Index of content:
Volume 118, Issue 2, August 2005
- GENERAL LINEAR ACOUSTICS 
118(2005); http://dx.doi.org/10.1121/1.1954587View Description Hide Description
Calculation of acoustic axes in triclinic elasticanisotropy is considerably more complicated than for anisotropy of higher symmetry. While one polynomial equation of the 6th order is solved in monoclinic anisotropy, we have to solve two coupled polynomial equations of the 6th order in two variables in triclinic anisotropy. Furthermore, some solutions of the equations are spurious and must be discarded. In this way we obtain 16 isolated acoustic axes, which can run in real or complex directions. The real/complex acoustic axes describe the propagation of homogeneous/inhomogeneous plane waves and are associated with a linear/elliptical polarization of waves in their vicinity. The most frequent number of real acoustic axes is 8 for strong triclinic anisotropy and 4 to 6 for weak triclinic anisotropy. Examples of anisotropy with no or 16 real acoustic axes are presented.
Parametrization of acoustic boundary absorption and dispersion properties in time-domain source/receiver reflection measurement118(2005); http://dx.doi.org/10.1121/1.1954567View Description Hide Description
Closed-form analytic time-domain expressions are obtained for the acoustic pressure associated with the reflection of a monopole point-source excited impulsive acoustic wave by a planar boundary with absorptive and dispersive properties. The acoustic properties of the boundary are modeled as a local admittance transfer function between the normal component of the particle velocity and the acoustic pressure. The transfer function is to meet the conditions for linear, time-invariant, causal, passive behavior. A parametrization of the admittance function is put forward that has the property of showing up explicitly, and in a relatively simple manner, in the expression for the reflected acoustic pressure. The partial fraction representation of the complex frequency domain admittance is shown to have such a property. The result opens the possibility of constructing inversion algorithms that enable the extraction of the relevant parameters from the measured time traces of the acoustic pressure at different offsets, parallel as well as normal to the boundary, between source and receiver. Illustrative theoretical numerical examples are presented.
118(2005); http://dx.doi.org/10.1121/1.1953247View Description Hide Description
This paper concerns a time-domain model of transient wave propagation in double-layered porous materials. An analytical derivation of reflection and transmission scattering operators is given in the time domain. These scattering kernels are the medium’s responses to an incident acoustic pulse. The expressions obtained take into account the multiple reflections occurring at the interfaces of the double-layered material. The double-layered porous media consist of two slabs of homogeneous isotropic porous materials with a rigid frame. Each porous slab is described by a temporal equivalent fluid model, in which the acoustic wave propagates only in the fluid saturating the material. In this model, the inertial effects are described by the tortuosity; the viscous and thermal losses of the medium are described by two susceptibility kernels which depend on the viscous and thermal characteristic lengths. Experimental and numerical results are given for waves transmitted and reflected by double-layered porous media formed by air-saturated plastic foam samples.
118(2005); http://dx.doi.org/10.1121/1.1945470View Description Hide Description
The method of superposition may be applied to reconstruct the field on a partial surface on a radiating structure from measurements made on a nearby limited surface. Unlike conformal near-field holography, where the measurementsurface surrounds the entire structure, in patch holography the measurementsurface need only be approximately as large as the patch on the structuresurface where the reconstruction is required. Using the method of superposition, the field on and near the measurementsurface may be approximated by the field produced by a source distribution placed on a surface inside the structure. The source strengths are evaluated by applying boundary conditions on the measurementsurface. The algorithm requires the inversion of the Green’s function matrix which may be ill-conditioned. Truncated singular value decomposition is used to invert it. The field on the structuresurface is then approximated by the field produced by the source distribution. The algorithm is easier to implement than the boundary elements method because it does not require integrations over singular integrands and may be applied to flat or curved surfaces.
118(2005); http://dx.doi.org/10.1121/1.1940427View Description Hide Description
The gradient vector (e.g., of the acoustic pressure) indicates the direction to the source of a wave, but it is easily corrupted by interference from other directions. However the gradient concept, even for higher orders, can be applied rigorously to a beamforming aperture that shields against interference, thereby allowing precise determination of the direction of sound echoes or emissions, especially for very brief, broadband transient sounds. In this treatment there is no gradient sensorper se; the aperture weighting supplants that function. Various geometric shapes can be used as apertures, but simple plates are often best, and the required weightings can be realized by patterned electrodes. The method is shown to be a natural extension of earlier techniques and inventions, and useful interpretations and generalizations are provided, such as compound and steered apertures, instantantly re-steerable nulls, and an equivalence to tracking acoustic particle motion after acoustical shielding from interference. There are two stages: aperture signal extraction, and ratio processing based upon Watson–Watt concepts, for which statistically based formulas are useful. In-water test results are provided.