Quantifying the effects of fast-acting compression on the envelope of speech.

Fast-acting compression has several effects on the envelope of speech signals, especially when a target and a background are mixed together. Three measures of the envelope are described, which can be used to quantify these changes: (1) Within-signal modulation correlation or coherence, the degree of correlation (or coherence) of the envelope (on a dB scale) of a single source across different frequency regions, which is reduced by fast-acting compression; (2) fidelity of envelope shape, the degree to which the envelope shape of the target speech in different frequency channels is preserved following compression; and (3) across-signal modulation correlation or coherence (ASMC), the extent to which the target and background acquire a common component of modulation when they are compressed together, which becomes greater in absolute value (more negative) when the target and background are compressed together. Values of these measures are presented and compared with intelligibility scores obtained using stimuli processed (with a noise-vocoder) so as to preserve mainly envelope cues in a limited number of frequency channels. The results suggest that the dominant factor affecting intelligibility is ASMC.