Volume 120, Issue 2, August 2006
Index of content:
- ARCHITECTURAL ACOUSTICS 
Auralization study of optimum reverberation times for speech intelligibility for normal and hearing-impaired listeners in classrooms with diffuse sound fields120(2006); http://dx.doi.org/10.1121/1.2216768View Description Hide Description
Speech-intelligibility tests auralized in a virtual classroom were used to investigate the optimal reverberation times for verbal communication for normal-hearing and hearing-impaired adults. The idealized classroom had simple geometry, uniform surface absorption, and an approximately diffuse sound field. It contained a speech source, a listener at a receiver position, and a noise source located at one of two positions. The relative output levels of the speech and noise sources were varied, along with the surface absorption and the corresponding reverberation time. The binaural impulse responses of the speech and noise sources in each classroom configuration were convolved with Modified Rhyme Test (MRT) and babble-noise signals. The resulting signals were presented to normal-hearing and hearing-impaired adult subjects to identify the configurations that gave the highest speech intelligibilities for the two groups. For both subject groups, when the speech source was closer to the listener than the noise source, the optimal reverberation time was zero. When the noise source was closer to the listener than the speech source, the optimal reverberation time included both zero and nonzero values. The results generally support previous theoretical results.
120(2006); http://dx.doi.org/10.1121/1.2216559View Description Hide Description
An acoustical radiosity model was evaluated for how it performs in predicting real room sound fields. This was done by comparing radiosity predictions with experimental results for three existing rooms—a squash court, a classroom, and an office. Radiosity predictions were also compared with those by ray tracing—a “reference” prediction model—for both specular and diffuse surface reflection. Comparisons were made for detailed and discretized echograms, sound-decay curves, sound-propagation curves, and the variations with frequency of four room-acoustical parameters—EDT, RT, , and . In general, radiosity and diffuse ray tracing gave very similar predictions. Predictions by specular ray tracing were often very different. Radiosity agreed well with experiment in some cases, less well in others. Definitive conclusions regarding the accuracy with which the rooms were modeled, or the accuracy of the radiosity approach, were difficult to draw. The results suggest that radiosity predicts room sound fields with some accuracy, at least as well as diffuse ray tracing and, in general, better than specular ray tracing. The predictions of detailed echograms are less accurate, those of derived room-acoustical parameters more accurate. The results underline the need to develop experimental methods for accurately characterizing the absorptive and reflective characteristics of room surfaces, possible including phase.
120(2006); http://dx.doi.org/10.1121/1.2211467View Description Hide Description
The qualification of anechoic and hemianechoic chambers requires selection of signal type and acquisition bandwidth to be used. This work demonstrates that, while a broadband random source may be used, the signal at points removed from the source are not composed of random combinations of direct and reflected waves, except in the limit of infinite bandwidth. It is shown that chamber qualification may be represented as finding the ratio of two transfer functions, where one includes the reflected waves, and the other is the free-field transfer function between the source and receiver. Consideration of this approach leads to a generic representation for the deviation from free-field performance where bandwidth is demonstrated to suppress the dominant modulation contributions of reflections. It is demonstrated that pure-tone qualifications will always exhibit a higher deviation from free-field performance than a broadband qualification. Finally, it is shown that the use of an incoherent source model in method-of-images simulations for the broadband performance of anechoic chambers is fundamentally flawed.