Volume 120, Issue 2, August 2006
Index of content:
- NONLINEAR ACOUSTICS 
Measurement of the frequency dependence of the ultrasonic parametric threshold amplitude for a fluid-filled cavity120(2006); http://dx.doi.org/10.1121/1.2214457View Description Hide Description
By driving a transducer at one end of a fluid-filled cavity parallel to a rigid plane reflector at the other end, standing ultrasonic waves can be generated. Variations in the cavity length resulting from transducer motion lead to the generation of resonant frequencies lower than the drive frequency (known as fractional harmonics). This excitation of fractional harmonics in a liquid-filled cavity by ultrasonic waves was described previously as a parametric phenomenon [Laszlo Adler and M. A. Breazeale, J. Acoust. Soc. Am.48, 1077–1083 (1970)]. This system was modeled by using a modified Mathieu’s equation whose solution resulted in the prediction of critical threshold drive amplitude for the excitation of parametric oscillation. The apparatus used by Adler and Breazeale was recently refined for accurate measurements of the threshold amplitude for parametric excitation at frequencies ranging from 2 to . The measurements showed that in this range the threshold amplitude increases with increasing drive frequency in apparent discrepancy with the results of Adler and Breazeale. Analysis of the theory indicates, however, that both past and current results lie in two different stability zones and each is in agreement with the existing theory.
120(2006); http://dx.doi.org/10.1121/1.2215228View Description Hide Description
Coupled equations describing the radial and translational dynamics of an encapsulated gas bubble in an ultrasound field are derived by using the Lagrangian formalism. The equations generalize Church’s theory [J. Acoust. Soc. Am.97, 1510 (1995)] by allowing for the translation motion of the bubble and radiation losses due to the compressibility of the surrounding liquid. The expression given by Church for the inner bubble radius corresponding to the unstrained state of the bubble shell is also refined, assuming that the shell can be of arbitrary thickness and impermeable to gas. Comparative linear analysis of the radial equation is carried out relative to Church’s theory. It is shown that there are substantial departures from predictions of Church’s theory. The proposed model is applied to evaluate radiation forces exerted on encapsulated bubbles and their translational displacements. It is shown that in the range of relatively high frequencies encapsulated bubbles are able to translate more efficiently than free bubbles of the equivalent size.
Translation of bubbles subject to weak acoustic forcing and error in decoupling from volume oscillations120(2006); http://dx.doi.org/10.1121/1.2214132View Description Hide Description
A microbubble in a sound wave oscillates in volume and translates unsteadily. The two motions are coupled. In large-scale simulations of the structure of bubble clouds driven by acoustic fields, it has been of significant convenience to decouple volume oscillations and translation, as an approximation. The errors of this decoupling approximation were considered in an earlier presentation [A. J. Reddy and A. J. Szeri, J. Acoust. Soc. Am.112, 1346–1352 (2002)], in the parameter range of interest in medical ultrasound. In this work, the approximation is reexamined for a much broader range of driving frequencies and bubble sizes. Solving the equation of motion for linearly oscillating bubbles, it is found that even for weak acoustic forcing, the translation speed obtained with the decoupling approximation can be in error as much as 30% relative to the translation speed in the full equations. The error depends on the bubble size, the driving frequency, and the liquid properties. The results are presented in a form convenient for applications. The principal utility of the analysis is for bubbles in microgravity, or in normal gravity driven by a soundfield with a horizontal wave-number vector.
120(2006); http://dx.doi.org/10.1121/1.2214131View Description Hide Description
The acoustic fields of a high intensity focused ultrasound (HIFU) transducer operating either at its fundamental or third harmonic frequency were measured by a fiber optic probe hydrophone (FOPH). At when the electric power applied to the transducer was increased from , the peak positive∕negative pressures at the focus were measured to be and . The corresponding spatial-peak pulse-average and spatial-average pulse-average intensities were and . Nonlinear propagation with harmonics generation was dominant at high intensities, leading to a reduced beam size of the compressional wave but an increased beam size of the rarefactional wave. Enhancement ratio of absorbed power density in water increased from 1.0 to 3.0. In comparison, the HIFU transducer working at produced higher peak pressures and with smaller beam size . Overall, FOPH was found to be a convenient and reliable tool for HIFU exposimetry measurement.