Monaural and binaural subjective modulation transfer functions in simple reverberation

The envelope of a signal is filtered by the transmission channel through which it passes. The amount of reduction for a given envelope, or modulation, frequency has been called the modulation transfer function (MTF) and can be derived from the impulse response of the transmission channel [Schroeder, M.R. (1981) Modulation transfer-functions: Definition and measurement, Acustica, 49, 179-182]. The envelope of a speech signal is critical for intelligibility, and the speech transmission index (STI) predicts the intelligibility of speech through a given transmission channel based on its MTF [Houtgast, T. and Steeneken, H.J.M. (1973) Modulation transfer-function in room acoustics as a predictor of speech intelligibility, Acustica, 28, 66-73]. In the present study, the results of intensity modulation detection experiments with broad-band noise carriers are reported in monaural and binaural conditions, with single reflections at different arrival times in the two ears and with a simulated room impulse response. The monaural data describe a subjective MTF, which is similar to the physical MTF. An interaural modulation phase difference can create an interaural intensity fluctuation, which can give a binaural advantage in detecting the intensity modulation. This binaural advantage could be used to enhance speech intelligibility over purely monaural listening.