摘要:

A formalism is presented that describes the response of a complex of coupled one‐dimensional dynamic systems to an impulse drive. The formalism is based on an impulse response operator that relates a drive applied to one point in the complex to the response at any point in the complex. The formalism is derived directly in the time domain and the impulsive drives which can be accommodated must be finite in time and applied at a spatial point. The constituent systems must be one‐dimensional and possess a pulse propagation velocity that is not a function of position within the system. Systems interact through junctions that also define their spatial extents. The junctions are characterized by reflection and transmission coefficients that modulate the amplitude of reverberant components and by delays in the reflections and transmissions. Propagation in the systems is characterized by losses. Several simplistic examples are calculated and presented to illustrate the type of information that the formalism can provide.

ISSN:0001-4966    

DOI:10.1121/1.400501

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

In this paper a practical method is proposed to calculate the acoustical response of a rigid sphere. This method combines the physical optic method, which gives good results at high frequencies, with the modal method, which is suitable at low frequencies. This gets around the problems that arise when only one of these methods is used, namely the failure of the physical optic method at low frequencies and the convergence difficulties of the modal method at high frequencies. Here, the impulse response (response to a Dirac pressure transmission) of a rigid sphere for a backscattering situation is calculated.

ISSN:0001-4966    

DOI:10.1121/1.400516

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Explicit expressions for the acoustic pressure and velocity scattered by two‐dimensionally rough surfaces are derived using a full‐wave approach. The conditions under which these solutions merge with the physical acoustic and small perturbation solutions in the high‐ and low‐frequency limits are given. The acoustic media on both sides of the rough interface are characterized by their bulk modulus, equilibrium density, and relaxation time (to account for dissipation); the Dirichlet and Neumann boundary conditions are treated as special cases. The closed‐form full‐wave expressions for the surface element scattering coefficients are significantly different for these special cases. However, the corresponding physical optics solutions differ only in the sign of the acoustic reflection coefficient. The full‐wave solution can be applied to composite surfaces with a broad range of roughness scales. Since it accounts for specular point and diffuse scattering in a unified self‐consistent manner, there is no need to adopt a two‐scale model of the rough surface. Thus the full‐wave expressions for the rough surface scattered fields are also more suitable for application to broadband (transient) excitation problems and for the solution of inverse problems.

ISSN:0001-4966    

DOI:10.1121/1.400500

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

An infinite periodic system of groups of equidistant metal strips deposited on a piezoelectric half‐space is considered. Such a system may be useful for analyzing a single group of strips if the period of the groups is sufficiently large. A dispersion relation for generally slanted surface acoustic wave (SAW) propagation with respect to electrodes is derived and analyzed. Numerical examples are given for a wave propagating along, and perpendicular to the electrodes. A functional relationship for strip currents dependent on strip potentials has been derived. This can be applied for analyzing interdigital transducers (IDT) having a finite number of periodic fingers. Bragg’s reflection of SAW from metal strips is also analyzed.

ISSN:0001-4966    

DOI:10.1121/1.400510

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A wave‐vector–time‐domain method that was used to obtain the transient acoustic radiation loading on vibrating finite cylinders is extended to include the effects of the presence of inviscid axial flow. The method is based on the use of theinvacuomodes of vibration of the shell which have infinite rigid extensions. The solution to the boundary value problem for the pressure is first obtained in wave‐vector–time space. The time‐dependent modal forces are then expressed as a sum of the modal radiation impulse responses of the finite cylinder convolved with the time‐dependent modal velocities. The modal radiation impulse responses are obtained by using the impulse responses of an infinite cylinder and the modes in wave‐vector space. The approach can thus be easily used to obtain the radiation impulse responses for various boundary conditions, length to radius ratios, and arbitrary space‐ and time‐dependent surface velocities. Numerical results are presented for a wide range of Mach numbers and for length to radius ratios of simply supported cylindrical shells.

ISSN:0001-4966    

DOI:10.1121/1.400474

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A numerical technique for simulating the behavior of an acoustic wave propagating through a turbulent medium is introduced. The technique involves two elements: the generation of 3‐D, random, hypothetical, isotropic velocity fields in terms of a collection of discrete Fourier velocity modes; and the integration of the ray‐trace equations to describe the trajectories of points tagging an acoustic wave front. The propagation times for these points to travel fixed distances through each of an ensemble of random velocity fields are recorded, and the variance of travel time (or acoustic phase) over the ensemble is calculated. In numerical ray‐trace experiments through fields having average perturbation indices ≊0.01, acoustic travel‐time variances are obtained that have a higher‐order dependence on travel distanceRthan the classical Chernov prediction—a linear increase withR. The Chernov result is obtained, however, when the rays are confined to axial trajectories. Additional numerical experiments integrating the stochastic Helmholtz equation and its parabolic approximation yield time‐variance estimates consistent with the ray‐trace results. Predictions from these simulations are then applied to the laboratory experiments of Blanc‐Benon and found to be in qualitative agreement. Finally, a set of 2‐D travel‐time experiments are presented to identify differences between source–receiver eigenray propagation and preassigned initial direction ray propagation.

ISSN:0001-4966    

DOI:10.1121/1.400415

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Gaussians are useful models for high‐frequency source field inputs into complex environments because they approximate the outputs of certain transducers, have favorable spectral and filtering properties, and can be propagated similar to ray fields. By recent analytic developments, any source field can be expressed exactly as a self‐consistent superposition of Gaussians on a discretized (configuration)–(wave number) phase space lattice. This extends the use of Gaussians systematically to realistic transducer outputs. The method is already being applied to electromagnetic and acoustic propagation. It is here extended to modeling the radiation from transducers into an elastic solid. Restricting to the two‐dimensional case, a distribution of forces over a finite, one‐dimensional planar aperture is expanded self‐consistently into Gaussian basis elements, which are then propagated into the unbounded medium. Numerical results reveal how successive addition of Gaussians for the compressional and shear potentials, as well as the displacements, homes in systematically on the assumed aperture profile, and on an independently generated numerical reference solution for the radiated near and far fields. Moreover, it is demonstrated how different self‐consistent choices of beams affect the convergence. Furthermore, the validity of complex‐source‐point modeling of the Gaussians is explored for later applications where the input will be required to propagate across interfaces, as in a layered medium.

ISSN:0001-4966    

DOI:10.1121/1.400369

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A thermodynamic method capable of determining theB/Avalue of 4‐ml sample volumes is described. The method involves a procedure in which the static pressure of the sample is altered in a short period of time, to approximate an adiabatic process, during which the ultrasonic velocity is measured. The velocity change so determined is used to calculate theB/Avalue. TheB/Ameasurement error is less than 0.7%.

ISSN:0001-4966    

DOI:10.1121/1.400370

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The influence of structural factors of biological media on the acoustic nonlinearity parameterB/Ahave been studied at the tissue, cellular, and molecular levels, using the thermodynamic and finite amplitude methods.B/Awas determined as the structural factors of the media were altered physically and biochemically, while chemical composition was maintained unchanged. Significant structural dependencies ofB/Awere observed at all three levels; 26% of the dry weight contribution to the totalB/A(theB/Avalue with water contribution subtracted) is due to the cell–cell adhesive force in liver tissue, 20% is due to the hepatocyte cellular structure, and 15% is due to secondary and tertiary protein structure.

ISSN:0001-4966    

DOI:10.1121/1.400371

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Muiretal. [T. G. Muir, C. W. Horton, Sr., and L. A. Thompson, J. Sound Vib.64, 539–551 (1979)] observed that the propagation direction of the sound wave in the sediment insonified by a parametric array departed significantly from the prediction of Snell’s law. The wave fronts penetrated more steeply into the sediment and the attenuation at postcritical incidence was less than that predicted by the plane wave theory. It was found that mainly due to the variation of the length of the parametric array, the direction of the refracted sound wave occurs at or near the line‐of‐sight between the projector and the hydrophone. It has been proved, both theoretically and experimentally, that Snell’s law is still valid when the length of parametric array is kept constant. At postcritical incidence, the penetration of a parametric array can be deeper due to the contribution of the lateral wave and secondary sources close to the boundary.

ISSN:0001-4966    

DOI:10.1121/1.400372

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP