Index of content:
Volume 125, Issue 4, April 2009
- STRUCTURAL ACOUSTICS AND VIBRATION 
Determination of the random anisotropic elasticity layer using transient wave propagation in a fluid-solid multilayer: Model and experiments125(2009); http://dx.doi.org/10.1121/1.3087428View Description Hide Description
The aim of this paper is to introduce a simplified model for an uncertain solid layer sandwiched between two acoustic fluid layers and using the ultrasonic characterization with an acoustic source placed in one fluid layer. Uncertainties are taken into account with a probabilistic model of the elasticitytensor. Its parameters are the mean value of the random tensor and a dispersion parameter that controls the statistical fluctuation level. The characterization of the solid layer given a database of actual measurements consists in the determination of the (i) elastic parameters of the mean elasticity model, (ii) the dispersion parameter, and (iii) mass density of the solid. This is performed with a numerical solver of wave propagation and for in vivo data collected previously. The model is representative of measurements of human bone properties with the so-called axial transmission technique. The capability of the model to predict the velocity of the first experimental arriving signal in the statistical sense is proved. The identified anisotropicelasticitytensor of cortical bone from actual data based on the simplified model is given.
125(2009); http://dx.doi.org/10.1121/1.3079774View Description Hide Description
A nonlinear modelequation in Rayleigh–Plesset form is developed for volume oscillations of a compliant object buried close to the surface in soil. The equation takes into account the stress-free boundary condition on the surface of the ground. The model is fully nonlinear given exact relations for the elastic potential energy stored in deformation of the object and the soil. Expansions of the potential energies for weak nonlinearity are provided in terms of elastic constants that can be determined experimentally. When the shear modulus is allowed to increase with depth below the surface, the natural frequency predicted by the model first decreases and thereafter increases with burial depth, in agreement with reported observations. Perturbation solutions are derived for the displacements on the surface of the ground at the second harmonic and difference frequency due to the nonlinear response of the object to acoustic excitation.
Elastodynamic wave scattering by finite-sized resonant scatterers at the surface of a horizontally layered halfspace125(2009); http://dx.doi.org/10.1121/1.3086290View Description Hide Description
The present paper deals with the multiple scattering by randomly distributed elastodynamic systems at the surface of a horizontally layered elastic halfspace due to an incident plane wave. Instead of solving this problem for a particular configuration of the system, multiple scatteringtheory is used to compute the ensemble response statistics. The Dyson equation is used to calculate the mean field, while the nonstationary second order statistics are obtained by means of the Bethe–Salpeter equation. This allows for the determination of the mean square response of the system in the time and frequency domains. This model is used to study multiple scattering between buildings under seismic excitation. The influence of multiple scattering on the seismic site response is verified. Furthermore, the influence of the footprint and the damping of the buildings are investigated. The results are compared to results of a coupled finite element/boundary element solution for a group of buildings.
125(2009); http://dx.doi.org/10.1121/1.3087430View Description Hide Description
This paper describes an original numerical prediction technique developed for the analysis of coupled vibro-acoustic problems in fluid waveguides. Specifically it is a wave-based method that adopts a spectral element approach. Unlike the conventional element-based methods, this technique uses wave functions that satisfy the governing equations to describe the dynamic variables exactly. One advantage is that fine domain discretizations, used by element-based methods near the fluid-structure interface typically, are no longer required. Hence the resulting model sizes are much smaller than element-based methods yielding a more time-efficient prediction technique that may allow handling of mid-frequency applications. Another advantage is that dispersion relations between propagating and excitation wavenumbers are easily obtained, and an example to show this, as well as a discussion on how the wave-based prediction technique can be used for two coupled vibro-acoustic problems: a cavity with a non-reflecting boundary and a silencer duct problem, is presented. Its beneficial characteristics compared to element-based methods are demonstrated through the validation study and transmission loss examples.