ASA 125th Meeting Ottawa 1993 May

4aPP7. The internal signal-to-noise ratio in hearing-impaired listeners.

Marjorie R. Leek

W. Van Summers

Mary T. Cord

Army Audiol. & Speech Ctr., Walter Reed Army Med. Ctr., Washington, DC 20307-5001

The reduced frequency resolution often demonstrated by listeners with sensorineural hearing impairment might have at least two related effects on their perception of speech in background noise. The internal representation of the peaks and valleys in the speech spectrum become smeared, resulting in less precise frequency analysis than can be achieved by normal-hearing persons. In addition, when the speech is embedded in noise, poorer frequency resolution will, in theory, result in a greater-than-normal decrease in the signal-to-noise ratio at the outputs of the auditory filters. This poorer-than-normal internal S/N might account for the common observation that noise is more degrading to speech understanding by hearing-impaired listeners than by normal-hearing listeners. To assess the validity of this notion, both normal-hearing and hearing-impaired subjects participated in an identification experiment using highly simplified vowel-like harmonic complexes presented in quiet and in three levels of broadband noise. After equating individual performance in terms of percent correct identification in quiet, trading relationships were established between amount of spectral contrast in the stimuli (level differences between peaks and valleys in the spectra) and levels of the background noise. When the effect of spectral smearing introduced by reduced frequency analysis was compensated by increasing the input spectral contrast, the effects of additional background noise were similar for normal-hearing and hearing-impaired groups of subjects. Results from individual listeners will be discussed in light of the degree of impaired frequency resolution as determined by auditory filter shape measurements. [Work supported by NIH.]