Signal properties reducing intelligibility of speech after noise reduction

The effect of noise reduction on the intelligibility of speech in noise is poorly understood. Although the SNR of noisy speech is improved by the removal of more noise than speech from the signal, the expected increase in intelligibility does not typically occur. To account for these deleterious effects we present an orthogonal decomposition of the signal intensity envelopes at the output of a filterbank. The noisy speech envelopes are decomposed into components indicating (1) the coherence of speech across audio bands; (2) the distortion of the speech envelope; and (3) the speechiness of the noise. By modelling the results of a listening experiment we show that envelope distortion can largely account for the deleterious effects of noise reduction; although reduced coherence could also play a role at low SNRs. There was little evidence for the idea that increased speechiness of the noise contributed to the poorer intelligibility after noise reduction.