Permissible number of synchronous averaging times to obtain reverberation time from impulse response under time-variance conditions

Abstract

In the measurement of room impulse response, the synchronous averaging technique and such new methods as the MLS and the swept-sine methods are being widely used to improve the signal-to-noise ratio. In actual measurement conditions, however, the air in a room is continuously moving and the temperature is changing to some degree. The measured value of the reverberation time in such a room tends to be shorter at higher frequencies when applying the synchronous averaging. Therefore, the assumption of a time invariant has to be carefully considered, and, on this point, some research has been conducted to date. We also have reported various research results concerning the impulse response measurement under the time-variance conditions. In this paper, the permissible number of synchronous averaging times for reverberation measurement is studied through some field experiments. In each field, many time impulse response measurements were taken between a fixed pair of sound source and receiving positions by the swept-sine method, without averaging. After the measurements, the characteristics and the extent of the time-variance under measuring were estimated by a short-term running cross-correlation function between each impulse response. The influence of the time variance on the synchronous averaging result was studied based on the estimated time variance.