Fluctuation of room acoustical parameters on small spatial intervals

Abstract

The Delft Acoustics group has introduced array technology in room acoustic measurement practice: instead of at a limited number of ‘‘representative’’ places, impulse responses are measured along an array of closely sampled microphone positions. By displaying the responses as a visual entity, the wave character of the sound field is clearly revealed. The data form a base for further processing, enabling the separation and analyzing of the different wave-field components. The sound fields in several concert halls have been measured this way. From impulse responses recorded with 0.05-m spatial separation, the common room acoustical parameters have been determined. It appears that, even between the ten positions in front of one and the same seat, unexpected fluctuations occur, e.g., when measured in octave bands, the clarity index varies over 1 dB on this interval, the lateral energy fraction 0.2 (i.e., between 0.1 and 0.3). For the broadband versions, these variations are approximately halved, still being significant. These results explain why the predictive value of these parameters for the perceptual quality of the acoustics on a (group of) seat(s) is limited. Instead of at one point, the values at several adjacent points should be considered.