On Frequency Response Curves in Rooms. Comparison of Experimental, Theoretical, and Monte Carlo Results for the Average Frequency Spacing between Maxima

The average frequency spacing 〈Δfmax〉 between adjacent maxima of the frequency response curve between two points in a room is determined by experiment, theory, and Monte Carlo computation. In earlier papers it had been shown that, above a certain critical frequency, 〈Δfmax〉 is reciprocally related to reverberation time and not dependent on other room characteristics—disproving a belief that 〈Δfmax〉 is a useful measure of the acoustical quality of rooms. Theory predicts 〈Δfmax〉 = 3.91/T60, where T60 is the reverberation time. Monte Carlo computation gives 3.90/T60. Measurements in two very different rooms using a vacuum tube voltmeter for reading the sound pressure are in good agreement with these predictions. Measurements with a logarithmic level recorder give, as in earlier investigations, much larger values. The discrepancy cannot be explained by the level recorder's 0.5-db quantization. This paper shows that: (1) The Monte Carlo method is a useful tool for solving complex problems in room acoustics. (2) Level recorder measurements of frequency (or space) irregularities of rooms must be taken with several grains of salt.