This paper presents an alternative model for predicting the reverberant sound field in empty rooms with diffusely reflecting boundaries, based on the generalization and the numerical implementation of a diffusion equation for the energy density. The paper focuses on the source term and the boundary conditions of the diffusion equation, both for the steady state and the time-varying state, in order to make computational use of the model. In addition, theoretical analysis of the diffusion equation shows that the diffusion model may be considered as an extension of the classical theory of reverberation to nondiffuse sound fields. The numerical model is first applied to a cubic room and shows a very good agreement with statistical theory. Two numerical applications are also given for a long room and a flat room; results are in good agreement with numerical results from a ray-tracing software. The main advantage of the present model is its capability to be applied regardless of the complexity of the room shape, and that it gives results at any receiver location, with a low calculation time.

Chronic exposure to noise levels typical of many workplaces was associated with excess risk for acute myocardial infarction death. Given the very high prevalence of excess noise exposure at work, this association deserves further attention.

A method has been developed for determining typical long-term speech and background-noise levels during lectures. Lectures are recorded and the recordings digitized and processed to obtain sound-pressure-level frequency distributions to which three normal-distribution curves are fit. The maximum values of these curves are associated with long-term sound-pressure levels associated with speech, ventilation noise, and student-activity noise. Recordings made during 18 university lectures in 11 classrooms have been analyzed. Average (standard deviation) A-weighted levels for the various sound components were determined as follows: ventilation noise, 40.9 (3.9) dB; student-activity noise, 41.9 (4.0) dB; total background noise, 44.4 (3.5) dB; and received speech-signal, 50.8 (3.9) dB. The average (standard deviation) A-weighted speech-signal to background-noise ratio was 7.9 (3.1) dB. That of the instructor sound-power level was 64.5 (4.2) dB. Empirical models have been developed to predict the room-average A-weighted results using multivariable regression analysis. Further analysis in the 63- to 8000-Hz octave bands confirmed the spectra of ventilation noise and of speech, and determined the spectrum of student-activity noise and of the speech-signal to background-noise ratio.

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.

Acoustical measurements were performed in 30 randomly chosen, unoccupied classrooms at the University of British Columbia (UBC). Tests had previously been done in 46 unoccupied UBC classrooms, as well as in 10 of these when occupied by students. The results for the 10 classrooms were used to correct the “unoccupied” results to the half-occupied and fully occupied conditions. The objective of the work was to characterize the 30 classrooms, which were used in subsequent studies, to determine the acoustical quality of the UBC classroom stock and how this depends on the classroom design and the presence of students, and to elucidate characteristics of classroom acoustics relevant to optimal design. The results showed that the UBC classroom stock is of far from optimum acoustical quality when unoccupied, but is much better in the occupied condition. Generally, many classrooms have excessive reverberation and result in low speech levels, especially at the back of the rooms; in addition, they have excessively noisy ventilation systems.