Sound propagation in enclosed spaces is characterized by reflections at the boundaries of the enclosure. Reflections can be wanted in the case when they support the direct sound or give a feeling of envelopment or they can be unwanted when they lead to echoes and colouration. When measuring multiple impulse responses in an enclosed space along an array the reflections can be mapped to the reflecting objects. Similar to seismic exploration, medical diagnostics, and underwater acoustics, an image of the reflecting objects is obtained in terms of reflected energy. The imaging process is based on inverse wave field extrapolation with the Kirchhoff–Helmholtz and Rayleigh integrals. The inverse of the imaging process recreates the measured impulse responses from the image and it allows one to remove or alter reflecting objects in the image and investigate their influence on the wave field in the enclosed space in a physically correct way. This can be verified by reimaging the altered wave field. Preliminary results from listening tests for the perceptual evaluation are presented. They indicate that the influence of a reflecting object can only be perceived in its close proximity.
In room acoustics, several measures have been defined that are supposed to quantify the apparent source width (ASW) in a hall, being one of the perceptual cues related to spaciousness. The most common ones are the lateral energy fraction (LF), i.e., the ratio between lateral and omnidirectional early energy, and the interaural cross correlation coefficient (IACC), all to be calculated from measured or simulated impulse responses. [Several versions of the LF are known in literature, having different names, generalized here as lateral energy fraction.] According to a method proposed by Berkhout et al. [J. Acoust. Soc. Am. 102, 2757–2770 (1997)], for a fixed source position impulse responses have been measured along an array of closely spaced microphone positions in several halls. The above measures, when calculated from these impulse responses, show large fluctuations with small variations in microphone position due to interference of the different components of the wave field to which the human ear is apparently insensitive. A revision of the measures is discussed, which contributes to the suppression of the interference effects. In order to assess their perceptual significance, the fluctuations have to be related to just-noticeable differences (jnd’s) in ASW. Since very different jnd values are given in the literature, the authors advise that new experiments should be conducted on this point.
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