Volume 109, Issue 1, January 2001
Index of content:
- GENERAL LINEAR ACOUSTICS 
109(2001); http://dx.doi.org/10.1121/1.1289924View Description Hide Description
The convergence of linear poroelastic elements based on Biot displacement formulation is investigated. The aim is to determine a mesh criterion that provides reliable results under a given frequency limit. The first part deals with 1D applications for which resonance frequencies can be related to Biot wavelengths. Their relative contributions to the motion are given in order to determine if the mesh criteria for monophasic media are suitable for poroelastic media. The imposition of six linear elements per wavelength is found for each Biot wave as a primary condition for convergence. For 3D applications, convergence rules are derived from a generic configuration, i.e., a clamped porous layer. Because of the complex deformation, the previous criterion is shown to be insufficient. Influence of the coupling between the two phases is demonstrated.
109(2001); http://dx.doi.org/10.1121/1.1331110View Description Hide Description
This paper presents an exact three-dimensional analysis of the general nonaxisymmetric free vibration of a piezoceramic hollow sphere by employing a state-space approach. By introducing three displacement functions and two stress functions, the basic equations of a spherically isotropic piezoelectric medium are eventually turned into two separated state equations with variable coefficients. The solutions of these two equations are then obtained by virtue of Taylor’s expansion theorem. Relationships between the state variables at the inner and outer surfaces of a laminated hollow sphere are established. Exact frequency equations corresponding to two independent classes of vibrations are then derived from the free conditions at the spherical boundary surfaces. Numerical results are finally presented.
Spatial analysis of torsional wave propagation in a cylindrical waveguide. Application to magnetostrictive generation109(2001); http://dx.doi.org/10.1121/1.1323717View Description Hide Description
A spatial analysis of the generation and propagation of torsional waves in a cylindrical rod is presented. Starting from the classical linear equation of propagation and assuming a linear medium of propagation, the eigenfunctions of the propagation operator are calculated. Under the hypothesis of separation-of-variables type of solution, two ways of deriving the associated modes are performed. Given the normal mode basis, the behavior of a wavefront generated into the rod is examined. The application to the magnetostrictive generation of torsional waves is studied. Including the influence of eddy currents on the excitation and the geometry waveguideeffects on the wave propagation, an analytical expression of mechanical losses during the first steps of propagation is given. A basic model of the interaction between a defect and the torsional guided waves is also proposed.
Influence of grazing flow and dissipation effects on the acoustic boundary conditions at a lined wall109(2001); http://dx.doi.org/10.1121/1.1331678View Description Hide Description
The problem of sound propagation near a lined wall taking into account mean shear floweffects and viscous and thermal dissipation is investigated. The method of composite expansion is used to separate the inviscid part, in the core of the flow, from the boundary layer part, near the wall. Two diffusionequations for the shear stress and the heat flux are obtained in the boundary layer. The matching of the solutions of these equations with the inviscid part leads to a modified specific acoustic admittance in the core flow. Depending on the ratio of the acoustic and stationary boundary layer thicknesses, the kinematic wall condition changes gradually from continuity of normal acoustic displacement to continuity of normal acoustic mass velocity. This wall condition can be applied in dissipative silencers and in aircraft engine-duct systems.
Structural-acoustic coupling in a partially opened plate-cavity system: Experimental observation by using nearfield acoustic holography109(2001); http://dx.doi.org/10.1121/1.1320476View Description Hide Description
In order to understand the cause and effect relation between a structure and a fluid, many studies on structural-acoustic coupling have been done. However, the studies were restricted to the interaction between only a structure and a fluid located on one or the other side of the structure. It is our aim to understand the coupling mechanism of a generally coupled system that has direct interaction between a finite interior fluid and a semi-infinite exterior one. We believe that this configuration allows the structure to interact with the fluid of the finite volume and that of the infinite one, thus providing a more general structure-fluid coupling (or structural-acoustic coupling) mechanism. For this purpose, we selected a partially opened plate-cavity system which has two different modally reacting boundary conditions: a plate and a hole. In order to understand the physical coupling phenomena of the selected system, visualization of the sound fields was performed experimentally. We used near field acoustic holography to estimate sound field variables, such as pressures and intensities. Examining the acoustic variables, we found that there are two types of coupling mechanisms depending on frequency and associated wavelength. One is where the plate and the cavity are so strongly coupled that the plate can be considered as a source. In this case, the system radiates acoustic energy effectively through the plate. The other is where the coupling interaction behavior decreases the radiation efficiency. The frequencies that determine whether the plate is a good or bad radiator are found to be around the natural frequencies of the plate.