Volume 119, Issue 6, June 2006
Index of content:
- NONLINEAR ACOUSTICS 
119(2006); http://dx.doi.org/10.1121/1.2197804View Description Hide Description
Nonlinear acousticproperties of a composite medium, consisting of hollow microspheres suspended in Castor Oil, at elevated hydrostaticpressure are experimentally investigated. The acoustic nonlinear parameter of the medium is found to be highly dependent upon hydrostaticpressure. varies from a small negative value near ambient pressure to a very large negative value (around ) in the vicinity of Pascals, above ambient pressure. With a further increase in hydrostaticpressure the magnitude of decreases passing through zero and finally becomes positive. Finite amplitude wave propagation in this medium at low hydrostaticpressures is characterized by waveform steepening in the backward direction leading to rarefactive shockwaves and, at high hydrostaticpressures, by steepening in the forward direction leading to compressive shockwaves. Estimates of are obtained from both the measurement of thermodynamic properties and from waveform distortion during propagation.
119(2006); http://dx.doi.org/10.1121/1.2197805View Description Hide Description
We examine the case of focusing transducers with a strong initial (in linear regime) shift of the main on-axis peak pressure maximum from the geometrical focal point toward the transducer. Such transducers are characterized, using the concepts introduced in this paper, by a low Fresnel number. The displacement of this initially shifted on-axis pressure maximum point toward the geometrical focus, and its backward motion as the driving transducer voltage increases until highly nonlinear regimes, has been experimentally observed. The simultaneous monitoring of the temporal wave-form distortion determines the real roles and interplay between different nonlinear effects (refraction and attenuation) in the observed dynamics of the on-axis pressure maximum. The numerical solution of the corresponding mathematical model confirms the physical interpretation of the observed phenomenon.
The effect of reflector geometry on the acoustic field and bubble dynamics produced by an electrohydraulic shock wave lithotripter119(2006); http://dx.doi.org/10.1121/1.2195074View Description Hide Description
A theoretical model for the propagation of shock wave from an axisymmetric reflector was developed by modifying the initial conditions for the conventional solution of a nonlinear parabolic wave equation (i.e., the Khokhlov–Zabolotskaya–Kuznestsov equation). The ellipsoidal reflector of an HM-3 lithotripter is modeled equivalently as a self-focusing spherically distributed pressure source. The pressure wave form generated by the spark discharge of the HM-3 electrode was measured by a fiber optic probe hydrophone and used as source conditions in the numerical calculation. The simulated pressure wave forms, accounting for the effects of diffraction, nonlinearity, and thermoviscous absorption in wave propagation and focusing, were compared with the measured results and a reasonably good agreement was found. Furthermore, the primary characteristics in the pressure wave forms produced by different reflector geometries, such as that produced by a reflector insert, can also be predicted by this model. It is interesting to note that when the interpulse delay time calculated by linear geometric model is less than about , two pulses from the reflector insert and the uncovered bottom of the original HM-3 reflector will merge together. Coupling the simulated pressure wave form with the Gilmore model was carried out to evaluate the effect of reflector geometry on resultant bubble dynamics in a lithotripter field. Altogether, the equivalent reflector model was found to provide a useful tool for the prediction of pressure wave form generated in a lithotripter field. This model may be used to guide the design optimization of reflector geometries for improving the performance and safety of clinical lithotripters.
119(2006); http://dx.doi.org/10.1121/1.2197799View Description Hide Description
This paper presents, in one dimension, the general analytical solution of the acoustic phase conjugation in an active medium in contact with passive media of arbitrary impedance. The homogeneous case where no impedance jumps exist at the edge of the active zone is obtained as a particular case. This homogeneous case was the only one treated explicitly in the literature but mostly in the frame of Brillouin scattering. In contrast to this previous work, the present theory is based on a preliminary straightforward analysis using a dual-time-scale method and provides very practical results like the threshold of the supercritical modes, the rate of amplification, and its link with the stress repartition in the conjugator.