Reverberation time measurement issues

Abstract

The current ISO attempt to revise ISO 354 has ended in failure because the experts on the working group could not reach agreement in the required time frame. However, several important issues were identified. Some of these issues are discussed in this paper. The current ISO 354:2003 changed from using 30 dB of decay to using 20 dB of decay to determine the reverberation time. Unfortunately, the conversion of the theoretical formulae for the spatial and ensemble standard deviation from 30 dB to 20 dB of decay was not correct. The current version of the standard incorrectly changed the maximum linear averaging time and changed from specifying the maximum exponential averaging time to specifying the maximum exponential time constant but did not halve the maximum value. There is also evidence that the maximum exponential averaging time can be increased by 50% without biasing the measured reverberation time. When a discrete decay curve is analyzed, there are issues when deciding the first and last point to use when determining the reverberation time. During the attempted revision of the standard, it was proposed to specify a minimum amount of empty room sound absorption. This did not reduce the spread of results in the round robin and this paper gives theoretical results which suggest that this is to be expected.