A detailed study on the effects of noise on speech reception

Abstract

The effect of adding continuous noise to a speech signal was studied by comparing, for a series of quarter octave bands, the band output for the original speech and for the speech-plus-noise. Three separate effects were identified. (a) Average envelope-modulation reduction: the original intensity-envelope is, on average, raised by the mean noise intensity, resulting in a reduction of the original modulation index. (b) Random instantaneous envelope fluctuations: on an instantaneous basis, the speech-plus-noise envelope shows random variations, caused by the stochastic nature of the noise, and by the instantaneous changes in the phase relation between the speech and the noise. (c) Perturbations of the carrier phase: in the band output carrier signal the addition of the noise causes random phase changes. By applying signal processing techniques, we were able to either include or exclude each of these three effects separately. The results of intelligibility measurements indicated the following order of importance of the three different effects: (1) the average envelope-modulation reduction, (2) the perturbation of the carrier phase, and (3) the random envelope fluctuations. The results will be discussed in the light of modeling and enhancing (noise suppression schemes) speech reception in noise.