Room Impulse Response Variation Due to Thermal Fluctuation and Its Impact on Acoustic Echo Cancellation

In the field of acoustic echo cancellation, it is well known that the amount of cancellation attainable in real-world systems is limited. Euphemistically this limitation has been referred to in the acoustic echo cancellation community as the “20 dB rule” [1]. This rule reflects the observation that, typically, one can obtain only about 20 to 30 dB of acoustic echo cancellation in actual physical settings. 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 significantly limit the amount of attainable cancellation. This paper investigates the effect of room thermal fluctuations on the loudspeaker-microphone impulse response. Both a theoretical model and actual experimental measurements are given that show 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.