An influence of binaural hearing aids on positioning of sound images.

An influence of binaural hearing aids on positioning of sound images produced by signals from two loudspeakers was investigated with normally hearing and hearing-impaired subjects. The image can be positioned along a horizontal line connecting the centers of the sources by adjusting the relative sound pressure levels of signals. When the listener is a symmetrical receiver, the image appears on the midline (0 degrees azimuth) for equal levels or away from the midline for unequal levels. Any asymmetry of the receiver causes a shift of the image away from the midline. In the study, the level differences, delta Ls, necessary to position sound images at 0 and +/- 12 degrees azimuths were measured. The baseline data were collected for 10 normally hearing subjects in unaided and aided conditions. In the aided conditions binaural aids were balanced (equal gains) or unbalanced (10 dB disparity in gains). The normally hearing subjects positioned the image on the midline with a group mean delta L of 0 dB in both unaided and balanced aided conditions. To position the image at +12 or -12 degrees azimuth they needed mean delta Ls that could be predicted from a theoretical equation. With unbalanced aids, a group mean delta L of +3 or -3 dB was needed for the midline image. The delta Ls for the side images also were shifted. For 12 hearing-impaired subjects with bilateral lossses, the midline delta Ls in unaided condition were within the normal range (+/- 2 dB). For two monaural hearing aid users with long experience, the midline delta Ls were greater than normal. The delta Ls for the side images were equal to or greatr than delta Ls for normally hearing subjects. In aided balanced and/or unbalanced conditions, some hearing-impaired subjects needed delta Ls for the midline image outside the normal range, and some could not position the image at the side azimuths. It was concluded that the binaural hearing aids can be a source of receiver asymmetry.