Forms of binaural summation and the implications of individual variability for binaural hearing aids.

The advantages which a two-ear system has over a one-ear system appear to be manifold, but not all understood in detail. All probably play some role in the generally recognised advantage of binaural aids in offsetting auditory disabilities. Several possible forms of binaural summation are distinguished here and their possible role discussed. Experimental results with normally-hearing listeners and with aid users show that binaural summation is likely to lead to gain settings of about 6 dB lower than with otherwise equivalent monaural amplification. In some cases this may be a direct cause of benefit. However individuals differ systematically in the extent to which they show this effect, which may be relevant to prognosis for binaural aiding. Evidence is reported of an association between binaural summation and binaural advantage. Individuals also differ systematically in binaural summation for uncomfortable loudness level.