The combined effect of source directivity and binaural listening on near-field binaural speech transmission index evaluation

Abstract

The binaural speech transmission index (BSTI) is mainly affected by the effect of binaural listening, background noise, and acoustic conditions in a given space, as well as source characteristics such as directivity. However, little attention has been paid to the combined effect of source directivity and binaural listening on the BSTI in near-field regions. This study thus investigates this issue using BSTI measurements in an anechoic environment to exclude the influence of acoustic conditions. Three directional loudspeakers are adopted as the sources, and the impulse responses under different radiation directions in the horizontal plane are measured in an anechoic chamber. These impulse responses are then convolved with the near-field head-related transfer functions of a KEMAR manikin to obtain the binaural impulse responses, which are further used to calculate the BSTIs. The results show that the BSTI decreases considerably with an increasing deviation of the axial direction of the source from the listener (i.e., an increasing radiation angle) due to the directivity pattern of the source, which is almost independent of the azimuth and the distance between the source and listener. Finally, the BSTI correction functions of the source radiation angles are established with third-order polynomial regression for the three sources and can be integrated into the azimuth-dependent BSTI model to yield a comprehensive model that considers the source radiation angle as well as the azimuth and distance between the source and listener.