摘要:

The axisymmetric motions in an infinitely long circular rod made of a transversely isotropic material are studied, and the frequency equation governing such motions is established. The relationship between frequency and wavelength contained in the equation is not explored in detail. Instead, exploration is limited to information that will be useful in a subsequent study in developing an approximate theory that will govern the same motions. By studying the distribution of cutoff frequencies on the frequency wave‐propagation constant plane, the conclusion is reached that the three lowest modes of motion strongly influence each other and are in turn influenced to a much lesser degree by the fourth and higher modes. Accordingly, numerical analysis is made of only the three lowest spectral lines.

ISSN:0001-4966    

DOI:10.1121/1.1912321

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A realistic computer simulation of reverberation from a moving sea surface has been used to compare time and ensemble averages. The experiment simulated has a directional source and an omnidirectional receiving hydrophone located at a depth of 350 yd (320 m). The surface is insonified in a strip of constant width independent of range. The reverberation is calculated in three “windows” of width 0.1 sec located at 0.5, 1.0, and 1.5 sec for a fully developed sea resulting from wind velocities of 10 and 20 kt (5.1 and 10.3 m/sec). The pulse waveform is two cycles of a 1‐kHz sinusoid. The sea surface is generated by using random phases in a Fourier polynomial of 100 terms. It is shown that, when the reverberation is reduced in the manner suggested by Ol'shevskii, the derived process is not stationary, but behaves like a narrow‐band noise of constant amplitude and random phase. The lack of stationarity arises because the statistical properties of the phase change with time. The backscattering strengths of the sea surface, computed for six combinations of grazing angle and sea state, agree only moderately well with experimental measurements at sea.

ISSN:0001-4966    

DOI:10.1121/1.1912322

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The absorption of a small‐amplitude sound wave in a liquid can furnish a method for studying the size distribution of microcavities produced by energetic particles. In contrast to ultrasonic cavitation, this method is nondestructive and therefore allows the following of time variations of the distributions. The method has been improved to detect cavities of radii of the order of 2 μm, such as those created in water by fast neutrons.

ISSN:0001-4966    

DOI:10.1121/1.1912323

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A large fluid lens with accompanying reservoir and pump arrangement is described, with emphasis on its underwater acoustic‐imaging application. Pertinent properties are cited of the neutrally buoyant lens fluid, the elastomeric containment membranes, and the composite lens. The liquid‐filled lens potentially provides better over‐all acoustic transmissivity than does a comparable solid lens and has an equivalent acoustic refractive index. Focusing, achieved without physical motion of the lens proper, occurs in a manner analogous to the operation of the human eye. Introduction or removal of fluid changes the curvature of the lens‐membrane surfaces. Theoretical consideration reveals the stretched‐membrane surface configuration to be approximately spherical. Thus, standard spherical lens equations are combined with appropriate mensuration formulas to obtain lens calibration equations. A hydroacoustic image taken with the liquid lens is displayed. This image is compared with that formed under identical circumstances by means of an equivalent Lucite lens. Beamwidth and spherical aberration of the fluid lens are discussed. A field stop for use with the liquid lens and a technique for immunizing the lens to water‐wave action are noted.

ISSN:0001-4966    

DOI:10.1121/1.1912324

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The radiation from an infinite phased planar array of rectangular pistons with nonrigid interstices is considered. Numerical calculations for exact equations are compared with a simple approximation, derived for small pistons close together. The comparisons show that the approximate expressions agree well with the exact results even for pistons of practical size, if (1) the array is steered to an angle near broadside, (2) the interstitial impedance is not very low, and (3) the interstices occupy a small fraction of the total array area.

ISSN:0001-4966    

DOI:10.1121/1.1912325

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The effect of a stochastic ocean bottom on underwater acoustic propagation is examined in two previous papers by the authors [J. Acoust. Soc. Amer43, 1395–1403 (1968);44, 1103–1114 (1968)]. In those investigations, the first and second moments of the random acoustic intensity are determined for various source and receiving‐point geometries. In a more recent study by the authors [J. Stat. Phys.2, 279–289 (1970)], the amplitude and decibel amplitude, as well as the intensity, of a random acoustic field are examined. There, each of the three measures of acoustic strength is shown to differ from the others in the analytic properties of its mean value and standard deviation. Therefore, in the present paper, the random‐bottom problem of the two earlier studies is reexamined. In particular, the first and second moments of the total‐field amplitude and decibel amplitude are studied under certain restrictions and are compared with those of the intensity. Furthermore, the first and second moments of the total‐field phase are treated.

ISSN:0001-4966    

DOI:10.1121/1.1912326

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Trials have been conducted in the Western Mediterranean to confirm the excess attenuation of sound at frequencies between 200 Hz and 10 kHz. As sound propagation along the deep refracted path in the isothermal deep water of the Mediterranean is affected only by geometrical spreading and attenuation, it is possible to measure the attenuation separately for each frequency. The trials used explosive sound sources at different depths and at accurately determined ranges between 200 m and 30 km. Special care has been taken to separate the effects of finite amplitude from the attenuation and it is shown that the former are not responsible for the excess attenuation found at low frequencies. The authors have reported the analysis more thoroughly in SACLANTCEN Technical Report No. 156, “Sound Attenuation Between 200 Hz and 10 kHz.”

ISSN:0001-4966    

DOI:10.1121/1.1912327

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

An approach is presented to computing the time‐dependent force acting on a piston as a result of the velocity of an adjacent piston which may be any specified time‐dependent function. The method is based on a Green's function solution to the time‐dependent boundary‐value problem for an impulsive piston velocity motion. An asymptotic expression for the mutual radiation impedance between square pistons has been obtained using the method. A numerical approach is also presented to obtain the mutual radiation impedance coefficients from the time‐dependent solution of the boundary‐value problem. Numerical results obtained from the asymptotic expression are shown to agree with previously published numerical results and the results obtained using the numerical approach indicated in the paper. Numerical results for time‐dependent interaction forces resulting from sinusoidal piston velocities are also presented and discussed.

ISSN:0001-4966    

DOI:10.1121/1.1912328

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

This investigation is concerned with the three‐dimensional interaction between an infinitely long elastic circular cylindrical shell, subjected to point loading and an acoustic medium. The formal solution to the problem was obtained by solving the governing field equations of the shell and the acoustic medium through the application of integral‐transform and series‐solution techniques. The resulting Fourier Laplace inversion integral was evaluated by means of numerical‐inversion methods. Detailed numerical results are presented for the dynamic response of a steel shell immersed in water when it is subjected to a point loading having the form of a step function of finite duration. Since there are no approximate relations introduced in the fluid‐shell phenomena, the result obtained from this investigation is the exact solution to the problem within the scope of linear‐acoustic and thin‐shell theory, and the approximation errors introduced by the numerical techniques.

ISSN:0001-4966    

DOI:10.1121/1.1912329

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

This communication presents the results of an investigation of the response of a nonlinear form of the Mathieu equation in the first unstable region. The equation of interest is a Mathieu equation plus a cubic nonlinearity. The response consists of a modulated one‐half subharmonic of the parametric‐excitation frequency. The envelope of the subharmonic is approximately a sinusoid. The magnitude and frequency of this sinusoid are determined by the method of harmonic balance. The results are compared with the results of computer simulation.

ISSN:0001-4966    

DOI:10.1121/1.1912330

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP