Comparison of time reversal acoustic and prefiltering methods of focusing of tone burst signals

Abstract

The concept of time reversal acoustics (TRA) provides an elegant possibility of both temporal and spatial concentration of acoustic energy in highly inhomogeneous media. TRA-based focusing is typically used for generation of short acoustic pulses, however, in some medical and industrial applications, longer pulses are required. TRA focusing of longer signals leads to an increase of side lobes in temporal and spatial domains. Another method for focusing, known as prefiltering, is based on measurements of the impulse response, which relates the signal at the TRA transmitter to that at the focusing point. After evaluating the impulse response, the excitation signal may be calculated to generate the desired waveform in the focus point. This method allows signal generation with any desired form including long tone-burst signals. Experiments on comparison TRA and prefiltering methods of ultrasound focusing were conducted in the frequency band of 200–1000 kHz. In the experiments, focused acoustic pulses with various forms and duration were generated: triangular, rectangular, and amplitude-modulated tone burst signals. The prefiltering modes provide better temporal compression of the focused signal, and the signal energy outside the main pulse in the prefiltering mode was shown to be much lower than that in standard TRA focusing.