Time reversal focusing of high amplitude sound in a reverberation chamber

Abstract

Time reversal (TR) is a signal processing technique that can be used for intentional sound focusing. While it has been studied in room acoustics, the application of TR to produce a high amplitude focus of sound in a room has not yet been explored. The purpose of this study is to create a virtual source of spherical waves with TR that are of sufficient intensity to study nonlinear acoustic propagation. A parameterization study of deconvolution, one-bit, clipping, and decay compensation TR methods is performed to optimize high amplitude focusing and temporal signal focus quality. Of all TR methods studied, clipping is shown to produce the highest amplitude focal signal. An experiment utilizing eight horn loudspeakers in a reverberation chamber is done with the clipping TR method. A peak focal amplitude of 9.05 kPa (173.1 dB peak re 20 μPa) is achieved. Results from this experiment indicate that this high amplitude focusing is a nonlinear process.

A comparison of impulse response modification techniques for time reversal with application to crack detection

“…The term c meanðjRðxÞj 2 Þ is a regularization term used to keep the deconvolution TR operation finite, a process described in more detail by Anderson et al 23 Optimization of c for reduction of the energy on either side of the peak focusing (termed side lobes 39 ) followed the process described by Willardson et al 21 and determined an optimal c value of 0.9, which was also the value reported by Anderson et al for TR focusing of waves in solid media. 23 As c approaches infinity, the deconvolution TR process returns a traditional RIR because the c term dominates in the denominator of Eq.…”

“…18,19 Others have used similar techniques wherein the instantaneous amplitude of the impulse response as time progresses is increased to compensate for the natural decay from geometric spreading losses and propagation losses in attenuating media, thereby increasing the SNR of an impulse response in strongly attenuating media. 20,21 Deconvolution TR, or inverse filtering, was introduced as a means of compensating for resonances in the impulse response to optimize the impulsive nature of a TR focus of energy. 22,23 Studies have examined the reliance of focal amplitude on the length of the initial pulse (i.e., the bandwidth).…”

“…31 Recently, Willardson et al published experimental research manipulating TR processing in order to maximize the focal amplitude of audible sound in a reverberation chamber. 21 The work by Willardson et al experimentally compared five different techniques that adjust the RIR to ultimately determine which provided the highest peak focal amplitude. The study was conducted in a reverberation chamber using a loudspeaker and a microphone, constituting a wide bandwidth system, and examined various attributes of the focal signals, including peak focal amplitude and temporal quality.…”

“…h)]. Like one-bit TR, clipping TR amplifies later reflections, but it also amplifies all low-level signals in the recorded impulse response, potentially amplifying background noise.Decay compensation TR attempts to compensate for the exponential decay of the impulse response 20,21. As explained by Willardson et al, the envelope of the RIR is obtained through a Hilbert transform operation, after which the envelope is smoothed through the use of a moving average filter [seeFig.…”

Time reversal acoustics applied to rooms of various reverberation times

“…Willardson et al 21 explored the application of TR to produce high amplitude focusing in a reverberant environment. They investigated how TR focusing changed depending on different signal processing strategies applied to hðÀtÞ prior to the backward step of the TR process.…”