EXPERIMENTS WITH CLASSROOM FM AMPLIFICATION

Objectives: 1) To quantify the benefits of FM amplification for person s with severe and profound hearing loss; 2) to compare a body-worn and a behind-the-ear FM system; 3) to measure the effects of reducing FM microphone sensitivity relative to hearing aid sensitivity. Design: Re cognition of phonemes in lists of consonant-vowel-consonant words was measured in 13 teenage students with severe and profound hearing loss. Presentation was by live voice at 10 feet from the listeners and 12 i nches from the PM microphone/transmitter. Students listened: a) via a body-worn and a behind-the-ear system; b) with the FM microphone/trans mitter on and off; c) in noise and in quiet. Systems were adjusted so that sinusoidal inputs of 65 dB SPL gave equal gains via the FM and he aring aid microphones. In a follow-up study, the gain via the FM micro phone was reduced so that a sinusoidal input of 65 dB SPL into the hea ring aid microphone produced the same output as a sinusoidal input of 80 dB into the FM microphone las recommended in American Speech-Langua ge-Hearing Association, 1994). Results: 1) Addition of the FM micropho ne signal to that available from the hearing aid microphone was equiva lent, on average, to doubling the number of independent channels of in formation available to the listeners. 2) FM benefit was present in bot h quiet and noise but was somewhat greater in noise. 3) Contrary to pr ediction, however, noise interfered with phoneme recognition even unde r the aid+FM condition. 4) Differences between the body-worn and behin d-the-ear systems were small, but there was a measurable advantage for the body-worn system under the aid+FM condition. 5) Reducing FM micro phone sensitivity by 15 dB virtually eliminated the FM benefit. 6) For ty-four percent of the variance in phoneme recognition (averaged acros s listening conditions) could be explained by better-ear, three-freque ncy average pure-tone threshold. 7) Vowels were recognized more easily than consonants, and initial consonants were recognized more easily t han final consonants, but the FM benefit was present for all three pho nemes. Conclusions: The findings confirm the value of FM amplification for persons with severe and profound hearing loss, in both quiet and noise. The negative effects of noise were not completely eliminated, h owever, under the aid+FM condition. This finding can be attributed to a reduction of gain in the FM channel, when speech input was used, bec ause of compression limiting in the microphone transmitter. The superi ority of the body-worn system under the aid+FM condition suggests a ne ed for higher saturation sound pressure level in the behind-the-ear sy stem when used with persons having severe and profound hearing loss. T he findings do not support use of an ''equal output'' criterion for ad justing relative gains via the FM and hearing aid microphones-at least for persons with very severe and profound hearing loss operating unde r the conditions tested in this study.