摘要:

The influence on ray propagation of including both attenuation and beam displacement at the bottom of a shallow‐water isospeed channel is determined. Attenuation is incorporated using the Mackenzie‐bottom model, rather than the commonly used Rayleigh reflection theory. Properties of displacement are studied as a function of launch angle and parameters such as bottom‐to‐water density, channel depth in wavelengths, and channel aspect ratio. Differences in beam displacement for the two bottom models are shown to affect the number and geometry of rays between surfaced source and receiver. Formulas for per‐ray amplitude and phase shift and for incoherent total‐field intensity are developed. Comparisons are made between these quantities for both modified and classical ray theory with a Mackenzie bottom, and also for both Mackenzie‐ and Rayleigh‐bottom models using modified rays. Numerical computations show significant phase and amplitude differences arising in both types of comparisons.

ISSN:0001-4966    

DOI:10.1121/1.394789

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Ray theory with beam displacement is used to simulate the propagation of pulses in shallow water. Comparisons show very close agreement between simulated time series and time series measured in an indoor tank having a sand bottom. The six normal modes present are successfully extracted from both the simulated and the experimental sets of depth‐dependent time series. The ray model accounts for the frequency dependence of beam displacement by interpolating eigenray characteristics at three frequencies in the spectrum of the source pulse. Also, the model corrects ray theory when receivers are close to caustics formed by beam displacement, in which case eigenrays must be found at six frequencies. Mode theory phenomena such as mode cutoff and dispersion can be observed in the simulations produced by this method.

ISSN:0001-4966    

DOI:10.1121/1.394790

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Analytical methods of Marsh [J. Acoust. Soc. Am.33, 330–333 (1961)] and Kuo [J. Acoust. Soc. Am.36, 2135–2142 (1964)] are applied to high‐frequency acoustic wave scattering from an ocean bottom of roughness amplitude typically quite small in comparison with the acoustic wavelength of interest. The bottom surface variation in acoustic velocity is considered negligible but the bottom surface variation in density is considered appreciable and included in the model. The analytical result indicates the possibility of a new scattering effect in grazing incident directions due to correlated roughness and density variations. When there exists an identifiable dominant roughness wavenumber, the theory predicts discrete backscattering. This phenomenon was observed in an experiment reported by Roderick and Dullea (TD 7181, Naval Underwater Systems Center, 1984). When the dominant roughness wavenumber is zero, the result is that of Rayleigh’s flat boundary. When the dominant roughness wavenumber is low compared to acoustic wavenumber, the result is that of the Kirchhoff approximation.

ISSN:0001-4966    

DOI:10.1121/1.394791

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The amplification of reflected bulk acoustic waves (RBW) and the convolution of two oppositely traveling incident bulk acoustic waves is analyzed. The case of a normal field degenerate convolver is considered. It is shown, on an orientation of the bismuth germanium oxide (BGO) crystal in which only one mode is excited and in which no mode conversion occurs upon reflection, that RBWs can be amplified by the drifting carriers in a semiconductor. It is also shown that convolution resulting from the mixing of two incident bulk acoustic waves can be obtained. The overall behavior of the amplified RBW and of the convolver open‐circuit voltage with the semiconductor parameters and the applied drift field and air gap is found to have some similarities to those of observed results in SAWs. A dependence of the acoustoelectric interactions on the angular distribution of the bulk wave radiation, hence the applied signal frequency, is shown. There may, however, exist some critical angles for the RBW for which no interaction exists due to the absence of an electric field in the gap. Results are presented for a BGO/Si structure and characteristics of the proposed RBW acoustoelectric devices are discussed.

ISSN:0001-4966    

DOI:10.1121/1.394792

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The dynamic Young’s modulus and loss factor of a viscoelastic material may be calculated as functions of frequency from data on the relative motions of the two ends of a bar of the material that is in harmonic oscillation at that frequency. Most investigators using this technique have confined their measurements to resonant frequencies, but it would be useful to find the moduli of the material at regular, narrowly spaced intervals of frequency. The characteristic equation, from which the moduli of the material are calculated, is investigated with respect to numerical solvability and stability. It is shown that this equation has infinitely many solutions. An apparently effective method of choosing the physically relevant solution is developed. It is found in the case of low‐loss materials that at certain frequencies solutions to the characteristic equation lie near zeros of the Jacobian, which are the solutions in the limiting case of a perfectly elastic material. Consequently, low‐loss viscoelastic materials will exhibit great sensitivity to errors in measurement at these frequencies. An error analysis is developed in order to estimate the magnitude of this instability. A microcomputer program written to solve these equations is applied to various simulated samples to illustrate the effects of this instability.

ISSN:0001-4966    

DOI:10.1121/1.394742

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The acoustic damping for single modes of a finite rectangular panel, simply supported in an infinite baffle, is theoretically determined from the ratio of the acoustic energy radiated per cycle to the vibratory energy of the panel. Asymptotic solutions for the low‐frequency region are presented for a panel mode driven at an arbitrary frequency and for a panel mode vibrating at its natural frequency. Curves of acoustic damping for a panel mode vibrating at resonance, as a function of the panel thickness‐to‐length ratio, are presented for various panel aspect ratios. For panels vibrating below the critical frequency, the damping depends on the aspect ratio with square panels developing the smallest value. For panels vibrating above the critical frequency, the damping is nearly independent of the aspect ratio.

ISSN:0001-4966    

DOI:10.1121/1.394743

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

Free vibration is analyzed for unsymmetrical joined plate or shell structures with a closed member by means of the transfer matrix method. For this purpose, the continuity and equilibrium relations for the displacements and forces at the joints are written with use of the joint matrices. The connection matrix between the closed member and other members at the joints is derived by introducing the structure matrix of the closed member, and the entire structure matrix is obtained by the product of the connection matrix of the closed member and the transfer matrices of other members. This method is applied to a plate structure with an unsymmetrically sited duct and a box‐type structure with an aslant interior plate, and the natural frequencies are calculated numerically together with the mode shapes of vibration giving the results.

ISSN:0001-4966    

DOI:10.1121/1.394744

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

The standard approach to vibration isolation analysis represents the source, isolator, and receiver by their mobilities. However, the choice of appropriate descriptor for each element is not arbitrary. Causality arguments show that whether the isolator is treated as a one‐port or a two‐port element, at least one of the system components must be represented by its impedance.

ISSN:0001-4966    

DOI:10.1121/1.394745

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

A comprehensive analytical model of the wayside noise generated by a railroad wheel rolling on straight track is presented. The model assumes that the small‐scale roughness on the running surfaces of the wheel and rail is the primary mechanism for the noise generation. Included in the model are such effects as the spatial filtering of the roughness due to the finite area of contact between the wheel and rail; the interaction between the wheel and rail, including local contact stiffness; axial response of the wheel; the radiation efficiencies of the wheel and rail; and the influence of sound propagation, including finite ground impedance.

ISSN:0001-4966    

DOI:10.1121/1.394746

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP

摘要:

An analytical model for the prediction of wheel/rail noise is validated through comparison with measurements made using the state‐of‐the art car (SOAC) on a test track. Predictions of wheel and rail vibrations and wayside noise are seen to agree reasonably well with measurements although areas of uncertainty remain. Model improvements to reduce discrepancies between theory and measurement are discussed.

ISSN:0001-4966    

DOI:10.1121/1.394747

出版商:Acoustical Society of America    

年代:1987

数据来源: AIP