Audible reflection density for different late reflection criteria in rooms

For reasonably accurate but practical auralizations, some simplifications and approximations are needed. The main issue in the present investigation is that the reflection density of a room impulse response, in theory, increases so fast as a quadratic function of the elapsed time, even assuming only specular reflections. Therefore in this study, the upper threshold for audible reflection density is investigated for four different transition times of 25, 50, 75, and 100 ms through a headphone listening test. Binaural impulse responses and speech signals simulated in three rooms with different characteristics (an empty office, a lecture room, and an auditorium) are used as stimuli. Subjects are asked to increase/decrease the reflection density of a stimulus until they cannot distinguish it from the stimulus that follows the theoretical reflection density for the different transition times in the three rooms. When using binaural impulse responses, the upper limit of the audible reflection density turns out to be limited to 2800 reflections per second. For speech signals, the maximum audible reflection density is shown to be as low as 300 reflections per second, regardless of the room and transition time.