Noise reduction with FM systems in FM/EM mode.

Commercially available noise reduction hearing aids have failed to deliver improved speech recognition in noise over properly fit conventional devices under typical listening conditions (Fabry & Van Tasell, 1990; Gabrielsson, Schenkman, & Hagerman, 1988; Tyler & Kuk, 1989; Van Tasell, Larsen, & Fabry, 1988). The primary factor related to this finding is that speech and noise are mixed at the input of a single microphone; spectral changes are applied equally to both speech and noise, and the signal-to-noise ratio remains unchanged from unprocessed conditions. The purpose of this study was to evaluate a prototype frequency-modulated (FM) auditory trainer that allowed use of a remote FM microphone and/or an ear level environmental microphone (EM). For each of five subjects with moderate to severe sensorineural hearing loss, the frequency response of the EM was configured either to match that of the FM response, or to provide a high-pass filter characteristic similar to a noise reduction hearing aid. Speech recognition threshold (SRT) testing noise was measured for five test conditions: 1) FM only; 2) EM only with high-pass filter response (EM-HP); 3) EM only with "standard" response (EM-S) matched to FM; 4) FM/EM-HP combined mode; and 5) FM/EM-S mode. Results averaged across subjects indicated that SRTs for the FM only condition were 9 to 10 dB better than those for either EM only condition; data from the combined FM/EM-HP mode averaged 4 dB better than for FM/EM-S conditions.