摘要:

The mean‐value theory predicts the mean line through the logarithmically recorded frequency‐response curve of a complex vibrator, the height of the resonance peaks, and the height of the minima that occur between every two resonance peaks. It predicts the mean and the extremes in the frequency‐ response curve from the first resonance of the vibrator to very high frequencies. The computations are based on the mode masses and on the density of the resonances. They are simple and they give considerable insight into the dynamics of complex vibrators. Part I of this paper presents the basics of the mean‐value theory. Part II deals with the variation of the vibration velocity with the distance from the driver and with the frequency. Part III is concerned with shells without and with stiffeners and with rib‐stiffened plates. Because it is always the component of the characteristic velocity that is transmitted by discontinuities (as though the parts of the vibrator behind them were infinitely large) that determines the geometric mean values of the response, the effect of ribs and other appendages on the vibration can be predicted without much computation. As an example of the application of the theory, the vibration isolation that can be obtained by spring mounting a motor on a ring welded into a shell is computed in detail and the predictions for the ring excited shell are compared with the measurements.

ISSN:0001-4966    

DOI:10.1121/1.384169

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The sound field produced by plane baffled transducers of highkacan be described by treating the radiated space as a waveguide of infinite size. Under the Fresnel approximation for cylindrical geometries, the modes of this waveguide are known to be the Gaussian–Laguerre functions. The general Gaussian–Laguerre formulation for expanding acoustic fields is presented and numerical results are shown for the circular piston. The Gaussian–Laguerre expansion coefficients for this example are obtained analytically, thus providing a new solution in series form.

ISSN:0001-4966    

DOI:10.1121/1.384170

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

We consider the inverse problem of determining small variations in propagation speed from remote observations of signals which pass through an inhomogeneous medium. Under the conditions (1) that the variations can be written as a small perturbation from a known reference value and (2) that the medium of interest varies in one direction only, an integral equation has been developed for the variations which can be solved in closed form. Here, a technique is presented to obtain and process synthetic data from a scattering profile of arbitrary shape. The results of numerical testing show that, as long as a velocity variation is indeed ’’small,’’ both its size and its shape can be reproduced with negligible error by this method.

ISSN:0001-4966    

DOI:10.1121/1.384171

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Some previous discussions of phase shifts at caustics have employed the WKBJ method of solution to the wave equation for a point source in a vertically stratified medium where a single sound‐speed minimum exists. Geometrical acoustic techniques are used in this paper to show for the same problem that a ray encounters only one caustic between turning points (starting with the first and second points), and that turning points and points of encounter with caustics approach one another with increasing range from the source.

ISSN:0001-4966    

DOI:10.1121/1.384172

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A numerical integration method is devoloped to calculate the intensity profile of an ultrasonic beam reflected from a liquid–solid interface. This numerical treatment is used to calculate nonspecular reflectivity at a range of angles of incidence near and at the Rayleigh angle. Calculations for a water–stainless steel interface are compared to a known approximate analysis for various beamwidths and frequencies. The theoretical predictions of the reflected beam profile near the Rayleigh angle of incidence are compared to experimental results.

ISSN:0001-4966    

DOI:10.1121/1.384173

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Acoustic tones generated by turbulent flows and shear‐layer‐instability interactions with side‐branch resonator pipes were investigated experimentally using fast Fourier transform techniques. The experimental values of resonant frequencies and instability frequencies were compared with predictions for two stages of shear‐layer interaction. Relative tonal amplitudes are shown to demonstrate cut‐in and cut‐out phenomena. The qualitative differences between turbulent‐flow‐generated tones and instability‐generated tones are also noted.

ISSN:0001-4966    

DOI:10.1121/1.384174

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

This paper analyzes the diffraction of Lamb waves by a finite crack situated on the plane of symmetry of an elastic layer. The surface of the crack and of the layer are assumed to be stress‐free. The problem is solved by the modified Wiener–Hopf technique. The field of the reflected and transmitted waves, and also the field in the vicinity of the crack, are given as an expansion in natural waves of the elastic layer. The amplitudes of these waves are expressed in terms of certain generalized quantities, which are found from exponentially converging infinite systems of equations. The solution converges at any value of the parameterL/H≳0 (whereLis the length of the crack andHis the thickness of the layer) and is particularly effective as this parameter increases. The strongly resonant phenomena in the region of the layer occupied by the crack are identified and discussed.

ISSN:0001-4966    

DOI:10.1121/1.384175

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The authors study the amplitudes of the pulses delivered by a piezoelectric sensor immersed in a cavitation field. A 1024‐channel analyzer shows that the amplitude distribution of these pulses includes peaks. They go on to show the pattern of evolution of this distribution as a function of the ultrasonic power and the distance between the piezoelectric sensor and the transducer.

ISSN:0001-4966    

DOI:10.1121/1.384176

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Nonstationary and nonuniform distributions of acoustic background in the ocean are important statistically when detection performance predictions are carried out for high‐gain receiving arrays. A high‐gain acoustic array, ADA (Acoustic Distribution Array), designed for measurement of high‐order temporal and spatial distributions of acoustic backgrounds in the ocean, was used to obtain envelope spectra of directional noise in octave bands centered at 1130 and 2260 Hz. The directional noise was measured through an infinitely clipped DIMUS beamformer and thus, because of the clipped normalization, the envelope spectra represent, solely, variations in the directional structure of the background noise, not its absolute level.

ISSN:0001-4966    

DOI:10.1121/1.384177

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

By means of a corrected split‐step parabolic‐equation numerical algorithm, acoustic propagation through an ocean region characterized by a sound‐speed distribution produced by an analytic model of an eddy is investigated. Parameter values for a moderate sized, cyclonic, Gulf Stream eddy are used. It is found that the presence of an eddy causes significant changes, both in nature and level, in the received acoustic field produced by an omnidirectional cw source. The eddy causes major changes in the arrival structure seen by a vertical array. The percentage of energy arriving at angles less than 5° from broadside increases from 5% to 50% in the presence of the eddy under consideration at 100 Hz. Greatly increased energy corresponding to horizontal arrival is noted. In addition, the effects of an eddy progressing through the region between an acoustic source and receiver is studied. The position of the eddy relative to the source causes changes in transmission loss of as much as 20 dB.

ISSN:0001-4966    

DOI:10.1121/1.384178

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP