Rob Drullman
Dept. of Otolaryngol., Free Univ. Hospital, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands
A number of listening experiments were run to investigate the relative contribution of temporal envelope modulations and fine structure to speech intelligibility. Firstly, the amplitude envelopes of 24 1/4-oct bands (100--6400 Hz) were processed in several ways (e.g., fast compression) to assess the importance of modulation peaks and troughs. Results show that reduction of modulations by the addition of noise is more detrimental to sentence intelligibility than the same degree of reduction achieved by direct manipulation of the envelope; in some cases the difference in speech-reception threshold (SRT) is almost 10 dB. Here, L[sub eq]-3 dB defines a perceptual cross-over level for which removing modulations either x dB below or above yields the same intelligibility score, whereas L[sub eq]-15 dB defines the median envelope level (acoustic cross-over level). Secondly, the fine structure of each 1/4-oct band was altered by manipulating the distribution of speech and noise over the sentences. Results indicate that noiseless peaks do not increase intelligibility; removing the speech fine structure in the troughs, however, yields a 2-dB increase of the SRT. In general, no one-to-one relation between the modulation-transfer function (MTF) and the intelligibility scores could be established. [Work supported by NWO.]