摘要:

A finite element method (FEM) is presented for the study of nonlinear acoustic standing waves inside a chamber. The method is developed using the Galerkin–Bubnov weighted residual formulation and applied to solve the Lagrangian second-order wave equation, including thermal and viscous dissipation terms. A one-dimensional problem is studied in the frequency domain. Numerical data are compared with analytical results obtained by using a perturbation scheme for the cases of resonance and antiresonance in a rigid-walled tube with one-sided rigid or pressure release boundary condition. The FEM algorithm is shown to be well suited for the study of high-frequency standing wave fields in which the effect of absorption cannot be described by simple analytical expressions.

ISSN:0001-4966    

DOI:10.1121/1.422750

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A general theory of the (Langevin) acoustic radiation force in three-dimensional fields in lossless fluids in Eulerian and Lagrangian coordinates is given. It is based on Brillouin’s approach of an acoustic radiation stress identified in Eulerian coordinates with the negative time average of the momentum flux density and in Lagrangian coordinates with the time average of the Piola–Kirchhoff–Boussinesq stress. It is shown in comparison to other statements from the literature that these stresses are not in general identical but that in the linear-acoustics approximation, the difference between the results in the two representations vanishes if the radiation force acting on an object entirely surrounded by the sound-propagating fluid is considered. It is just this situation which is of experimental relevance.

ISSN:0001-4966    

DOI:10.1121/1.422751

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

It is demonstrated that seismic interface waves on the surface of a natural beach can be used to identify the position of a buried object. For this experiment, the waves were created with a sediment-coupling transducer and received on a three-element horizontal line array of triaxial geophones. The source and its coupling to the medium provided a high degree of signal repeatability, which was useful in improving signal-to-noise ratio. Reception of all three directions of particle velocity made it possible to augment conventional beamforming techniques with polarization filters to enhance interface-wave components. Reverberation in the beach was found to be large, though, and coherent background subtraction was required to isolate the component of the sound field reflected by the target. Propagation loss measurements provided comparisons of reflected signal power with predictions made previously, and the two were found to agree closely.

ISSN:0001-4966    

DOI:10.1121/1.422752

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A method is presented for efficiently computing or estimating low-order elastic wave scattering, from a pointlike inhomogeneity in a bounded medium, into interface as well as bulk modes. The particular system considered is a half-space of fluid overlying a half-space of a linear elastic solid, with the obstacle placed in the solid very near the interface. Boundary constraints are enforced as an improvement to the bulk scattering vertex, so that the stratified-medium Green’s function is used to propagate scattered waves at each perturbative order. Analytic approximation of the first-order scattering amplitude, based on the symmetries of the scattering vertex and the interface-wave pole structure, makes it easy to identify the qualitatively different components of the scattered wave, and their dependence on medium properties.

ISSN:0001-4966    

DOI:10.1121/1.422753

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Boundary value integral equations and the moment method solution technique provide a succinct and elegant strategy for the solution of acoustic surface scattering problems. This paper explores the application of a subdomain basis set, based on the phase of the incident radiation, which is used to solve for the unknown normal pressure gradient on the surface of the scatterer using the moment method. The advantage of this basis set lies in the fact that it can accurately represent the fast phase character of the pressure gradient on the surface of the scatterer. This allows for far fewer discretizations/basis elements in the moment method formulation and therefore significantly reduces the density of unknowns that must be numerically computed. The main contribution of this paper is to show how to efficiently employ this natural basis and to explore the physical justification for using this approach. The method is compared with other techniques including the Kirchhoff solution technique, high frequency single scattering techniques, the Nussenzveig–Fock technique, and the parabolic equation approximation. Unlike these methods no approximation to the physical system need be introduced.

ISSN:0001-4966    

DOI:10.1121/1.422754

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Acoustic radiation produced by end-fired explosive line charges is examined experimentally and theoretically. Three explosive compositions are studied: PETN detonating cord, RDX detonating cord, and thin ribbons of HLX. Specific output levels are formulated for each of the three explosive materials over the four 1-oct bands spanning 50–800 Hz. Bubble period relationships are derived for each material and compared to previous studies. In terms of directivity effects, long line charges are found to behave like beam-steered continuous arrays over moderately wide bands. A math model is developed which predicts acoustic levels and spectrum shapes for the shock wave component over a wide range of view angles and linear charge densities. This model is driven by two parameters which are extracted from the experimental data for the three compositions studied. The result is a practical means of designing explosive line arrays to desired source levels, bandwidths, and beam patterns.

ISSN:0001-4966    

DOI:10.1121/1.422755

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Nonlinear scattering due to bubble oscillations was theoretically and experimentally investigated for bubble size distribution measurements in water. If two primary acoustic waves of different frequencies are incident on a bubble, the difference frequency component of primary waves is produced in the scattered field. The incoherent scattering at the difference frequency from bubbles was theoretically studied. It was shown that the difference frequency amplitude is proportional to the density of bubbles having resonance frequencies close to the difference frequency. It allows the use of difference frequency measurements to determine the bubble density in water. For experimental investigations two focused transducers were used to increase the levels of primary waves in the interaction zone. One of the primary frequencies was kept constant 2.25 MHz while another was changed from 2.22 to 1.93 MHz, so that the observed difference frequency was varied from 30 to 320 kHz. This frequency band corresponds to the resonant bubble radii from 109 to 10 μm. For the bubble clouds produced in a laboratory tank by an electrolysis-type and a slit-type bubble maker the bubble densities were well estimated with the present nonlinear acoustic method.

ISSN:0001-4966    

DOI:10.1121/1.422756

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The pressure dependence of the resonance frequency of several resonant ultrasound spectroscopy modes in a sample of fused silica has been measured at UCLA in atmospheres of air, helium, and argon near ambient temperature. For both compressional and torsional modes, the radiation resistance is linearly dependent upon pressure and increases with the molecular mass of the surrounding gas. The effects are larger for breathing modes than for torsional modes. They also increase with the molecular mass of the gas. A radiation impedance model is presented which explains some of these data qualitatively and quantitatively.

ISSN:0001-4966    

DOI:10.1121/1.422757

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

In many cases, it is convenient to have approximate relations to evaluate thermoacoustic gain and efficiency. G. W. Swift [J. Acoust. Soc. Am.84, 1145–1180 (1988)] has provided such relations for short stacks in a standing wave. This paper describes an approximation method based on a finite difference form of the thermoacoustic equations and provides results for thermoacoustic engines or refrigerators for both standing and traveling waves. The short stack assumption is retained. The method is limited at present to linear temperature gradients across the stack. The calculations are compared to measurements and in the appropriate limits to Rott’s boundary conditions for a temperature discontinuity [N. Rott, Adv. Appl. Mech.20, 135–175 (1980)]. The approximation is applied to both thermoacoustic prime movers and thermoacoustic refrigerators.

ISSN:0001-4966    

DOI:10.1121/1.422758

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The time-reversal process is applied to focus pulsed ultrasonic waves through the human skull bone. The aim here is to treat brain tumors, which are difficult to reach with classical surgery means. Such a surgical application requires precise control of the size and location of the therapeutic focal beam. The severe ultrasonic attenuation in the skull reduces the efficiency of the time reversal process. Nevertheless, an improvement of the time reversal process in absorbing media has been investigated and applied to the focusing through the skull [J.-L. Thomas and M. Fink, IEEE Trans. Ultrason. Ferroelectr. Freq. Control43, 1122–1129 (1996)]. Here an extension of this technique is presented in order to focus on a set of points surrounding an initial artificial source implanted in the tissue volume to treat. From the knowledge of the Green’s function matched to this initial source location a new Green’s function matched to various points of interest is deduced in order to treat the whole volume. In a homogeneous medium, conventional steering consists of tilting the wave front focused on the acoustical source. In a heterogeneous medium, this process is only valid for small angles or when aberrations are located in a layer close to the array. It is shown here how to extend this method to aberrating and absorbing layers, like the skull bone, located at any distance from the array of transducers.

ISSN:0001-4966    

DOI:10.1121/1.422759

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP