Intelligibility prediction for speech mixed with white Gaussian noise at low signal-to-noise ratios

The effect of additive white Gaussian noise and high-pass filtering on speech intelligibility at signal-to-noise ratios (SNRs) from -26 to 0 dB was evaluated using British English talkers and normal hearing listeners. SNRs below -10 dB were considered as they are relevant to speech security applications. Eight objective metrics were assessed: Short-Time Objective Intelligibility (STOI), a proposed variant termed STOI+, Extended Short-Time Objective Intelligibility (ESTOI), Normalised Covariance Metric (NCM), Normalised Sub-band Envelope Correlation metric (NSEC), two metrics derived from the Coherence Speech Intelligibility Index (CSII), and an envelope-based regression method Speech Transmission Index (STI). For speech and noise mixtures associated with intelligibility scores ranging from 0% to 98%, STOI+ performed at least as well as other metrics, and under some conditions better than STOI, ESTOI, STI, NSEC, CSIIMid and CSIIHigh. Both STOI+ and NCM were associated with relatively low prediction error and bias for intelligibility prediction at SNRs from -26 to 0 dB. STI performed least well in terms of correlation with intelligibility scores, prediction error, bias and reliability. Logistic regression modelling demonstrated that high-pass filtering, which increases the proportion of high to low frequency energy, was detrimental to intelligibility for SNRs between -5 and -17 dB inclusive