摘要:

Conventional normal mode theory cannot be applied to the nonseparable problem of wave propagation in an ocean with sloping bottom. For small bottom slopes, the sound field may be expressed approximately in terms of adiabatic modes, but this description fails when a mode propagating upslope passes through cutoff. An alternative solution by ray acoustics, valid at high frequencies, contains many multiply reflected contributions, and also undergoes difficult trapped to leaky ray transitions upslope. To address these transition problems, the conventional ray solution is used here as a convenient starting point for collective treatment of ray fields and their conversion into local modes. First, the ray fields are generalized by associating with each ray trajectory from source to observer a bundle of local plane waves that is multiply reflected between the boundaries and remains valid also in the transition regions. When the generalized ray series is subjected to Poisson summation and subsequent asymptotics on the transform integrals, it is found to furnish local modes which coincide with the conventional adiabatic modes where these exist. The local mode integrals also yield transition functions which smoothly continue an originally trapped adiabatic mode through cutoff to the leaky regime. The transition behavior agrees with that found by Pierce [J. Acoust. Soc. Am.72, 523–531 (1982)] by an entirely different approach, and with that predicted by the spectral Green’s function method of Kamel and Felsen [J. Acoust. Soc. Am.73xxx–xxx (1983)]. The ultilization of rays and local modes in each step of the analysis here grants physical insight and therefore clarifies the mechanism of propagation in this range‐dependent environment.

ISSN:0001-4966    

DOI:10.1121/1.389281

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Spectral representations based on the theory of characteristic Green’s functions (resolvents) have been used effectively for studying sound propagation in a coordinate separable ocean environment. Such representations are here generalized to accommodate weak nonseparability as represented by a homogeneous water channel separated from a homogeneous sediment by a gradually and monotonically sloping bottom. The generalization involves the use of adiabatic invariants for the spectral integration variable and of symmetrizing factors in order to insure that the Green’s function, so expressed, reduces by residue calculus to the conventional adiabatic trapped mode expansion whenever that is valid. However, subject to ignoring coupling between the adiabatic modes, the generalized Green’s function contains all of the spectral information (discrete and continuous) for upslope propagation from a source to arbitrary observation points in the water or the bottom, and it can be used to derive ray, hybrid ray‐mode, and other formulations in this configuration. Numerical evaluation of the spectral integral and comparison with results obtained independently by the parabolic equation algorithm have confirmed its validity for a typical range of observer locations. Asymptotic considerations have been employed to clarify the mechanism of adiabatic mode transition from trapped to radiating in terms of lateral and leaky waves, and to reduce the spectral integral to a simpler canonical transition function similar to those of Pierce [J. Acoust. Soc. Am.72, 523–531 (1982)] and of Arnold and Felsen [J. Acoust. Soc. Am.73, xxx–xxx (1983)] which were derived by entirely different methods.

ISSN:0001-4966    

DOI:10.1121/1.389282

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

This work presents both theoretical predictions and experimental measurements of attenuation and progressive phase constants of sound in open‐cell, highly porous, elastic polyurethane foams. The foams are available commercially in graded pore sizes for which information about the static flow resistance, thermal time constant, volume porosity, dynamic structure factor, and speed of sound is known. The analysis is specialized to highly porous foams which can be efficient sound absorbers at audio frequencies. Negligible effect of internal wave coupling on attenuation and phase shift for the frequency range 16–6000 Hz was predicted and no experimentally significant effects were observed in the bulk samples studied. The agreement between predictions and measurements in bulk materials is excellent. The analysis is applicable to both the regular and compressed elastic open‐cell foams.

ISSN:0001-4966    

DOI:10.1121/1.389283

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

This work presents a comprehensive study of the surface acoustic admittance properties of graded sizes of open‐cell foams that are highly porous and elastic. The intrinsic admittance as well as properties of samples of finite depth were predicted and then measured for sound at normal incidence over a frequency range extending from about 35–3500 Hz. The agreement between theory and experiment for a range of mean pore size and volume porosity is excellent. The implications of fibrous structure on the admittance of open‐cell foams is quite evident from the results.

ISSN:0001-4966    

DOI:10.1121/1.389284

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The liquid‐filled target sphere is a high target strength reflector which, since its first use in 1971, has proven to be of great utility in sonar research. The spherical metal shell filled with a low‐velocity liquid is being used as a calibrated, passive, aspect‐independent target. To date, the characteristics of this target have only been described in a narrowkarange by simple ray theory models. In this paper, a comprehensive treatment for 0

ISSN:0001-4966    

DOI:10.1121/1.389285

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Doubly Asymptotic Approximations (DAAs) are differential equations for boundary‐element analysis of the interaction between a complex structure and a surrounding infinite medium. In this paper, the use of first‐ and second‐order DAAs for steady‐state vibration analysis of submerged structures is examined. First, the governing discrete‐element equations for the general problem are set down and discussed. Then the accuracy of three DAA forms is studied through the generation of numerical results for a submerged spherical shell. Although the first‐order DAA is found to be inadequate, the two second‐order forms show considerable promise.

ISSN:0001-4966    

DOI:10.1121/1.389286

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The theory of ultrasonic propagation in polycrystals presented in a previous paper is used to calculate the scattering coefficient and the phase and group velocities of plane shear waves in polycrystals of cubic symmetry with randomly orientated grains. The calculation was done in second‐order perturbation theory using the assumption that the changes in the elastic constants and in the density from grain to grain are small. The asymptotic values at low κa(Rayleigh scattering) are exactly the same as the well‐known results from Bhatia and Moore. Numerical calculations are carried out for some examples.

ISSN:0001-4966    

DOI:10.1121/1.389206

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A general expression has been derived and evaluated for the backscattered echo energy of an acoustic pulse due to a collection of identical randomly distributed isotropic scatterers. Excess attenuation of the signal due to the extinction cross section of the scatterers as well as second‐order scattering have been taken into account. Special attention is focused toward the numerical evaluation of second‐order scattering effects. The expression is evaluated for three scattering geometries. It is shown in each geometry that when the absorption cross section of the scatterers is small, second‐order scattering can be a factor in the backscattered energy. In this case, second‐order scattering at least partially offsets effects due to excess attenuation in the low‐to‐moderate attenuation region. When applied to fish‐echo processing, it was shown that in most cases the results represent an upper bound for the processed signal from a school of fish. The directional characteristics, acoustic frequency of the pulse in relation to the resonance frequency of the swimbladder (if any), and degree of randomness of the spatial distribution of the fish determine the degree to which second‐order scattering plays a role in this area.

ISSN:0001-4966    

DOI:10.1121/1.389287

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Cavity resonances in engine cylinders are caused by combustion events such as the rapid rate of pressure rise that occurs during compression ignition in diesels or from knock in gasoline engines. These resonances generally occur at frequencies greater than 4 to 5 kHz where the engine structure is not an efficient acoustical radiator. However, when they occur at lower frequencies such as in engines with a large bore or in indirect injection diesels, they can be important in the noise generation process. They are also important for knock detection in gasoline engines. Current knock detection systems are tuned to the frequency band of the lowest cavity resonance in the combustion chamber. It is shown in the paper that higher order resonances can also be detected by a knock vibration sensor on the surface of the engine. Another use for the cavity resonances is to determine the bulk temperature of the gas in the combustion chamber as a function of crank angle. This technique is demonstrated in the paper for a heavy‐duty two‐stroke diesel. Also, the results of several fundamental investigations of cavity resonances in engine combustion chambers are reported briefly. Good agreement is obtained between theoretical prediction of the resonant frequencies and experimental observation. The splitting of degenerate modes into two components is discussed.

ISSN:0001-4966    

DOI:10.1121/1.389261

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Three methods for the reconstruction of inhomogeneities in a one‐dimensional lossless medium from incident and reflected wave signals are presented in this paper: the method of characteristics, the Gel’fand–Levitan method, and the newly developed method of iterative local regulization. Reconstructed results from numerical experiments are compared with emphasis on inversion in the presence of noise, and on general excitations. An error analysis and a study of computation requirements are also presented.

ISSN:0001-4966    

DOI:10.1121/1.389262

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP