摘要:

The Rayleigh‐Ritz method of eigenvalue approximation is used to obtain a solution for the free vibrations of an elastic solid. The resonant frequencies of an isotropic cube, calculated by this method, agree with the frequencies of the Lamé modes, for which an exact solution is possible, up to eight significant digits. Experimental measurements of the 13 lowest‐frequency modes of a cube of fused quartz are all within 0.5% of the results computed by using the elastic constants determined by another method. These constants differ from the elastic constants determined directly from the experimental frequencies by less than 0.2% for the shear modulus and by less than 0.003 for the value of Poisson's ratio. The cube‐resonance method of elastic constant determination is also applicable to substances of more general crystallographic symmetry.

ISSN:0001-4966    

DOI:10.1121/1.1912415

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The equations of geometrical optics governing the ray shape and the energy transport along the rays are obtained for flexure waves propagating on an inhomogeneous finite weight plate. Krasil'nikov's results for the energy flux vector and the amplitude relations along the rays are generalized both in the absence and in the presence of liquid loading.

ISSN:0001-4966    

DOI:10.1121/1.1912416

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The stationary value problem of the dynamic action for the determination of the natural frequencies of the polygonal plates is transformed by a holomorphic function into an equivalent problem of another plate with a circular boundary. The equivalent problem is then solved by the Rayleigh‐Ritz method. Since the selected coordinate functions are independent of the actual shape of a plate, the formulation is valid for one plate and for all others with boundaries which are the conformal images of a unit circle. The numerical results for the fundamental natural frequencies of plates of various shapes are calculated and compared with the available data in good agreement.

ISSN:0001-4966    

DOI:10.1121/1.1912417

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Magnetic tape recordings of seismic profiler signals were made across an acoustically transparent wedge of sediments varying from 80 to 500 m in thickness on the flanks of Bermuda. The data were processed using13‐octbandwidth filters. The average attenuation coefficient of the sediment, α, was determined from the equationd(y)/d(z) =α, whereyis the ratio expressed in decibels of the peak pressure amplitude of the bottom to the peak pressure amplitude of the subbottom returns, andzis twice the sediment thickness. Possible errors resulting from spreading losses and changes in subbottom reflectivity due to increasing velocity and density with depth were considered and appropriate corrections made to the computed attenuation coefficients. For filters centered at 200, 250, and 315 Hz the average attenuation coefficients for the sediment were found to be 0.0031, 0.0069, and 0.0126 dB/m, respectively. For a filter centered at 400 Hz, the minimum value of the attenuation coefficient is 0.0110 dB/m. Outside this frequency range, the attenuation is too high or too low to be determined by this method. With attenuation expressed as being proportional to the frequency raised to the powern, the value ofnis close to 3 for the frequency range 200–400 Hz. This high value ofnis attributed to the effect of intermediate reflecting layers.

ISSN:0001-4966    

DOI:10.1121/1.1912418

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Acoustic intensities up to 1400 W/cm2have been generated at the focus of a ceramic bowl. The rf voltage applied to the transducer has been rectified and filtered and the signals produced from cavitation events have been examined. A directional hydrophone simultaneously monitoring the focal point aided interpretation of the signals detected by the driven bowl. Low‐amplitude low‐frequency signals appeared on the driven bowl synchronously with the onset of first subharmonic emission in the water at an intensity of 13 W/cm2. The subharmonic increased rapidly with increasing sound intensity and then leveled off. Just before this occurred, the scattering properties of the focal region increased markedly and a cyclic signal, interpreted as indicating the production of clouds of microbubbles when surface oscillations set in on the cavitation bubbles, was detected on the filter circuit. Strongly radiating single cavitation events lasting for the order of milliseconds were also detected. The amplitude, duration, and rate of occurrence of these events were all one to two orders of magnitude greater in tap than in degassed water.

ISSN:0001-4966    

DOI:10.1121/1.1912419

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Frequency shifts in surface reverberation were consistently observed when a directional transducer was pointed in the direction of ocean wave propagation. The magnitude of the Doppler shift systematically decreased to zero as the transducer was rotated 90° to point toward the cross‐wave direction, and the sign of the shift reversed as the transducer was horizontally rotated from the up‐wave to down‐wave direction. Doppler‐shift data were obtained mainly for 110‐msec tone transmission at 15 and 60 kHz. The measured shifts were generally in accord with first‐order resonance theory and may be attributed to ultragravity waves. Higher‐order shifts corresponding to large gravity waves were not observed. The spectral width of surface reverberation was essentially independent of wind and wave directions and roughly three times that of the transmitted pulse for the 60‐kHz data, and less wide in the case of 15‐kHz transmission. It is hypothesized that orbital motions of the water particles forming the larger‐scale ocean wave can impart a radial to‐and‐fro motion of the ultragravity wavetrain riding on it. Rapid up and down Dopplers induced by this motion are believed to contribute to the observed spectral broadening.

ISSN:0001-4966    

DOI:10.1121/1.1912420

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The first known measurement of the attenuation of sound in deep water at frequencies below 1 kHz in the South Pacific Ocean is described. The receiver was situated off the east coast of the North Island of New Zealand. It was found that the attenuations are in most cases comparable with the values of attenuation measured in the North Pacific Ocean and the Atlantic Ocean between Bermuda and South America. However, on one of the paths over which the attenuation was measured, there was an abrupt change in the quality of sound transmission, which is thought to be a result of the path crossing the Subtropical Convergence; south of this discontinuity the attenuations were found to be comparable with those obtained previously in the Southern Ocean.

ISSN:0001-4966    

DOI:10.1121/1.1912421

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

Experiments are described in which the dorsal‐aspect target strengths of a number of individual teleostean fishes of eight species were measured at various frequencies. The results of these experiments indicate that the variations of target strength with frequency are different for fishes in two major teleostean groups, the malacopterygians and the acanthopterygians. These results are combined with results from eight other sources and an empirical equation approximating the dorsal‐aspect target strength of an individual fish determined for 0.7⩽L/λ⩽90, whereLis the fish length and λ is the incident acoustic wavelength. The combined results are compared to similar results for the maximum side‐aspect target strength of an individual fish, and curves showing the trend of dorsal‐aspect and maximum side‐aspect acoustic cross sections of an individual swimbladder‐bearing fish are presented for allL/λ⩽90.

ISSN:0001-4966    

DOI:10.1121/1.1912422

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

In media for which the speed of sound is position dependent, propagating sound will be refracted and, in some cases, focused. In the focusing regions, usually referred to as caustics or convergence zones, significant amplification of the pressure levels above those predicted by spherical spreading has been observed for continuous waves as well as for explosive pulses. In addition, the waveforms of explosive pulses undergo drastic distortion. In the present paper, an asymptotic theory of the refraction and focusing of sound originating from a point source in a stratified medium is presented. It is applicable to realistic velocity profiles and encompasses both transient pulses and harmonic waves. A comparison with Barash's and Goertner's recent experiment involving explosive pulses indicates that the theory gives reliable estimates of the peak pressure levels at caustics, but reproduces only qualitatively the shape of the focused pulse. The discrepancy is attributed mainly to the neglect of finite‐amplitude effects in the theory's formulation. The inaccuracies inherent in the high‐frequency asymptotic methods employed in the theory are discussed in some detail.

ISSN:0001-4966    

DOI:10.1121/1.1912423

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

An approach is presented to compute the time‐dependent force acting on a piston in a rigid infinite planar baffle as a result of the specified velocity of the piston. The approach to computing the force is applicable to both sinusoidal and nonsinusoidal velocity pulses and is valid for all piston shapes. The approach, which is based on a Green's‐function solution to the time‐dependent boundary value problem, utilizes a transformation of coordinates to simplify the evaluation of the double surface integrals. An impulse response function is defined such that the time‐dependent force can be obtained by differentiating the convolution of the impulse response and piston velocity time functions. A closed‐form expression for the impulse response of a circular piston is derived and discussed. Numerical results for the impulse response and the forces on large square pistons resulting from sinusoidal piston velocities are then presented and discussed to compare the transient and steady‐state behavior of the forces. Finally, an approach is presented to compute the radiation impedance as a function of normalized frequency from the impulse response data, and the approach is used to obtain the normalized radiation resistance and reactance for square pistons.

ISSN:0001-4966    

DOI:10.1121/1.1912424

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP