Signal Processing to Improve Speech Intelligibility in Perceptive Deafness

Abstract

Many deaf persons with recruitment cannot use enough hearing-aid gain to bring weak consonants into the useful dynamic range of their hearing, because this amount of gain would make lower-frequency, high-amplitude vowels intolerably loud. Whether or not from this cause, such subjects commonly find amplified speech to have poor intelligibility. In the present experiments, speech is processed by a two-channel amplitude compressor whose frequency-dependent compression ratio is adjusted to compensate the frequency-dependent recruitment of the individual subject, and the compressed speech is subjected to frequency equalization similarly adapted to the subject. The aim is to amplify each acoustical element of speech, at each frequency-amplitude coordinate of the speech band, to a relative loudness for the deaf subject corresponding to the relative loudness of that speech element perceived by normals, This processing improved speech recognition, both in quiet and in the presence of competing speech introduced before processing, for six perceptively deaf subjects. Each subject showed an improvement in either initial- or terminal-consonant recognition of at least 22% and as much as 160% at optimum levels in quiet, and from 10% to 229% with speech interference 10 dB below the pre-processed signal.