Retrodirective array performance case studies and implications for mine countermeasures

The use of focused acoustic fields for the mechanical destruction of fluid-borne targets is applied zn. the medical field for the comminution of kidney stones by lithotripsy. Inhomogeneities in the propagation medzum are of practical concern to both the medical and the ocean acoustics communities. The use of phase conjugation to compensate for medium inhomogeneities and multipath efjects has been demonstrated for linear acoustics in both medical and ~nderwater contexts. The feasibility of focusing an intense acoustic field onto ~aterborne targets has implications for mine countermeasures because of the possibilitu of remotely neutralizing mines. The demonstration of the concept in actual mine countermeasure applications has yet to be realized. We present the results of a numerical study which investigates the performance of linear arrays using time .reversal as a means of focusing acoustic fields onto targets in an arbitrary medium modeled after a shallow water channel. The simulation investigates the cases where jitter exists in the initial phase of the time signals. The existence of tight focal wzdths, narrower than the free-space diffraction limit prediction, sometimes called "super-focusing", was observed for simulations containing small-scale inhomogeneitaes. </p

Simulations of the thermo-acoustic lens effect during focused ultrasound surgery

Laboratory measurements of soft tissue properties show a dependence of background propagation properties on temperature. For typical focused ultrasound surgery (FUS) applications, only the slow variations in tissue background parameters need to be accounted for when computing the outcome of a FUS sonication. The cumulative effect of slowly varying sound speed has been referred to in the literature as a thermal lens, or a thermo-acoustic lens because of its beam-distorting properties. An algorithm to solve the coupled acoustic-thermal problem is described, and numerical results are presented to illustrate the effects of dynamic sound-speed profiles in layered tissues undergoing FUS. The results of simulations in liver with and without a fat layer indicate that the thermal-acoustic interaction results in more complex dynamics in FUS than a simple model will predict. Both the size and the position of the lesions predicted from the simulations are affected by the thermo-acoustic lens effect. However, the overall effect from short sonications at high power from sharply focused single element sources (F-no. from 0.8 to 1.3) around 1 MHz similar to those used in clinical setups is found to be small. © 2001 Acoustical Society of America

Retrodirective array performance case studies and implications for mine countermeasures

The use of focused acoustic fields for the mechanical destruction of fluid-borne targets is applied zn. the medical field for the comminution of kidney stones by lithotripsy. Inhomogeneities in the propagation medzum are of practical concern to both the medical and the ocean acoustics communities. The use of phase conjugation to compensate for medium inhomogeneities and multipath efjects has been demonstrated for linear acoustics in both medical and ~nderwater contexts. The feasibility of focusing an intense acoustic field onto ~aterborne targets has implications for mine countermeasures because of the possibilitu of remotely neutralizing mines. The demonstration of the concept in actual mine countermeasure applications has yet to be realized. We present the results of a numerical study which investigates the performance of linear arrays using time .reversal as a means of focusing acoustic fields onto targets in an arbitrary medium modeled after a shallow water channel. The simulation investigates the cases where jitter exists in the initial phase of the time signals. The existence of tight focal wzdths, narrower than the free-space diffraction limit prediction, sometimes called "super-focusing", was observed for simulations containing small-scale inhomogeneitaes. </p

Amplitude degradation of time-reversed pulses in nonlinear absorbing thermoviscous fluids.

The linear wave equation in a lossless medium is time reversible, i.e., every solution p(x, t) has a temporal mirror solution p(x, -t). Analysis shows that time reversal also holds for the lossless nonlinear wave equation. In both cases, time-reversal invariance is violated when losses are present. For nonlinear propagation loses cannot normally be ignored; they are necessary to prevent the occurrence of multivalued waveforms. Further analysis of the nonlinear wave equation shows that amplification of a time-reversed pulse at the array elements also leads to a violation of time reversal even for lossless nonlinear acoustics. Numerical simulations are used to illustrate the effect of nonlinearity on the ability of a time-reversal system to effectively focus on a target in an absorbing fluid medium. We consider both the amplitude and arrival time of retrodirected pulses. The numerical results confirm that both shock generation (with the accompanying absorption) and amplification at the array, adversely affect the ability of a time-reversal system to form strong retrodirective sound fields

The acoustic emissions from single-bubble sonoluminescence

Detailed measurements of the acoustic emissions from single-bubble sonoluminescence have been made utilizing both a small 200-μm aperture PVDF needle hydrophone, and a focused 10-MHz transducer. Signals obtained with the needle hydrophone show a fast (5.2 ns), probably bandlimited rise time and relatively large pulse amplitude (≈1.7 bar).. Below the sonoluminescence threshold, the emissions are observable, but considerably smaller in amplitude (≈0.4 bar).. Several signals are observed with the 10-MHz transducer and correspond to acoustic emissions from the bubble during the main collapse, as well as from the rebounds. Experiments reveal that the acoustic emissions occur at or near the minimum bubble radius. Calculations of the peak pressures and pulse widths are compared with experimental data. </p