Signal processing to improve speech intelligibility in perceptive deafness

Abstract

A deaf person with recruitment perceives sound as though listening through a volume expander followed by an attenuator, the expansion ratio and attenuation be'rag typically frequency dependent. (Other perc•tive aberrations may also be present, of course.) The subject is often prevented from using enough hearing-aid gain to bring weak consonants into the useful dynamic range of his hearing, because this amount of gain would make lower-frequency, high-amplitude vowels intolerably loud. Such subjects commonly find amplified speech to have poor intelligibility. In a preliminary experiment it is established that recruitment in normal subjects, induced by masking or simulated by expansion of the signal, reduces the intelligibility of amplified speech severely, and that this intelligibility can be largely restored by signal processing. The implication is that recruitment in deaf subjects is a sufficient cause for loss of intelligibility, whether or not there are other causes. In the pre•ent experiments, speech is processed by a two-channel amplitude compressor whose frequency-dependent compression ratio is adjusted to compensate the recruitment of the individual subject, and the compressed speech i• subjected to frequency-selective amplification 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 1oudne• for the deaf subject corresponding to the relative loudness of that speech element perceived by normals. Thi• processing improved speech recognition, both in quiet and in the presence of competing speech introduced before processing, for six perceptively deaf subjects. Subjects 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.The compressor characteristics that can be controlled by the designer include the attack and release times, the compression ratio (the ratio, expressed arithmeti-cally, of input-level change in decibels to output-level change), the compression linearity (constancy of the compression ratio over the dynamic range), and the compression threshold (the input level at which gain begins to decrease with input). A. Attack and Release TimesThe reduction of compressor gain in response to an increase of signal voltage--the compression attack-can be accomplished in a small fraction of 1 msec without creating audible distortion. The release of compression, however, must be slowed up to prevent compressor action from following the instantaneous amplitude of individual cycles, an operating mode that would introduce severe waveform distortion.Commercial compressors rarely provide a release time 2 shorter than several hundred milliseconds. A delay of this length following a high-amplitude vowel would keep the compressor in its low-gain state for the duration of a succeeding consonant, leaving the vowelconsonant amplitude ratio unchanged...

Effects of hearing-aid dynamic range compression on spatial perception in a reverberant environment

“…To compensate for recruitment and thereby restore the normal dynamic range of audibility, multi-band fast-acting dynamic range compression (DRC) algorithms for hearing aids have been developed (Allen, 1996;Villchur, 1973). DRC algorithms amplify soft sounds and provide progressively less amplification to sounds whose level exceeds a defined compression threshold (CT).…”

Auditory-Filter Characteristics for Listeners With Real and Simulated Hearing Impairment

“…Because of the levels of required masking noise, however, this simulation technique is limited to losses less than roughly 70 dB HL. A second technique, known as multi-band amplitude expansion (MBE), employs level-dependent attenuation of the input signal to achieve elevated thresholds and recruitment (Villchur, 1973, 1974; Duchnowski, 1989; Moore & Glasberg, 1993; Duchnowski & Zurek, 1995; Graf, 1997; Lum & Braida, 1997). This method can thus be used to simulate impairments that are more severe than those that can be comfortably simulated using masking noise alone.…”

Temporal Resolution With a Prescriptive Fitting Formula

“…The dynamic range available to a listener can be increased with wide dynamic range compression (WDRC) by decreasing the gain as the input level increases, which is sometimes referred to as input-controlled compression (e.g. Villchur 1973). In contrast to WDRC, linear amplification provides the same gain for all input levels up to the listener’s upper limit of comfort.…”