Effect of stimulus level on speech masking by multitalker noise in a cochlear model

Previous research shows that intelligibility of speech in noise (SIN) with a constant speech-to-noise ratio (SNR) changes in dependence on overall stimulus level. It has been hypothesized that these changes were associated with nonlinear responses of the cochlear basilar membrane, however, the exact role of auditory periphery in this behavior is not clear. We explore this phenomenon using a nonlinear cochlear model with adjustable gain to examine how multi-talker (babble) noise affects the speech signal at the output of the auditory periphery. The amount of distortion at various intensities caused by babble noise is calculated by correlating the model responses to speech alone and to SIN with a constant speech-to-noise ratio of -5 dB. The model shows that both 5% quantile and the mean value of the cross-correlation coefficient increase with stimulus level from 30 to 60-70 dB SPL and then decline for levels between 70 dB and 90 dB SPL, indicating that babble noise represents a less effective masker at intermediate intensities and its influence is strongest at both low and high levels. In addition, reduction of the gain of the cochlear amplifier decreases the cross-correlation, leading to worsened SIN intelligibility. Altogether, while not ruling out possible central contributions, our results offer a plausible peripheral mechanism suggesting that the level dependence of SIN intelligibility has its origins already in peripheral processing and that cochlear amplification plays an important role in this process.