Volume 118, Issue 1, July 2005
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
118(2005); http://dx.doi.org/10.1121/1.1937205View Description Hide Description
Cavitation plays a varied but important role in lithotripsy. Cavitation facilitates stone comminution, but can also form an acoustic barrier that may shield stones from subsequent shock waves. In addition, cavitation damages tissue. Spark-gap lithotripters generate cavitation with both a direct and a focused wave. The direct wave propagates as a spherically diverging wave, arriving at the focus ahead of the focused shock wave. It can be modeled with the same waveform (but lower amplitude) as the focused wave. We show with both simulations and experiments that bubbles are forced to grow in response to the direct wave, and that these bubbles can still be large when the focused shock wave arrives. A baffle or “suppressor” that blocks the propagation of the direct wave is shown to significantly reduce the direct wavepressure amplitude, as well as direct wave-induced bubble growth. These results are applicable to spark-gap lithotripters and extracorporeal shock wave therapy devices, where cavitation from the direct wave may interfere with treatment. A simple direct-wave suppressor might therefore be used to improve the therapeutic efficacy of these devices.
118(2005); http://dx.doi.org/10.1121/1.1938528View Description Hide Description
The modes of vibration of an elastic plate are usually divided into propagating and nonpropagating kinds. While nonpropagating modes characterize local vibration nearby a perturbation source, which can be either an external force or a geometrical discontinuity, propagating modes carry energy along the waveguide and account for the vibration far away from the perturbation source. In this paper, by considering that the modes of an absorbing plate are always propagating, it is shown that each elastic mode consists of propagating and nonpropagating branches, which turn into a single propagating mode as soon as internal absorption is considered. Moreover, it is shown how introducing a little material damping leads to a rigorous differentiation of elastic modes when they are connected. A similar result can be obtained by loading the plate with a light fluid [Rokhlin et al. , J. Acoust. Soc. Am.85, 1074–1080 (Year: 1989)].