Contributions of comodulation masking release and temporal resolution to the speech-reception threshold masked by an interfering voice.

Two experiments are presented to explain the difference in speech-reception threshold (SRT) between conditions with a steady-state noise masker or an interfering voice. Literature shows for normal hearing a masking release of 6-8 dB with an interfering voice and a substantial reduction of this release with hearing impairment. In experiment I the possible role of comodulation masking release (CMR) is investigated by manipulating the comodulation in the interfering voice by the introduction of temporal shift among filter bands of various width. The spectral spread of masking from the manipulated interfering voice was controlled by interleaving the mutually shifted speech bands with 1/3-octave bands of noise. Although comodulation in the interfering speech appears to be very important for the low SRT, the contribution of across-frequency processing of masker fluctuations--commonly considered as the origin of CMR--is only 1.3 dB. In experiment II the level dependence of masking release with an interfering voice is investigated. The data fit in with the hypothesis by Festen and Plomp [J. Acoust. Soc. Am. 88, 1725-1736 (1990)] that the release from masking with an interfering voice is limited by forward masking. It appears that up to about 55 dBA the release from masking increases with level up to about 7 dB. Above 55 dBA the difference in SRT obtained with a noise masker or an interfering voice is constant due to the limited average modulation depth of speech.