Index of content:
Volume 107, Issue 4, April 2000
- NONLINEAR ACOUSTICS 
107(2000); http://dx.doi.org/10.1121/1.428473View Description Hide Description
The results of experimental and theoretical investigations of nonlinear acoustic phenomena (nonlinear losses, shift of resonance frequency, generation of the third harmonic, and nonlinear sound-by-sound damping) in polycrystallinezinc nonannealed and annealedresonating rods are presented. The measurements were carried out in the strain range at a frequency of about 3 kHz; the frequency of the weak ultrasonic pulse was about 270 kHz. The experimentally observed phenomena are described in frames of phenomenological equations of state containing elastic hysteresis and dissipative nonlinearity. The nonlinear acoustic parameters of these equations are determined by comparison between theoretical dependencies and experimental results. The influence of structural changes in zinc due to annealing on the nonlinear acoustic phenomena is shown.
A method for estimating time-dependent acoustic cross-sections of bubbles and bubble clouds prior to the steady state107(2000); http://dx.doi.org/10.1121/1.428474View Description Hide Description
Models for the acoustic cross-sections of gas bubbles undergoing steady-state pulsation in liquid have existed for some time. This article presents a theoretical scheme for estimating the cross-sections of single bubbles, and bubble clouds, from the start of insonation onward. In this period the presence of transients can significantly alter the cross-section from the steady-state value. The model combines numerical solutions of the Herring–Keller model with appropriate damping values to calculate the extinction cross-section of a bubble as a function of time in response to a continuous harmonic sound field (it is also shown how the model can be adapted to estimate the time-dependent scatter cross-section). The model is then extended to determine the extinction cross-section area of multiple bubbles of varying population distributions assuming no bubble–bubble interactions. The results have shown that the time taken to reach steady state is dependent on the closeness of the bubble to resonance, and on the driving pressure amplitude. In the response of the population as a whole, the time to reach steady state tends to decrease with increasing values of the driving pressure amplitude; and with the increasing values of the ratio of the numbers of bubbles having radii much larger than resonance to the number of resonant bubbles. The implications of these findings for the use of acoustic pulses are explored.