Nonlinear focusing of high amplitude sound using time-reversal

Abstract

Time reversal (TR) is a signal processing technique that may be used to intentionally generate high amplitude focusing of sound. The use of time reversal in room acoustics has been studied by others, but the application to generating high amplitude focusing has not previously been explored. The purpose of this study is to generate high amplitude sound waves in order to mimic a virtual spherical source with enough intensity to observe nonlinear wave propagation. Experiments have been carried out in a reverb chamber with eight compression horn drivers. Using these drivers, the impulse response is calculated, reversed in time, and modified using the clipping technique. When these signals are broadcast from the sources, a focus is generated at the receiver location with peak levels reaching 198 dB (ref 20 μPa). As the waves superpose at the focus, the amplitudes observed do not scale linearly when the experiment is repeated at increasing amplification levels. The compression peaks are higher in amplitude than expected, and the rarefaction troughs are lower in amplitude (less negative) than expected, when compared to linear scaling. Additionally, the diverging waves from the focus resemble the propagation of a single spreading shockwave.