Effect of peak clipping on speech recognition threshold

Speech recognition thresholds (SRTs) were measured for a closed set of spondees that had been processed by peak clipping and compression. Both hearing-impaired and normal-hearing subject groups showed progressively higher SRTs with increasing levels of peak clipping, with significant threshold shifts occurring for clipping levels greater than 18 to 24 dB. Neither subject group showed significantly elevated SRTs for the compression processed stimuli. Magnitude-squared coherence analysis of the speech stimuli revealed high levels of distortion generated by peak clipping and relatively low levels generated by the compression processing. Subsequent analysis suggested that the addition of distortion products and not the alteration of the speech waveform envelope was responsible for the observed threshold shifts, and that coherence analysis may be a valuable tool for predicting the effects of distortion on speech intelligibility. Judgments of sound quality showed that the clipping level where SRTs began to be significantly affected coincided with the clipping level at which the quality of the speech was judged to be unacceptable.