摘要:

Field patterns of short‐pulse ultrasonic transducers are quite different from those of continuous‐ or quasicontinuous‐wave sources and depend critically on the actual waveform emitted. The observed patterns, including those provided by using a small target to probe the field in the transmit–receive mode, are also dependent on the method of detection used—whether full‐ or half‐wave. The structure of the field and the marked difference between the pulsed pressure beam pattern and the corresponding transmit–receive mode beam pattern are explained in terms of the plane and edge waves radiated by the source. The field radiated by a transducer is compared with measurements of the field structure of a circular aperture insonified by a plane wave. Consequences of the pulsed beam structure for ultrasonic pulse‐echo evaluation are pointed out.

ISSN:0001-4966    

DOI:10.1121/1.391411

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A theoretical investigation of the farfield secondary radiation present in sediment due to a parametric array incident on a water–sediment boundary is conducted, based on the assumption that the interface completely truncates the parametric array in the nearfield of its primary radiation. The field is found to be due to two apertures, one coincident with the transducer, the second with the truncation. At subcritical angles of primary beam incidence the field is similar to that produced by a conventional beam. At postcritical angles of primary beam incidence the presence of the truncation aperture results in a steeper and deeper penetration of the sediment than in the conventional case. It is concluded that the presence of virtual sources near the interface is responsible for the postcritical penetration of sediment by a parametric beam.

ISSN:0001-4966    

DOI:10.1121/1.391412

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This paper initiates the derivation of a general analytical model for nonlinear effects in sound beams driven at high source pressure levels. The excitation is generated by a planar transducer that is in harmonic motion in an arbitrary axisymmetric pattern. The analysis develops a perturbation solution of a nonlinear equation for the velocity potential. The first‐order term, which is derived with the aid of a Hankel transform to represent the transverse dependence, is the King integral for a linear sound beam. Using this integral to form the source terms exciting the second‐order potential leads to a dual Hankel transform. Reduction to a single integral is achieved with the aid of an asymptotic integration following Laplace’s method. The second‐order term that is derived in this manner describes the tendency for the second harmonic to grow with increasing distance from the source. This result is an intermediate step in the overall development, because the integrand loses validity in the spectrum of transverse wavenumbers near the transition between evanescent and propagating wavelets, as well as for increasing distance from the transducer.

ISSN:0001-4966    

DOI:10.1121/1.391413

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The first part of this investigation [J. Acoust. Soc. Am.76, 1201–1207 (1984)] derived a perturbation representation of the velocity potential for an axisymmetric, monochromatic sound beam in the form of a complex function. That result, which used Hankel transforms to describe the dependence on transverse position, lacked uniform validity in the corresponding wavenumber, as well as the distance from the transducer. Expressions for the state variables derived from the potential have the same behavior. The example of a planar wave is used to adapt the singular perturbation method of renormalization to the case where the potential is in complex form. The resulting technique is used to obtain a coordinate straining transformation that makes the state variables uniformly accurate. The expression for the pressure is similar to the King integral in linear theory, except that the integrand is a function of the strained coordinates. Comparison of the predicted waveform properties with experimental data [Gallego‐Juarez and Gaete‐Garreton, J. Acoust. Soc. Am.73, 761–765 (1983)] shows good correlation. Further evaluations disclose some new features of the distortion phenomena in both the time and frequency domains.

ISSN:0001-4966    

DOI:10.1121/1.391414

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Exterior boundary‐value problems are considered for the class of cylinders defined byr=r(ϑ), with 1/rexpressible as a finite Fourier series, so thatr(ϑ) is meromorphic in the complex ϑ plane. Upon transforming the extended boundary condition integrals into the complex plane, simple and apparently exact asymptotic behavior is determined for the surface field expansion coefficientsbm; e.g., for a charged cylinder with no applied field one hasbm∼2∑exp(imϑp), form≫1, where the sum is over poles ofr(ϑ) in the upper half‐plane. These results are shown to have significant impact on convergence for solutions to the infinite systems of equations involved. We consider potential theory for Dirichlet and Neumann boundary conditions, and acoustic scattering in the Rayleigh limit.

ISSN:0001-4966    

DOI:10.1121/1.391415

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Comparisons are presented for acoustical scattering from fluid‐loaded prolate spheroidal sound soft scatterers and elastic shells of the same shape, ranging in thickness from 0.1% to 2.5% of the semi‐major axis. TheKaregion being studied varies from 3 to 12, whereKis the wavenumber of the incident wave andais one‐half the length of the major axis. Exact theoretical calculations are performed using a modifiedT‐matrix approach which for the first time allows one to perform calculations on very thin nonspherical elastic shells. The calculations show that, with the exception of the very thin shell, one does not observe a sound soft background. Furthermore, resonances for these objects are very broad and tend to dominate the form function plots. Of particular interest is the fact that by the time the thickness reaches 2.5%, the background begins to appear rigid for suitably highKawith a shift in resonance nulls, which is related to the fact that rigid scattering is out of phase by 180° with sound soft scattering

ISSN:0001-4966    

DOI:10.1121/1.391416

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This work is concerned primarily with the acoustic structure and propagation of sound in highly porous, layered, fine fiber materials. Of particular interest is the utilization of the Kozeny number for determining the static flow resistance and the static structure factor based on flow permeability measurements. In this formulation the Kozeny number is a numerical constant independent of volume porosity at high porosities. The other essential parameters are then evaluated employing techniques developed earlier for open‐cell foams. [J. Acoust. Soc. Am.72, 879–887 (1982)]. The attenuation and progressive phase characteristics in bulk samples were measured and compared with predicted values. The agreements on the whole were very satisfactory.

ISSN:0001-4966    

DOI:10.1121/1.391417

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The integral relations equivalent to Helmholtz’s equation with closed‐boundary conditions possess nonunique exterior solutions at eigenwavenumbers of the corresponding interior problems. If the Green’s function for Helmholtz’s equation can be expanded into a series of orthogonal functions, then an infinite set of integral equations at the boundary results. These ‘‘null‐field’’ equations are shown to possess unique solutions for all wavenumbers. Furthermore, these solutions can be used to represent the solutions in the exterior domain as algebraic series.

ISSN:0001-4966    

DOI:10.1121/1.391418

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Nyborg’s [J. Acoust. Soc. Am.70, 310–312 (1981)] derivation of heat generation density has been examined. An alternative approach has been developed to yield the same expression by assuming a complex wavenumber with the linear acoustic equations.

ISSN:0001-4966    

DOI:10.1121/1.391374

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Two experiments are presented which examine whether the voice bar in [b] and [d]contains information relevant to the identity of the stop consonant and the following vowel. The results indicate that listeners can exploit consonantal place‐of‐articulation information contained in the voice bars. The ability of the subjects to ascertain the identity of the vowel following the voice bar was extremely limited, even after a training period; the confusions again point to the influence of consonantal place‐of‐articulation information.

ISSN:0001-4966    

DOI:10.1121/1.391375

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP