摘要:

This paper describes a new algorithm based on a frequency‐division method by which a solution to the inverse source problem of acoustic emission can be obtained. The algorithm is applicable to cases in which the acoustic emission signal can be expressed as a bilinear equation involving weighted Green’s functions or its spatial derivatives, convolved with a common source–time function. Through division of the detected signals in the frequency domain, the common source–time function is eliminated and the ratios of the weighting constants can be recovered by solving a set of linear equations. Using these recovered ratios, the source–time function can be found from the signal at any receiver point. Application of this algorithm is made to process the signals simulating the formation of a microcrack in an elastic plate modeled with a second‐order moment tensor. Dynamic information about the source is recovered by processing the signals at two receiver points. If four receivers are used, the second‐order tensor can be recovered at each frequency of the recorded signals. Additional information about the source can be extracted by diagonalizing and rotating the recovered moment tensor. The effect of noise on the inversion scheme is also considered.

ISSN:0001-4966    

DOI:10.1121/1.393710

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The application of a coupled finite element–boundary integral approach to the analysis of vibrating elastic, arbitrary‐shaped, three‐dimensional structures in an infinite media is presented. A feature of the methodologies implemented is the utilization of the isoparametric concept from finite element theory. The same shape functions are used both to model the surface of the structure and to interpolate the acoustic variables over the radiating surface. The boundary integral formulation used for the exterior acoustic field is a formulation that ensures uniqueness and existence of solutions for the entire frequency range. An assessment of the accuracy was made after comparing the solutions attained for a vibrating submerged sphere for various excitation loads and frequencies with the exact solution.

ISSN:0001-4966    

DOI:10.1121/1.393711

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Blast walls and barriers are often proposed for reducing noise from explosives or artillery fire. The calculation of blast noise reduction by three‐dimensional finite difference flow field codes is expensive and computer time intensive. For many cases, the source strength at the barrier is neither very strong nor very weak (acoustical). In this paper, a combined theoretical and experimental model study to investigate the range of validity of linear transient diffraction theory is described. Also, this paper further investigates the use of a finite wave propagation program to extend the utility of the linear calculation. This study demonstrates that the linear theory, with an accurate input waveform, can predict the insertion loss to within 1.5 dB for peak sound‐pressure levels less than 162 dB at the barrier. For the source levels greater than 162 dB, the prediction significantly underestimates the insertion loss.

ISSN:0001-4966    

DOI:10.1121/1.393659

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The quantitative determination of the shape of liquid drops levitated in an ultrasonic standing wave has provided experimental data on the radiation pressure‐induced deformations of freely suspended liquids. Within the limits of small deviations from the spherical shape and small drop diameter relative to the acoustic wavelength, an existing approximate theory yields a good agreement with experimental evidence. The data were obtained for millimeter and submillimeter drops levitated in air under 1g, wheregis the sea level gravitational acceleration.

ISSN:0001-4966    

DOI:10.1121/1.393660

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

It is the purpose of this paper to present the effect of air absorption on the intensity of sound produced by a moving point source, particularly the role of the Doppler effect in the absorption. It has been assumed that a nondirectional point source is moving along a straight line, with subsonic speed, in a homogeneous atmosphere. The investigation is based on a geometrical approach.

ISSN:0001-4966    

DOI:10.1121/1.393661

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The surface of the ground is modeled as that of an air‐filled poroelastic soil layer of known thickness overlying a semi‐infinite nonporous elastic substrate. Using a modified form of the Biot–Stoll differential equations for wave propagation in fluid‐saturated porous media, propagation constants for the two possible dilatational waves and the shear wave in the poroelastic layer are determined. The dilatational waves are identified as a fast wave, moving predominately in the solid frame, and a slow wave, moving predominately in the pore air. The elastic moduli in the substrate are assumed to be those of the solid grains of which the poroelastic soil layer is composed. Intergranular friction in the soil and substrate is assumed to be negligible. Boundary conditions at the air–soil interface and at the porous soil–substrate interface are applied to determine, numerically, the displacement amplitudes of the allowed wave motions. From the incident and reflected amplitudes at the air–soil interface, the normalized ground surface impedance is calculated as a function of angle of incidence and of frequency. In this paper, the response of the pore fluid and frame to airborne acoustic waves is considered and those ideas will be pursued in a later publication. The predicted impedance at normal incidence is compared with measurements of the impedance of a sandy soil for which measurements of the various parameters required by the theory are also available. The predictions of impedance are found to be in tolerable agreement both with measured data and with predictions of a simpler model of the surface as that of a rigid porous semi‐infinite homogeneous medium. Calculations of the surface impedance as a function of angle of incidence suggest that the porous medium is locally reacting.

ISSN:0001-4966    

DOI:10.1121/1.393662

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A model for the response of a poroelastic layered soil to an incident plane wave developed in a previous paper [Sabatieretal., J. Acoust. Soc. Am.79, 1345–1352 (1986)] is used to predict the complex sound pressure within the upper poroelastic layer. The predictions both of phase velocity and attenuation of the slow wave associated primarily with propagation in the pore fluid are compared with measurements made with a specially constructed probe microphone. The agreement between theory and experiment is good. The predictions for a layered poroelastic soil model are compared numerically with those of a semi‐infinite rigid porous soil model and are found to differ only at frequencies higher than 1000 Hz. Analysis of the sensitivity to the theoretically predicted propagation constants in the poroelastic soil to the assumed value for the bulk rigidity modulus of the soil predicts that over the known range of rigidity moduli for soils it is possible to obtain a switchover between fast and slow propagation modes. This switchover occurs at the lower end of the possible range of values of the shear modulus. It is suggested that probe microphone measurements in air‐filled soils offer a way of measuring flow resistivity and of deducing the structural parameters required for application of the Biot–Stoll model to water‐saturated sediments.

ISSN:0001-4966    

DOI:10.1121/1.393663

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

This paper describes the calculation of a new value for sound speed (c0=331.29 m s−1) in standard dry air at 0 °C and at a barometric pressure of 101.325 kPa. The maximum uncertainty is estimated to be approximately 200 ppm. The theory of the calculation is based on the equation of state, and includes the knowledge of γ/Mwhich is derived from published theoretical and experimental thermodynamic data on the constituents of the standard atmosphere. Investigations which led to the general acceptance of the previous sound speed are examined, and there is good evidence to conclude that, in previous sound speed assessments, the maximum possible uncertainties were sufficient to encompass the above new sound speed. The variation of sound speed with carbon dioxide concentration and temperature is also discussed.

ISSN:0001-4966    

DOI:10.1121/1.393664

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A general formalism is presented for the study of the transient wave fields in the vicinity of caustics of arbitrary complexity. The caustics are classified using catastrophe theory. In the present study, a general introduction to the subject is presented and explicit results (numerical and analytical) for the cuspoid family of catastrophes are given. Numerical results are presented for the fold, cusp, and swallowtail catastrophes which show the smooth variation of the acoustic field as the control parameters are independently varied. These results vividly demonstrate the phenomenon of ray focusing, showing the number of rays which converge at each catastrophe, the resulting phase shifts they undergo, as well as their decay in shadow regions. Analytical results are given which describe the time‐dependent acoustic field on and at large distances away from each of the cuspoid catastrophes. Four sets of numbers are introduced which describe the behavior of the wave field in the vicinity of each catastrophe. These are: (1) an exponent which describes the temporal decay of the acoustic field on the most singular point of each catastrophe; (2) a number which describes the asymmetry of the time‐dependent acoustic field on the most singular point of each catastrophe; (3) an exponent which describes the time separation as a function of control parameter between individual ray arrivals as each of the control parameters are independently varied; and (4) an exponent which describes the asymptotic decay as a function of control parameter of each constituent ray arrival. Sets (1) and (3) are shown to be simply related to thesingularityandfringeindices previously introduced by Arnold [V. I. Arnold, Usp. Mat. Nauk.30(5), 1–75 (1975)] and Berry [M. V. Berry, J. Phys.A10, 2061–2081 (1977)], respectively. It is argued that many features of the frequency domain representation of the acoustic field in the vicinity of the catastrophes are most easily understood by examining the time domain representation.

ISSN:0001-4966    

DOI:10.1121/1.393665

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The caustics of high‐frequency wave propagation may be classified using catastrophe theory. In this paper, we examine the transient wave fields in the vicinity of the elliptic, hyperbolic, and parabolic umbilic catastrophes. Analytical results are presented which describe the time‐dependent wave field due to an impulsive source on and at large distances in each control direction away from the most singular point of each of these three catastrophes. Numerical results are presented which show the smooth variation of these transient wave fields as each control parameter is independently varied. These results are compared to previously derived results on the transient wave fields in the vicinity of the cuspoid catastrophes (the cuspoid and umbilic catastrophes have coranks 1 and 2, respectively). It is found that the transient wave fields in the vicinity of the cuspoid and umbilic catastrophes differ with regard to the temporal structure of the wave field on the most singular point of each catastrophe, the manner in which these temporal structures unfold, and the phase shifts which individual rays undergo as a result of touching one of the catastrophes.

ISSN:0001-4966    

DOI:10.1121/1.393666

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP