Effect of envelope signal-to-noise ratio on the intelligibility of speech in speech-spectrum shaped noise

In attempts to improve speech intelligibility, the envelope of speech in noise is commonly processed in the time domain and used to amplitude modulate the corrupted speech. In this study the envelope was constructed from the noisy speech, but separately and independently of the SNR of the noisy speech. Envelopes corresponding to 0, −3, −6, and −9 dB SNR were applied to the noisy speech when the latter had an SNR of either −9, −6, −3, or 0 dB, to evaluate the benefit to intelligibility of improving the SNR in the modulation domain. Signals from 200 Hz to 6 kHz were processed by MATLAB into sixteen contiguous subbands with bandwidth approximately 1.5×ERB of an auditory filter. The subband envelopes were formed from the absolute value of the signals and low-pass filtered. Eleven subjects aged 29 ± 8 years (mean and range) with normal hearing underwent the Modified Rhyme Test to assess speech intelligibility. The stimuli were presented diotically over earphones with the subject seated in an audiometric room. Statistically significant increases in mean word score of up to 35% could be obtained by improving the envelope SNR, suggesting this processing may benefit speech intelligibility. [Work supported by NIOSH.]