Implication of speech level control in noise to sound quality judgement

Relative levels of speech and noise, which is signal-to-noise ratio (SNR), alone as a metric may not fully account how human perceives speech in noise or making judgement on the sound quality of the speech component. To date, the most common rationale in front-end processing of noisy speech in assistive hearing devices is to reduce “noise” (estimated) with a sole objective to improve the overall SNR. Absolute sound pressure level of speech in the remaining noise, which is necessary for listeners to anchor their perceptual judgement, is assumed to be restored by the subsequent dynamic range compression stage intended to compensate for the loudness recruitment in hearing impaired (HI). However, un-coordinated setting of thresholds that trigger the nonlinear processing in these two separate stages, amplify the remaining “noise” and/or distortion instead. This will confuse listener's judgement of sound quality and deviate from the usual perceptual trend as one would expect when more noise was present. In this study, both normal hearing (NH) and HI listeners were asked to rate the sound quality of noisy speech and noise reduced speech as they perceived. The result found that speech processed by noise reduction algorithms were lower in quality compared to original unprocessed speech in noise conditions. The outcomes also showed that sound quality judgement was dependent on both input SNR and absolute level of speech, with a greater weightage on the latter, across both NH and HI listeners. The outcome of this study potentially suggests that integrating the two separate processing stages into one will better match with the underlying mechanism in auditory reception of sound. Further work will attempt to identify settings of these two processing stages for a better speech reception in assistive hearing device users.