Model of loudness summation applied to impaired ears

The loudness of complex sounds composed of three or four pure tones was measured as a function of the over-all spacing ΔF between the lowest and highest components. The measured relation between loudness and ΔF was compared to calculations from Zwicker's model of loudness summation. In eight ears with a conductive impairment, loudness summated normally and as predicted by the model; loudness remained approximately constant as a function of ΔF near threshold and increased with ΔF beyond the critical band at higher sensation levels. In eight ears with a cochlear impairment, loudness did not change with ΔF at any tested sensation level. This invariance of loudness was not predicted by tire model nor was it found in six normal ears tested in the presence of a 90-dB uniform masking noise intended to simulate the cochlear impairment. Under masking, loudness summated as predicted. The unexpected results in cochlear pathology were ascribed, tentatively, to a possible widening of the critical band.