Low-level versus high-level amplitude compression

Abstract

Sensori-neural hearing-impaired individuals often exhibit a loudness function with a steeper slope at low levels than normally hearing people. At progressively higher levels the slope of the loudness function for the hearing-impaired approaches that of the normally hearing. Reducing the signal level differences at high levels (high-level compression, HLC) makes an abnormal loudness function, more so, while, reducing level differences at low levels (low-level compression, LLC) should result in a more normally appearing loudness function. CVC words (MRT) were processed under three conditions: (a) only levels between Lr −25 dB and Lr − 13 dB (Lr is the average maximal rms levels of the test words' vowels) were compressed (LLC) with a compression ratio CR = 4:1: (b) only levels above Lr −13 dB were compressed (HLC) with CR = 4:1: and (c) only levels above Lr −25 dB were compressed with the average compression ratio between Lr and Lr −25 dB in conditions “a” and “b” equal to 1.7. All three conditions had attack and release times of 1.5 and 30 ms. respectively. The performance of nine sensori-neural hearing-impaired subjects did not significantly differ among the three conditions investigated; this confirms Lippmann's (1978) findings with the compression system restoring normal equal loudness contours. [Work supported by NIH NS 12946.]