SNR is not enough: Noise modulation and speech quality

Speech quality metrics are used to predict quality under a variety of noise and distortion conditions. A simple metric for noisy speech is the signal-to-noise ratio (SNR), which gives the ratio of the long-term average speech power to the long-term average noise power. However, the average noise level fails to include the impact of the noise amplitude modulation on speech quality judgments. This paper reviews how the noise characteristics affect quality ratings even when the SNR is held constant. In one study, ratings for speech combined with stationary Gaussian noise are compared to ratings for noise having the same envelope modulation as the speech. In a second study, ratings for speech combined with continuous noise or multi-talker babble are compared to speech where the interference is gated off during the silent intervals in the stimulus. An accurate quality metric must take into account the modulation as well as the intensity of the noise, and the implications for speech quality metrics are discussed.