Theoretical and experimental analysis of the diffuseness in room sound fields
Final Report Abstract
A method to investigate the directional properties of transient sound fields using directional energy decay curves was developed. The developed framework is based a sound field decomposition of measurements performed with spherical microphone arrays. The directional decay curves allowed to estimate the sound field isotropy not only in steady state, but also during the decay process. Further, a method for the inference of directional decay times was developed. The methods were applied to the analysis and characterization of reverberation rooms, as well as multi-purpose venues such as churches, and a concert hall and a coupled volume system. The analysis of performance spaces and coupled volume systems showing new insights into its spatio-temporal characteristics. Anisotropic sound fields were found to be present in most rooms under consideration. Especially striking anisotropy was found in the presence of coupled volume systems, such as in a church with domed roof. The performance space analysis indicated that the existence of a mixing time may not apply to many rooms, highlighting the importance of directional reverberation for room acoustic design and the correct rendering of room acoustics in virtual acoustic environments. The major contribution of the reported project are new insights into the spatio-temporal characteristics of the sound field decay in reverberation rooms. A reverberation room with varying degrees of sound field diffuseness was investigated experimentally. Measurements were performed in the room equipped with and without panel diffusers, and in both cases with an absorbing test specimen present or absent. The analysis of the DEDCs clearly uncovered non-uniform energy flows in multiple room configurations. This effect is especially prominent in configurations with an absorber present, where a clear separation of decaying modes affected by the absorbing specimen and decaying modes with directions of propagation tangential to the absorbing sample is observed. Tangential energy flows even increased during the sound field decay for the configurations without panel diffusers and in all configurations at low frequencies, due to a non-uniform damping of modes decaying at different rates. These effects were well reflected by the developed isotropy estimation metric. The increase of tangential energy flows resulted in a reduction of sound field isotropy during the late sound field decay. As a result, the isotropy was found to be maximal only for short time intervals, smaller than the interval used for estimation of T30 or even T30. A fully isotropic sound field was found to be impossible to achieve when an absorbing specimen occupied the room, and even for the unoccupied room at low frequencies. In cases of distinctly anisotropic sound field decays, distributions of decay times per direction were inferred, supporting the hypothesis, that the decrease in estimated isotropy is linked to non-uniform damping of modes, decaying at different rates. Nevertheless, these findings allowed to draw conclusion on the revision of the international standard ISO 354, especially regarding the currently discussed extension of the reverberation time regression interval to 푇 instead of 푇 , thus extending the regression intervals to even less isotropic sound field decays. While the estimation of directional decay times resulted in interesting new insights, the method resulted in overestimation in some cases due overfitting caused by a mismatch between model and measured data. Hence, improvements to the model based on additional fundamental research is required.
Publications
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“Analysis of sound field isotropy based on directional energy decay curves,” J.Acoust.Am. 144, no. 3, pp. 1883–1883, Sep. 2018
M. Berzborn and M. Vorländer
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“Comparison of Isotropy Estimators for the Analysis of Reverberation Rooms,” in e-Forum Acusticum, 2020, pp. 139–146
M. Berzborn, M. Nolan, E. Fernandez-Grande, and M. Vorländer
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“Directional Grouping of Decaying Modes in a Reverberation Room,” in Proceedings of the 46th DAGA, Hannover, Germany, 2020, pp. 830–833
M. Berzborn and M. Vorländer
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“Isotropy in decaying reverberant sound fields,” J. Acoust. Soc. Am 148, no. 2, pp. 1077–1088, Aug. 2020
M. Nolan, M. Berzborn, and E. Fernandez-Grande
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“Directional sound field decay analysis in coupled spaces,” J. Acoust. Soc. Am 149, no. 4, pp. A116–A116, Apr. 2021
M. Berzborn, J. Balint, and M. Vorländer
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“Directional sound field decay analysis in performance spaces,” Build. Acoust., p. 1351010X2098462, Jan. 2021
M. Berzborn and M. Vorländer
