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.
This letter explores the effect of the directivity of a source on time reversal acoustic focusing of energy. A single loudspeaker produces an airborne focus of sound in a reverberation chamber and in a classroom. Individual foci are created at microphone positions that surround the loudspeaker. The primary axis of the loudspeaker is then rotated and experiments are repeated to average out the room response. Focal amplitude, temporal quality of the foci, and spatial focusing quality are compared to determine the optimal angle to aim a directional source axis relative to the desired focal position.
Time reversal (TR) focusing used for nonlinear detection of cracks relies on the ability of the TR process to provide spatially localized, high-amplitude excitation. The high amplitude improves the ability to detect nonlinear features that are a signature of the motion of closed cracks. It follows that a higher peak focal amplitude than what can be generated with the traditional TR process will improve the detection capability. Modifying the time-reversed impulse response to increase the amplitude of later arrivals in the impulse response, while maintaining the phase information of all arrivals, increases the overall focal signal amplitude. A variety of existing techniques for increasing amplitude are discussed, and decay compensation TR, a technique wherein amplitude is increased according to the inverse of the amplitude envelope of the impulse response decay, is identified as the best modification technique for nonlinear crack detection. This technique increases the focal signal amplitude with a minor introduction of harmonic content, a drawback in two other methods studied, one-bit TR and clipping TR. A final study employs both decay compensation TR and traditional TR, focusing on a rod with stress corrosion cracking, and compares the merits of each in detecting nonlinearity from cracks in a real system. V
This research optimizes a time reversal focus of energy in terms of amplitude and quality by exploring several signal processing techniques applied to the reversed impulse response. Techniques explored here include deconvolution or inverse filtering [M. Tanter et al., J. Acoust. Soc. Am. 108, 223-234 (2000)], one-bit time reversal [A. Derode et al., J. Appl. Phys. 85, 6343-6352 (1999)], and a new technique termed “clipping.” A parameterization study comparing the maximum focal amplitude and focal temporal quality of time reversal focal signals for these techniques with different settings will be presented. Focal signals are explored in two different systems. The first is a resonant system comprised of a steel sample with a piezoelectric transducer and a scanning laser Doppler vibrometer (SLDV). The second, nominally non-resonant system, comprises a studio monitor loudspeaker and a precision microphone in a reverberant space above the Schroeder frequency. Comparing the focal quality of various time reversal techniques in two starkly different systems shows the optimal conditions for focusing in each system.
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