Effects of envelope expansion on speech recognition.

This study investigated the effects of expanding the gross time-amplitude variations of 'speech-envelope noise' stimuli on speech recognition. The initial stimuli were VCV logatomes presented in quiet or against a steady white noise with a 0-dB signal-to-noise ratio. Their low-frequency temporal modulations (<500 Hz) were extracted in broad frequency bands, and raised to the power 2. The resulting envelopes were then used to modulate a white noise, and combined to produce the 'speech-envelope noise' stimuli. As a consequence, listeners were forced to identify speech using primarily temporal envelope cues. The results obtained with four normal-hearing listeners show small decrements in recognition performance of 1-15% when expanding the envelope of the speech stimuli presented in quiet. The results also show a small but consistent improvement in performance of 6-14% when expanding the envelope of the speech stimuli presented in noise. These results are consistent with those obtained by Fu and Shannon (J. Acoust. Soc. Am. 104 (1998) 2570-2577) with speech presented in quiet. They also suggest that the reduction in the modulation depth of the speech envelope caused by noise or reverberation could be compensated by expanding low-frequency temporal modulations.