Spatial fluctuations in measures for spaciousness

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.