Second Biennial Hearing Aid Research and Development Conference

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

Binaural Advantage for Speech in Noise With and Without Hearing Aids

Normal-hearing listeners enjoy an advantage of binaural over monaural listening when speech and noise originate from different azimuths. There are two components of this binaural advantage: Head shadow is the monaural improvement in intelligibility at the ear on the side of the head opposite the noise source, resulting from the acoustic "shadow" cast by the head for noise frequencies above about 1000 Hz. "Squelch" is the intelligibility improvement over the monaural performance of the shadowed ear resulting from addition of binaural information from the other ear. In the work to be reported, both components of unaided binaural listening advantage were measured in both normal-hearing and hearing-impaired listeners. For the hearing-impaired listeners, aided binaural listening was also assessed with two types of hearing aids: 1) digital hearing aids designed to correct for magnitude and phase effects of the hearing aid itself; and 2) custom analog hearing aids that were matched to the digital aid as closely as possible in frequency response and gain. Results were analyzed using the model proposed by Zurek [P.M. Zurek, in G. S. Studebaker & I. Hochberg, Acoustical Factors Affecting Hearing Aid Performance, Boston: Allyn & Bacon, 1993] for directional effects in speech intelligibility. As predicted by the model, access to head shadow is shown to be limited by peripheral hearing loss above 1 kHz, as well as the amount of insertion gain the hearing aid can provide. Binaural squelch for speech is: 1) observed in the unaided condition when signals are sufficiently intense; 2) preserved to some extent by both types of hearing aids; and 3) correlated negatively with measured head shadow. Implications of the binaural model for explaining observed effects will be discussed.

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