摘要:

A normal-mode scattering formulation that assumes a finite spatial correlation length in the distribution of scattering features is used to compute single-frequency bottom reverberation for bistatic and monostatic scattering geometries in a shallow water and deep water environment. Spatial correlation of the scattering features allows the superposition of modes scattered within each spatially correlated region and produces diffraction in the scattered field that is not predicted in the limit of a zero spatial correlation length (point scattering). For bistatic scattering geometries, the scattered field computed as a function of scattering location in the horizontal plane exhibits a pattern of diffractive maxima and minima for nonzero spatial correlation lengths. The spatial details of the diffraction pattern and its influence on the scattered energy depend on the frequency and spatial correlation length and can result in a significant reduction in the predicted levels of received reverberation. The greatest sensitivity to finite correlation effects occurs for monostatic scattering geometries because the strongest diffractive effects occur in the backscattering direction. The effects of including modal interference in the incident and scattered field propagation are also examined in this paper. The inclusion of modal interference in the propagating fields imposes an interference pattern on the spatial structure of the scattered field in the horizontal plane, and can cause the temporal dependence of the reverberated return to oscillate about the levels of return predicted when modal interference in the propagating fields is neglected. In agreement with previously published results for bottom backscattering, the effects of including modal interference in the propagating fields were found to be significant for deep water environments that exhibit convergent zone propagation and to be of limited importance for shallow water environments in which the energy incident on the bottom is characterized by a large number of multipaths. The present work includes results which show that the effects of including modal interference in the propagating fields can be important for shallow water environments that exhibit significant bottom penetration.

ISSN:0001-4966    

DOI:10.1121/1.419833

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The coupled phase approach is extended to model sound propagation in an emulsion of two compressible, thermoviscous liquids. This provides an alternative to scattering theory for modeling sound propagation in emulsions in the low-acoustic radius regime. Predictions of the new theory are compared with predictions of multiple-scattering theory and experimental results for ultrasonic velocity and attenuation in an emulsion. For the particular set of experimental data used here, the coupled phase theory gives predictions close to the scattering theory predictions, and a better prediction of the dependence of the attenuation on volume fraction. A similar difference between the two theoretical approaches has been found in earlier work on coupled phase models for sound propagation in suspensions.

ISSN:0001-4966    

DOI:10.1121/1.419745

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

When a Rayleigh wave becomes “supersonic” for a given geometry of propagation, a thin solid layer coated on the surface of the medium can bring about the transformation of this wave into a leaky wave. In such a case in the sagittal plane there emerges a narrow interval of angles of incidence within which the phase of the reflection coefficient suffers abrupt changes, and the reflection is accompanied by excitation of surface vibrations with amplitudes greatly exceeding the amplitude of the incident wave. The effect is due to the resonance excitation of the leaky wave. The critical angle of incidence and the width of the resonance interval are determined by the real and imaginary parts of the leaky wave speed. Approximate expressions for the leaky wave speed and for the coefficients of plane wave conversion are derived and analyzed without any assumptions about the crystallographic symmetry of both the substrate and the layer.

ISSN:0001-4966    

DOI:10.1121/1.419751

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper presents a theoretical investigation of the generation of Lamb waves inside an immersed plate by complex harmonic inhomogeneous plane waves. Whereas free modes of a fluid-loaded plate are classically assumed to be formed by inhomogeneous plane waves with real frequency (leaky waves), the nature of Lamb waves is searched in this study in terms of combinations of transient inhomogeneous plane waves characterized by a complex slowness vector and a complex frequency. It is shown that dispersion curves of transient Lamb waves (only the frequency is complex) can be very different from the dispersion curves of permanent leaky waves (only the slowness is complex), especially for theA0andS0modes for which there exists a cut-off frequency beyond which these modes do not exist. In the general case, when both the frequency and the slowness are complex, connections between modes clearly appear from the calculation of dispersion curves.

ISSN:0001-4966    

DOI:10.1121/1.419752

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The wave propagation in a long pyroelectric cylinder of crystal class 6 with inner and outer irregular shape is considered. The frequency equation has been derived for the traction free, earth connected, and thermally insulated surfaces. The boundary conditions at the arbitrarily shaped boundaries are satisfied using the Fourier series expansion collocation method. The longitudinal and flexural vibrations of barium titanate ceramic, which belongs to the (6 mm) class, have been analyzed numerically. The numerical results in the case of elliptical and cardioidal cross sections with an elliptical cavity are tabulated and are presented graphically.

ISSN:0001-4966    

DOI:10.1121/1.419753

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A unique low-frequency (900 Hz) class IV flextensional transducer that produces an enhanced far-field pressure on one side and canceled far-field pressure on the other side has been developed. The transducer radiating surface consists of a thick-walled elliptical aluminum shell and a U.S. Navy type III piezoelectric stack along its major axis with two active sections and one inactive section. The directionality is achieved by simultaneously exciting the shell into an omnidirectional and dipole operation by driving stack into both extensional and bending modes. Both measurements and modeling on this device show a front to back pressure ratio of more than 30 dB, producing cardioid-type radiation patterns over an octave band, for a single transducer element. The transducers measured mechanicalQis 8, coupling coefficient is 0.25, and electroacoustic efficiency is 80% and produced a source level of 215 dBre:1μPaat 1 m when driven at a field limit of 394 kV/m (10 kV/in.) at resonance. The uniqueness of this transducer is its directional beam patterns (directivity index=3.4 dB) and high acoustic output power from a small (less than a third of a wavelength) single element. Six of these transducers were placed in a closely packed line array two-wavelengths long. The array successfully produced narrow directional sound beams (directivity index=8.7 dB) with a front to back ratio greater than 30 dB and a source level of 225 dBre:1μPaat 1 m.

ISSN:0001-4966    

DOI:10.1121/1.419609

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper deals with the prediction of the dynamic response and vibrational power flow in general three-dimensional coupled beam structures. An arbitrary loading condition is assumed and flexural waves in two perpendicular planes and longitudinal and torsional waves, may be excited simultaneously in each beam. The interaction of different wave types at the coupling boundaries is therefore expected. Suitable for the low- and medium-frequency range, the modal receptance method used here allows the prediction of the detailed structural response and power flow components carried by the different wave types through joint boundaries and at any cross section of the beam structure. The mathematical description of the three-dimensional beam structure and the manipulation of receptance matrices are presented in such a form that the vibration of any complex beam structure may be described systematically and with clear physical interpretations. The prediction of the input and output power for each component beam quantifies the energy dissipation in the beam, which may be taken as a measure of the spatial average response level of the beam. Computational examples of the power flow components at the joint boundaries of the component beams in coupled three-dimensional structures are used to illustrate the significance of the theoretical results.

ISSN:0001-4966    

DOI:10.1121/1.419754

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A new solution to the problem of flexural vibration of an infinite wedge excited by a point harmonic load is obtained via a superposition of the geometric and diffraction fields, for simply supported and roller boundary conditions along the edges. The diffraction integrals are evaluated both in closed form in terms of the Faddeeva function and also as an asymptotic series using the steepest descent method. An eigensolution is also derived by expanding eigenfunctions in the angular direction and propagation terms in the radial direction. These two representations of the solution are compared numerically for accuracy and spatial distribution in different frequency regimes. The relative contribution of the geometric and diffraction fields to the overall solution is examined. This result is then compared to the second-order Helmholtz wave equation solution in order to demonstrate the near-field effects arising due to the fourth-order biharmonic formulation. The authors’ motivation for obtaining the solution for a wedge-shaped infinite plate is that it is an essential step toward devising numerical models for the vibration of arbitrary polygonal plates. By combining the contribution from diffraction at the vertices of a polygonal plate with the geometrical solution obtained by the method of images, the authors expect to obtain better predictions at mid and high frequencies than is possible by other available methods.

ISSN:0001-4966    

DOI:10.1121/1.419755

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

When an acoustic plane wave is incident on an elastic plate, energy propagates within the plate, as well as being reflected from and transmitted through the plate. The present study determines the structural intensity as a function of incident angle at different depths along the cross section. It is found that most peaks, but not all, in the structural intensity may be identified as free waves of the fluid–plate system yielding a form of “numerical spectroscopy.” It is also found that for particular frequencies and angles of incidence the direction of the structural intensity parallel to the plane of the plate is opposite than what would normally be expected. Various studies are performed to gain physical insight into this phenomenon.

ISSN:0001-4966    

DOI:10.1121/1.419756

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The geometric-mean drive-point admittance (or “mobility”) of a complex structure is given by the admittance of the corresponding infinite structure (i.e., the “characteristic admittance,”Yc). The frequency response of an infinite plate, for example, coincides with the geometric-mean response of a finite one. Eugen Skudrzyk’s “mean-value theorem” was derived and experimentally verified without consideration of fluid loading. This paper shows that Skudrzyk’s method can be applied to fluid-loaded plates well below the coincidence frequency. Skudrzyk’s general mathematical expression allows simplified modifications that account for fluid loading and result in an approximate fluid-loaded characteristic admittance that differs only by a small multiplicative factor(<2 dB)from a correct analytical expression derived by Crighton.

ISSN:0001-4966    

DOI:10.1121/1.419757

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP