摘要:

The path integral technique is applied to investigate the effects of internal waves and temperature fine structure on multipath propagation in the ocean. The novel feature of this study is the use of ray tracing to evaluate the stationary paths between source and receiver. An expression for the correlation of the acoustic pressure propagated along disjoint paths is derived which describes both deterministic and random effects. Numerical results are obtained for the interference of two rays in the presence of a bilinear sound‐speed profile. These results clearly demonstrate that internal waves rather than oceanic fine structure govern the decorrelation of acoustic energy propagated along the two paths.

ISSN:0001-4966    

DOI:10.1121/1.394949

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

In this paper acoustic scattering from a viscoelastic sphere accurately modeled by means of the Kelvin–Voigt model is studied. The approach based on asingleimpedance‐type or Cauchy boundary condition used in the past to account for losses in the body, is reconciled with the exact approach of viscoelasticity, which accounts for material losses via complex field equations, complex propagation vectors, and a set ofthreerealistic boundary conditions on the surface of the sphere. Using the exact approach of viscoelasticity theory, the effect of viscoelastic losses on the various quantities of interest is determined. The Resonance Scattering Theory (RST) is ideally suited to isolate features of the acoustic spectrum which are dependent upon material composition. In order to use the RST for the combination of a viscoelastic object in a liquid medium, an impedance‐matched background is required, and it is developed here for the first time. Subtraction of this background successfully isolates the resonances in the present case. Finally, an exact expression for the specific surface impedance of the sphere, which depends in a complicated way on frequency, on mode order, and on the four parameters controlling the viscoelastic properties of the sphere is derived. The effect of all these quantities on the sonar cross section of the sphere, or on the modal contributions contained within it is studied, and many pertinent results are displayed.

ISSN:0001-4966    

DOI:10.1121/1.394950

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The eigenfrequencies at which smooth convex objects resonate under the incidence of an acoustic wave correspond to the real parts of those complex frequency values at which circumferential waves generated by the incident signal phase‐match after repeated circumnavigations around the object [H. Überall, L. R. Dragonette, and L. Flax, J. Acoust. Soc. Am.61, 711 (1977)]. A resonance condition based on this principle is formulated, and applied to the case of elastic prolate spheroids and cylinders with hemispherical endcaps. Using then the known phase velocities of surface waves on elastic spheres, with a radius equal to the local radius of curvature along the surface path, the elastic resonance frequencies of these objects can be predicted. This was done for the Rayleigh wave on a prolate spheroid, where comparison with resonances in the scattering amplitude as obtained by aT‐matrix calculation led to good agreement.

ISSN:0001-4966    

DOI:10.1121/1.394951

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The generation of sound by a moving laser source is investigated both theoretically and experimentally. The analysis is restricted to sound waves generated exclusively through the thermal mechanism. The model is based on the impulse response of a thermoacoustic source as described in a previous paper [J. Acoust. Soc. Am.78, 2074–2082 (1985)]. The results presented here include pressure waveforms, directivity patterns, sound level dependence on source velocity, and spreading curves for subsonic, transonic, and supersonic source velocities. In general, experimental results are in good agreement with theoretical predictions.

ISSN:0001-4966    

DOI:10.1121/1.394952

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The intensity fields of disk, bowl, and conical transducers have been evaluated theoretically throughout their nearfield regions at a single frequency. The results have been compared to the fields resulting from the square of the pressure (P2). It is shown that the two types of fields differ most markedly on axis and close to the transducer face. The direction of energy flow has also been analyzed, and it is demonstrated that parallel flow lines occur in regions whereP2is the closest to intensity.

ISSN:0001-4966    

DOI:10.1121/1.394953

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

This paper is concerned with a method for solving response problems of an elastic solid of arbitrary shape with a circular cylindrical cavity subjected to a transient pressure along the cavity. In the analysis, the exact solutions which satisfy the boundary conditions along the cavity are obtained by applying the Laplace transformation to the equations of motion of elastic solids based on the two‐dimensional theory of elasticity. The boundary conditions along the outer surface of arbitrary shape are satisfied by means of the Fourier expansion collocation method. The Laplace transform inversion integral is obtained from the residue theorem. Reasonably accurate results are obtained when the integration for finding the coefficients of the Fourier series and the numerical differentiation are carried out appropriately. To discuss the accuracy of the present analysis, results are compared with those from an exact method for circular rings. Numerical calculations are carried out for sample cases.

ISSN:0001-4966    

DOI:10.1121/1.395134

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

This paper is concerned with a method for solving dynamic response problems of arbitrarily shaped plates in contact with a fluid. By utilizing the Fourier expansion and the Laplace transform methods for disposing of general dynamic pressures, and by applying the Fourier expansion collocation method for satisfying the boundary conditions, expressions for the dynamic response of displacement and bending moment are obtained in a general form which is applicable to general dynamic pressures and arbitrarily shaped plates. Numerical calculations have been carried out for circular plates, rectangular plates, rectangular plates with round corners, and oval plates. The results obtained in certain plates are compared with the exact ones.

ISSN:0001-4966    

DOI:10.1121/1.394900

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Consideration is given in this paper to the problem of a thick, spinning, annular disk constrained at its inner radius. Mindlin’s transverse shear deformation and rotary inertia terms are included in the eigenvalue formulation. In the modified Rayleigh–Ritz method used here, the ‘‘admissible’’ or ‘‘trial functions’’ are developed numerically by an iterative scheme so that all the forced, as well as natural, boundary conditions are fully satisfied. The natural frequencies for different mode shapes of the disk have been computed and are presented in a nondimensional form as a function of rotating speed.

ISSN:0001-4966    

DOI:10.1121/1.394901

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The sound attenuation characteristics of two military helmets were measured using an acoustic manikin as the test apparatus. The manikin results are compared to the results of attenuation measurements made on human subjects wearing identical helmets. The testing room and instrumentation were the same for both the manikin and human subjects. Procedures in ANSI S3.19‐1974 were used in the real‐ear attentuation at threshold (REAT) part of this study. The results are encouraging as they suggest that the manikin may be used in place of a panel of human subjects to evaluate the hearing protection characteristics of military head gear.

ISSN:0001-4966    

DOI:10.1121/1.394902

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Active noise attenuators have been developed for use in an industrial air duct. A theory is presented for calculating the attenuation of tight‐coupled attenuators, where the microphone and loudspeaker are in close proximity. Practical and theoretical aspects of a tandem system have been developed, utilizing a cascade of two simple monopole attenuators. A theoretical model suitable for use in the computation of the attenuation from both types of tight‐coupled attenuators is presented. The tight‐coupled tandem attenuator can provide a minimum attenuation of 20 dB for more than three and one‐half octave bands from 30 to 330 Hz. Also, the attenuators could provide significant attenuation in the presence of airflow. The attenuation patterns in front of the attenuator loudspeaker are investigated. Good agreement between the measured attenuation and the computed results validates the proposed theory and the model for the calculation of the attenuation from tight‐coupled systems.

ISSN:0001-4966    

DOI:10.1121/1.394903

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP