Volume 126, Issue 4, October 2009
Index of content:
- STRUCTURAL ACOUSTICS AND VIBRATION 
126(2009); http://dx.doi.org/10.1121/1.3206581View Description Hide Description
The static and dynamic transport properties of elastic wave propagation through two-dimensional random slabs without internal reflection were studied at two different scattering parameters: one for Rayleigh scattering and the other for Rayleigh–Gans scattering. The spatial distribution and temporal evolution of shear and compressional waveenergy densities inside the slabs were calculated by solving the radiative transfer equation and the generalized diffusion equation (GDE). The comparison of their results can determine the region of validity of the GDE. The process of energy equilibration between the two wave modes was demonstrated explicitly as well as the process of diffusion. The depth inside a slab that is needed to reach energy equilibration or diffusive behavior is found to be dependent on source polarization. The results also show that the bulk equilibration ratio can be found inside a sample only when the sample is sufficiently thick. Deviations of the equilibration ratio from its bulk value are found near the output surface due to the absence of in-flow energy flux. The behavior of the deviations is sensitive to the scattering parameter but independent of source polarization.
126(2009); http://dx.doi.org/10.1121/1.3203359View Description Hide Description
Theorems indicating that a fully equipartitioned random wave field will have correlations equivalent to the Green’s function that would be obtained in an active measurement are now legion. Studies with seismic waves, ocean acoustics, and laboratory ultrasound have confirmed them. So motivated, seismologists have evaluated apparent seismic travel times in correlations of ambient seismicnoise and tomographically constructed impressive maps of seismic wave velocity. Inasmuch as the random seismic waves used in these evaluations are usually not fully equipartitioned, it seems right to ask why it works so well, or even if the results are trustworthy. The error, in apparent travel time, due to non-isotropic specific intensity is evaluated here in a limit of large receiver-receiver separation and for the case in which the source of the noise is in the far field of both receivers. It is shown that the effect is small, even for cases in which one might have considered the anisotropy to be significant, and even for station pairs separated by as little as one or two wavelengths. A formula is derived that permits estimations of error and corrections to apparent travel time. It is successfully compared to errors seen in synthetic waveforms.
126(2009); http://dx.doi.org/10.1121/1.3203930View Description Hide Description
A method is introduced which is shown to predict radiated sound power from rectangular baffled panels. The method employs a filtered wavenumber transform to extract the power in the supersonic wavenumbers on the panel and a radiation factor to scale the supersonic power to match the actual radiated sound power. Although empirically derived, the radiation factor is shown to be related to the radiation efficiency of an infinite panel. The radiation factor is simple, depending only on the ratio of the wavenumbers of the panel to the radiation medium, and the method is straightforward to use, requiring only the panel normal velocities. The computation is efficient, as much as two orders of magnitude faster than a Rayleigh integration, thus providing a means of combining sound power predictions with finite element optimizations. A formula is derived which predicts the lowest frequency for which the method is valid as a function of the bin width of the wavenumber transform. The radiation factor method is shown to produce radiated sound power estimates which favorably compare to estimates derived from intensity measurements of physical test specimens and to Rayleigh integral estimates computed using both simulated and measuredvelocities.