Comparing the effects of scattered and specular sound reflections on speech intelligibility in rooms

Speech intelligibility in rooms is assumed to depend on the amount of reverberation, and on the distribution of sound energy in the impulse response between early and later reflections, the former being considered beneficial, the latter detrimental. This assumption is based on the analysis of a single channel, which is monaural. When the binaural capacities of the auditory system are considered, other phenomena come into play, supporting speech recognition mainly by comparing signal levels and times of arrival at the ears. Starting from these basic mechanisms, the present work shows that, for given monaural conditions, binaural cues are influenced by the type of sound reflections in a room via the correlations they produce on the signals at the ears. A fully scattering scenario, and a totally flat boundary scenario were used to investigate listeners' performance in a speech intelligibility task in a virtual room with spatialized noise conditions. The amount of correlation at the ears of the listener of the signals coming from both the source and the masker were found to affect speech intelligibility - the stronger either correlation, the greater the speech intelligibility - and their joint effect was larger than the two effects taken separately. A specular setting for the sound reflections was associated with both better reception thresholds and a better usage of spatial cues when deciphering speech. The implications for the acoustic design of rooms with a view to facilitating speech intelligibility are discussed.