication of the American Speech Hearing Association) the FM benefit was eliminated. These data confirm the benefits of FM amplification in both quiet and noise. The further suggest that, among the population of subjects with severe and profound hearing loss, the greater benefits go to those subjects with more residual hearing. They also support the use of behind-the-ear systems but indicate the desirability of higher saturation sound pressure level if used with this population. The data do not support the use of an ìequal outputî criterion for adjusting relative gains of FM and local microphones -- at least in subjects with severe and profound hearing loss.

[Research supported by NIDRR]

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