摘要:

The scattering of a symmetric pair of plane waves by a crack in an isotropic, homogeneous, linearly elastic solid is considered. For high frequency, an asymptotic solution to the integral equation for the normal crack‐opening displacement (COD) is found. The elliptical and penny‐shaped cracks are considered in some detail. For the penny‐shaped crack, excellent agreement with numerical results is found as well as agreement with other asymptotic results.

ISSN:0001-4966    

DOI:10.1121/1.399430

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The farfield backscattered form function is examined for the problem of plane acoustic pressure wave scattering by an empty elastic circular cylindrical shell. The analysis of the exactly computed total form function has been carried out. The approach of finding the contribution introduced by two peripheral Lamb‐type waves—the zero‐order symmetricS0and the zero‐order antisymmetricA0—is outlined. An approximate description of these contributions is used to synthesize the backscattered form function. The presentation is developed using the case of an Armco iron shell with moderate thickness, immersed in water.

ISSN:0001-4966    

DOI:10.1121/1.399431

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Previous work on the active control of sound has mostly used frequency domain formulations in order to establish the physical limitations of active methods. While entirely adequate for the prediction of the performance of active control systems designed to deal with deterministic primary fields, these methods cannot necessarily be applied in cases where the primary excitation is stationary random in nature. The application of frequency domain techniques often yields results for the optimal control strategy that require the secondary sources to act noncausally with respect to the primary sources. The work described here illustrates classical time domain methods for determining the performance limits of active noise control systems that are constrained to act causally. The first example considered is the minimization of the mean‐squared acoustic pressure at a position in the field of a point monopole primary source by the introduction of a point monopole secondary source. The primary source radiates a stationary random signal and the secondary source is constrained to act causally with respect to the primary source. The active control of low‐frequency random sound in enclosures is then addressed and the classical Wiener theory extended in order to deal with problems involving the minimization of multiple errors. The active control of a one‐dimensional enclosed sound field is presented as a simple example. This theory is also used in presenting a third example that consists of a primary/secondary source pair radiating in a free field. The minimum acoustic power output of the source combination is calculated when the primary source radiates random sound and the secondary source is again constrained to act causally with respect to the primary source.

ISSN:0001-4966    

DOI:10.1121/1.399432

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

From the pioneering works of Gericke [O. R. Gericke, J. Acoust. Soc. Am.35, 364–368 (1963)] and some other researchers [L. Adler, K. V. Cook, and W. A. Simpson, ‘‘Ultrasonic frequency analysis,’’ inResearchTechniquesinNondestructiveTesting, edited by R. S. Sharpe (Academic, London, 1977), Vol. 3; A. F. Brown, ‘‘Ultrasonic spectroscopy,’’ inUltrasonicTesting, edited by J. Szilard (Wiley, New York, 1982)], frequency spectra of ultrasonic returns form hidden flaws carry a rich amount of information usable for flaw characterization. With a proper modeling of these ultrasonic echoes, the effectiveness of such frequency analyses can be further enhanced by a process called deconvolution or inverse filtering. In this paper, the performances of several deconvolution algorithms when applied to ultrasonic pulse echoes from artificial flaws embedded in some aluminum blocks are investigated. The relative computational complexities of these algorithms are also analyzed and compared. Empirical results shall justify the applications of these algorithms for flaw characterization. Furthermore, on comparing the experimental results, simpler implementations and higher efficiencies should favor the use of the spectrum‐based deconvolution techniques over time‐domain techniques.

ISSN:0001-4966    

DOI:10.1121/1.399433

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Wave scattering by inhomogeneous scatterers in a waveguide is studied using a new method that combines the hybrid ray‐mode, boundary element, and finite element methods systematically in a single framework. This is a generalization of the previous work [I. T. Lu, ‘‘Analysis of acoustic wave scattering by scatterers in layered media using the hybrid ray‐mode‐(boundary integral equation) method,’’ J. Acoust. Soc. Am.86, 1136‐1141 (1989)]. The boundary element method is employed to model the interaction among scatterers and the coupling between interior and exterior of scatterers, the finite element method to formulate the interior responses of the scatterers, and the ray‐mode method to provide Green’s function of the waveguide. This hybrid combination optimizes the advantages of each method and hence, provides physical insights and numerical efficiency and accuracy. To further improve the computational efficiency, the Jacobi’s iteration solution is employed.

ISSN:0001-4966    

DOI:10.1121/1.399434

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

A general expression for the acoustic radiation force on a small compressible sphere has been derived and expressed in terms of the time‐averaged densitiesTiandViof kinetic and potential energies, respectively, in the incident sound field. The results have been applied to two focused beams: Gaussian beams and piston beams. For both cases, the analytical expressions of the radiation forces on a small compressible sphere placed on the axis of the beams are calculated. The pertinent applications in acoustic levitation and bubble dynamics are discussed.

ISSN:0001-4966    

DOI:10.1121/1.399435

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Finite‐amplitude pulses are examined acousto‐optically using a newly developed light‐diffraction apparatus. Based on an optical analysis of ultrasonic transducer response to continuous‐wave excitation at and near the fundamental frequency, pulse Fourier spectra are derived for input to a light‐diffraction model, providing quantitative agreement between experiment and theory. The diffraction theory predicts that a light‐diffraction pattern produced by a harmonically distorted acoustic pulse train will exhibit asymmetry in the intensity distribution with respect to the zero order. To simulate harmonic distortion, pulse frequency spectra are used for input to a computational model that is based on the Burgers’ equation for propagation of finite‐amplitude acoustic waves in a nonlinear medium. The spectrum, propagation, and light‐diffraction models give a complete description of light diffraction by finite‐amplitude pulses and provide good agreement with experimentally obtained diffraction patterns.

ISSN:0001-4966    

DOI:10.1121/1.398826

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

Using a modified Biot theory, employing open pore boundary conditions at the interfaces, and by seeking plane‐wave solutions, dispersion equations are derived for a rigid porous half‐space, for a poroelastic half‐space, and for a layered poroelastic half‐space. That for the rigid porous boundary has a single solution. Numerical search shows that, for parameter values characteristic of a dry soil, the dispersion equation for the interface between air and an air‐filled poroelastic half‐space has three possible solutions corresponding, respectively, to that for the rigid porous case, an air‐coupled pseudo‐Rayleigh wave, and a new fast surface wave with a speed slightly less than the bulkP‐wave speed. The sensitivities of these three surface waves to porosity and elastic parameters are investigated. Of the solutions to the dispersion equation for a system consisting of a single poroelastic layer above a poroelastic half‐space, with parameters typical of a dry soil, one is identifiable as an air‐coupled pseudo‐Rayleigh wave with a phase velocity near to the speed of sound in air. Slower and faster surface waves are predicted also on the layered system when the input parameters for the upper layer are relevant to either a soil or thick snow.

ISSN:0001-4966    

DOI:10.1121/1.398827

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

The presence of wind and temperature gradients causes sound to follow curved ray paths, changing the relative delay between direct and reflected components. The way this changes the excess attenuation of propagating acoustic impulses and their waveforms is considered by using ray‐bending theory, including intensities, for an atmosphere supporting either a linear or a nonlinear sound‐speed gradient. These predictions are contrasted with those from creeping‐wave theory, assuming a linear gradient, and both are compared with experimental results for impulses propagating close to the ground. Although the measured gradients are nonlinear, impulse behavior is found to be best predicted by assuming an effective linear gradient. A technique involving impulse rise times is used to determine shadow‐boundary distances and hence the profile of the boundary.

ISSN:0001-4966    

DOI:10.1121/1.398828

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP

摘要:

This paper addresses the problem of passive acoustic tracking from closest point of approach (CPA) time, amplitude, and frequency data at spatially distributed sensors in the case of finite sound‐propagation delay. A novel method is presented for the computation of source velocity (speed and direction) from the CPA times at three noncollinear sensors that lie on the same side of the source path. The method reduces the problem to the solution of a linear trigonometric equation. The method is used as the basis for determining the qualitative location of a straight source path with respect to a square grid of calibrated sensors, assuming that the amplitude of the sensor signal at CPA varies monotonically with the distance between source and sensor. In the case of a maneuvering source, qualitative relations are stated between the local shape of the source path, its relative position with respect to an acoustic sensor, and whether amplitude and frequency increase or decrease monotonically with (source) time at that sensor. These qualitative relations from multiple sensors are then used to segment the source path into straight and maneuvering segments (under negligible propagation delay). Experimental results with real data are presented for the source‐velocity computation, together with a sensitivity analysis.

ISSN:0001-4966    

DOI:10.1121/1.399533

出版商:Acoustical Society of America    

年代:1990

数据来源: AIP