Experiments with classroom FM amplification.

OBJECTIVES

1. To quantify the benefits of FM amplification for persons 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

Recognition 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 inches from the FM microphone/transmitter. Students listened: a) via a body-worn and a behind-the-ear system; b) with the FM microphone/transmitter 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 hearing aid microphones. In a follow-up study, the gain via the FM microphone was reduced so that a sinusoidal input of 65 dB SPL into the hearing aid microphone produced the same output as a sinusoidal input of 80 dB into the FM microphone (as recommended in American Speech-Language-Hearing Association, 1994).

RESULTS

1. Addition of the FM microphone signal to that available from the hearing aid microphone was equivalent, on average, to doubling the number of independent channels of information available to the listeners. 2) FM benefit was present in both quiet and noise but was somewhat greater in noise. 3) Contrary to prediction, however, noise interfered with phoneme recognition even under the aid+FM condition. 4) Differences between the body-worn and behind-the-ear systems were small, but there was a measurable advantage for the body-worn system under the aid+FM condition. 5) Reducing FM microphone sensitivity by 15 dB virtually eliminated the FM benefit. 6) Forty-four percent of the variance in phoneme recognition (averaged across listening conditions) could be explained by better-ear, three-frequency average pure-tone threshold. 7) Vowels were recognized more easily than consonants, and initial consonants were recognized more easily than final consonants, but the FM benefit was present for all three phonemes.

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, however, under the aid+FM condition. This finding can be attributed to a reduction of gain in the FM channel, when speech input was used, because of compression limiting in the microphone transmitter. The superiority of the body-worn system under the aid+FM condition suggests a need for higher saturation sound pressure level in the behind-the-ear system when used with persons having severe and profound hearing loss. The findings do not support use of an "equal output" criterion for adjusting relative gains via the FM and hearing aid microphones--at least for persons with very severe and profound hearing loss operating under the conditions tested in this study.