Room impulse response variation due to temperature fluctuations and its impact on acoustic echo cancellation

In the field of acoustic echo cancellation, it is well known that the amount of cancellation that can be obtained in real-world systems is limited. Euphemistically this has been referred to in the acoustic echo cancellation community as ”the 20 dB rule” [?]. This rule was formed from many real-world observations that one can typically obtain only about 20-30 dB of cancellation. The acoustic echo cancellation problem is generally approached as a linear systems identification problem. As such, one would expect that the acoustic coupling between the loudspeaker and microphone could be well identified and therefore result in very large cancellation values. Unfortunately, this is not the case as there are practical issues that can significantly limit the amount of attainable cancellation. This article investigates the effect of room thermal fluctuations on the the loudspeaker-microphone impulse response. Both a theoretical model and actual experimental measurements are given which show that thermal variations on the order of tenths of a degree Centigrade can lead to surprisingly large variations in the room impulse response. Thus, small thermal variations can be a limiting physical factor in acoustic echo cancellation performance.