Volume 111, Issue 2, February 2002
Index of content:
- ARCHITECTURAL ACOUSTICS 
111(2002); http://dx.doi.org/10.1121/1.1433808View Description Hide Description
A very interesting new phenomenon that we call a sweeping echo is described and investigated. When we clap hands in a regularly shaped reverberant room, we hear sweeping echoes whose frequency increases linearly with time. An example of sweeping echoes observed in a rectangular reverberation room is first described. Then, the mechanism that generated the sweeping echoes is investigated by assuming a cubic room and using number theory. The reflected pulse sound train is found to have almost equal intervals between pulses on the squared-time axis. This regularity of arrival times of the reflected pulse sounds is shown to generate the sweeping echoes. Computer simulation of room acoustics shows good agreement with the theoretical results.
Effect of noise and occupancy on optimal reverberation times for speech intelligibility in classrooms111(2002); http://dx.doi.org/10.1121/1.1428264View Description Hide Description
The question of what is the optimal reverberation time for speech intelligibility in an occupied classroom has been studied recently in two different ways, with contradictory results. Experiments have been performed under various conditions of speech-signal to background-noise level difference and reverberation time, finding an optimal reverberation time of zero. Theoretical predictions of appropriate speech-intelligibility metrics, based on diffuse-field theory, found nonzero optimal reverberation times. These two contradictory results are explained by the different ways in which the two methods account for background noise, both of which are unrealistic. To obtain more realistic and accurate predictions, noise sources inside the classroom are considered. A more realistic treatment of noise is incorporated into diffuse-field theory by considering both speech and noise sources and the effects of reverberation on their steady-state levels. The model shows that the optimal reverberation time is zero when the speech source is closer to the listener than the noise source, and nonzero when the noise source is closer than the speech source. Diffuse-field theory is used to determine optimal reverberation times in unoccupied classrooms given optimal values for the occupied classroom. Resulting times can be as high as several seconds in large classrooms; in some cases, optimal values are unachievable, because the occupants contribute too much absorption.