Sound focusing in reverberating rooms: The time-reversal approach

Abstract

Time reversal is already known as an efficient ultrasonic method to focus through inhomogeneous or multidiffusive media. The presence of pointlike reflectors in these media allows applications of this technique in fields like nondestructive testing, medical imaging, or underwater acoustics. An extension of this technique in audible range acoustics is presented. An array of 70 microphone/loudspeaker couples is used to refocus sound inside of a reverberating room. At a desired focal point, directivity patterns are measured and compared with those obtained by focusing with a cylindrical beamforming technique through the same antenna. Time reversal is shown to strongly improve the focal spot pattern (beamwidth and sidelobes level). These results are related to the ability of time reversal to compensate for reverberation and scattering induced in the studied room. The process acts as a spatio-temporally matched filter to the propagation transfer function of the desired focal point through the room. It makes this technique an auto-adaptive focusing system to a random 3-D cavity. Theory, experimental results, and future applications will be described.