Index of content:
Volume 116, Issue 2, August 2004
- NONLINEAR ACOUSTICS 
116(2004); http://dx.doi.org/10.1121/1.1766051View Description Hide Description
A model has been developed for the response of a rigid-porous hard-backed medium containing an arbitrary number of layers to high amplitude sound. Nonlinearity is introduced by means of a velocity-dependent flow resistivity in Johnson’s equivalent fluid model for the complex tortuosity of each layer. Numerical solution of the resulting system of algebraic equations allows prediction of the dependence of surface impedance and reflection coefficient on the incident pressure amplitude. Measurements have been made of the surface impedance of various triple layers, made from different diameters of spherical lead shot and double layers consisting of gravel with different mean particle size, subject to high-intensity continuous sound. Good agreement between the model predictions and data for these multiple-granular layers is demonstrated. Moreover it is shown both theoretically and experimentally that the layer configuration giving optimum performance at low sound intensities may not continue to do so as the incident sound level is increased and the response becomes increasingly nonlinear. It is shown also that the nonlinear behavior depends strongly on layering and that, in some cases, the behavior is changed simply by changing the top layer thickness.
116(2004); http://dx.doi.org/10.1121/1.1768251View Description Hide Description
In this paper, we consider the propagation of shear acoustic waves in a single spherical bead and in linear one-dimensional periodic chains of identical spheres. In both cases, normal force of interaction compresses the sphere(s) and obeys the nonlinear Hertz’ law. In the low-frequency domain, the spectroscopy of the transmitted impulses gives rise to peaks the existence and the origin of which are discussed.
116(2004); http://dx.doi.org/10.1121/1.1768253View Description Hide Description
In this paper we derive a general expression for the acoustic Casimir pressure between two parallel slabs made of arbitrary materials and whose acoustic reflection coefficients are not equal. The formalism is based on the calculation of the local density of modes using a Green’s function approach. The results for the Casimir acoustic pressure are generalized to a sphere/plate configuration using the proximity theorem.
116(2004); http://dx.doi.org/10.1121/1.1765198View Description Hide Description
The persistence of acoustic cavitation in a pulsed waveultrasound regime depends upon the ability of cavitation nuclei, i.e., bubbles, to survive the off time between pulses. Due to the dependence of bubble dissolution on surface tension, surface-active agents may affect the stability of bubbles against dissolution. In this study, measurements of bubble dissolution rates in solutions of the surface-active polymer poly(propyl acrylic acid) (PPAA) were conducted to test this premise. The surface activity of PPAA varies with solution and concentration of dissolved polymer molecules. The surface tension of PPAA solutions (55–72 dynes/cm) that associated with the polymer surface activity was measured using the Wilhelmy plate technique. Samples of these polymer solutions then were exposed to 1.1 MHz high intensity focused ultrasound, and the dissolution of bubbles created by inertial cavitation was monitored using an active cavitation detection scheme. Analysis of the pulse echo data demonstrated that bubble dissolution time was inversely proportional to the surface tension of the solution. Finally, comparison of the experimental results with dissolution times computed from the Epstein–Plesset equation suggests that the radii of residual bubbles from inertial cavitation increase as the surface tension decreases.