Diffuse sound fields, reverberation-room methods and the effectiveness of reverberation-room designs

The reverberation-room method, which assumes a diffuse sound field, has long been used for various standardized room-acoustical determinations – e.g. of surface-absorption coefficients, power levels of sound sources, transmission losses of acoustical partitions, etc. In this regard, a number of standards have emerged to offer some help by outlining necessary reverberation-room design guidelines to achieve sufficient sound-field diffuseness. However, unsatisfactory opinions regarding the prediction accuracy of the method, especially at low frequencies, have been reported over the years. This might be due to deviations from the assumed diffuse-field concept, which is very challenging to implement from an application point of view; also there are no straight-forward ways to characterize the degree of sound-field diffuseness. To investigate the problem and propose solutions, diffuse-field theory and existing standards have been revisited. Using numerical, finite-element-based, modal prediction, their capacity/effectiveness to achieve a diffuse sound field is analyzed by means of a number of descriptors (room-acoustical parameters). Because of time limitations, the concept regarding the design of a reverberation-room structure – i.e. size, shape, etc. – is mainly explored, rather than the internal test-setup arrangements. The prediction accuracy of different room-acoustical parameters are also determined by the reverberation-room standard methods, with respect to both the Sabine and Eyring versions of the diffuse-field formulae, and both in octave and third-octave bands. The minimum approachable frequencies of predictions and the quality of sound-field diffuseness are discussed in terms of the prediction accuracy of different room-acoustical parameters. Considering three room volumes prescribed by standards, and four room shapes for each of the volumes, it has been found that the reverberation room of volume 150 m³, as prescribed by the ISO 354 standard, with the typical dimensional orientation (longest x-dimension/shortest vertical dimension) yields better field diffuseness than the other rooms of different volumes and shapes. To check the possibility of further improvement of the field diffuseness, a number of additional features are integrated into that reverberation room. It is found that the rooms with diffusers and absorbent corner treatments yield improved sound-field diffuseness, while the rooms with diffuse surface reflection yield poor field diffuseness due to the increased surface absorption.