摘要:

A theoretical treatment of the reflection problem at a lossy liquid–lossless isotropic solid is given. The calculation results indicate that the reflection coefficient has a peak greater than unity. A physical explanation is proposed which also leads to the prediction that at some critical liquid attenuation a Rayleigh wave propagating on the solid surface will not leak into the liquid.

ISSN:0001-4966    

DOI:10.1121/1.382773

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Transmission and reflection losses (TL, RL) are calculated as functions of frequency thickness (F×D) and incidence angle (ϑ) for single and multiple viscoelastic plates (steel, ABS, syntactic foam, neoprene, and Plexiglas) using a transfer matrix method modified to include complex wave velocities. The calculation of these velocities depends on four measurable material constants: longitudinal and shear phase velocities (cL,cs) and absorption coefficients (αL, αS). A physical model is developed to relate αLto αS, and shown to be experimentally accurate for a wide range of materials, temperatures and frequencies, thus providing a new way to account for shear absorption when directly measured data is not readily available. Use of actual measured data is shown to be necessary to validly compare theory with the experimentally observed TL and RL. Analysis of maxima and minima in TL, RL versusF×Dor ϑ is made in detail using calculated dispersion curves for single plates, and these extrema are shown to be consistant with the coincidence rule. In the case of a plate with high loss, the maxima in TL versusF×D, e.g., observed where minima would be expected from the coincidence rule, are explained in terms of interfering ’’resonance’’ peaks using a recent theory of acoustic resonances in plates. The concept of interfering resonances is shown to be successful in interpreting some of the previously unexplained features of TL and RL observed in steel.

ISSN:0001-4966    

DOI:10.1121/1.382774

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Measurements are reported of the arrival times of broadband ultrasonic pulses scattered by a circular, cylindrical, solid inclusion imbedded in a matrix whose longitudinal wave speed is lower than that of the scatterer. The wavenumber‐inclusion radius productka, ranges from approximately 1 to 15. The matrix materials are water, polyester, and brass; and the inclusion materials are copper, quartz, stainless steel, and tungsten. A strong, scattered pulse, corresponding to a circumferential wave on the cylinder is observed forcs≳cP, wherecsis the shear wave speed of the inclusion material andcPis the longitudinal wave speed of the matrix material. These observations are in agreement with results published by others. The experiments also show that the pulse corresponding to the circumferential wave is not observed forcs

ISSN:0001-4966    

DOI:10.1121/1.382775

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

A method of calculating levels of farfield nonspecular acoustic backscattering from finite plates at frequencies above coincidence, due to scattering of flexural waves from the edges of the plate, is presented. For water‐load steel plates at frequency–thickness products above 20 kHz‐in., the flexural vibration can be adequately represented by the edge component and the supersonic wave. The amplitudes of these components are found by satisfying the boundary conditions, and the reradiated sound levels can then be calculated. The Timoshenko–Mindlin plate model is used, and comparison of calculated results with experimental results shows good agreement, even down to 15 kHz–in.

ISSN:0001-4966    

DOI:10.1121/1.382776

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The nonlinear two‐dimensional acoustic waves that occur within a rectangular duct of semi‐infinite length as the result of periodic excitation are determined by an asymptotic method. A regular perturbation expansion is employed to obtain the velocity potential as a solution of a nonlinear wave equation. Uniformly valid expressions for the particle velocity components and pressure are then derived with the aid of a coordinate straining transformation that features both spatial coordinates. The response is found to be formed from the linear superposition of nondispersive groups of propagating nonlinear waves, each of which corresponds in linear theory to a fundamental wave and all overtones having the same phase speed, together with exponentially decaying standing waves resulting from excitations below the cutoff frequency. Typical spatial profiles and temporal responses for one nondispersive group are discussed in an example.

ISSN:0001-4966    

DOI:10.1121/1.382777

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The propagation velocities in deformed isotropic elastic material are determined as functions of the strain or the stress. The velocities are expanded with respect to a strain measure and they are expressed by second‐order polynomials of it, where the coefficients are given by two second‐, four third‐, and six fourth‐order elastic constants. In the first‐order approximation the velocity difference between two transverse waves is proportional to the stress difference and the stress acoustical law holds. The isotropic hyperelastic material is treated and the three conditions are derived where a general isotropic elastic material is reduced to a hyperelastic material in the third‐order theory. The velocity relations in this material are depicted. The expansions of the velocity with respect to the displacement gradient are also treated. Two methods of expansion yield identical linear but different nonlinear relations.

ISSN:0001-4966    

DOI:10.1121/1.382778

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The self‐sustained oscillation of a circular jet coupled with a whistler nozzle has been investigated experimentally for a range of the controlling parameters, viz., nozzle length, collar length, step height, and jet speed. It is shown that these parameters are related through an empirical formula which can be used to predict pure‐tone frequencies at which a whistler nozzle can induce controlled excitation of a circular jet.

ISSN:0001-4966    

DOI:10.1121/1.382779

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

A prototype FM–CW radar for atmospheric sounding and some of its applications are described. The prototype generates a signal whose frequency increases linearly from 1830 to 2160 Hz during a time interval of 10 s, the excitation being at intervals of 20 s. The principle of operation is based on the fact that received echos reflected from inhomogeneities at height h, will have a frequency lower by an increment of (df/dt) (2h/c) than that currently being transmitted. Typical data taken with the prototype are compared with analogous data obtained with a pulsed SODAR.

ISSN:0001-4966    

DOI:10.1121/1.382780

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

A recently developed diagnostic technique for aerodynamic noise involves cross correlation between various types of source quantities and the radiated sound [T. E. Siddon, 7th ICA (1971)]. The technique is applied to a circular jet where—following H. S. Ribner’s dilatation model—the turbulent pressure is viewed as the basic source fluctuation (the ’’cause’’), and is cross correlated with the radiated acoustic pressure (the ’’effect’’). The cause–effect principle is applied in real time. This led to the adoption of the term ’’causality’’ correlation. Information on the volume distribution of source strength, the spectrum from unit volume, and the local correlation volume can, in principle, be obtained by moving a specially designed pressure sensor around the jet. The pressure sensor is of an airfoil type, designed to minimize the projected area in the direction of radiation, and thereby the additional noise from the interaction of the turbulence with the sensor. Also, pressure measurement errors due to turbulent interference are reduced substantially with respect to those for classical cylindrical pressure probes. By integrating the radial distribution of source strength a ’’slice‐wise’’ axial distribution is obtained; it is in qualitative agreement with results of other researchers. However, on integrating the axial distribution along the entire jet, the resulting total sound level was found to be substantially larger than that obtained by direct measurement. This paradox is explained as a consequence of spurious drag dipole radiation from the stem of the probe.

ISSN:0001-4966    

DOI:10.1121/1.382781

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

In March 1976, measurements were made of the ambient sea noise in the South Fiji Basin to the northeast of New Zealand. As part of the analysis, the standard deviation of the noise in each 1/3‐ octave band between 10–500 Hz was computed. The effect of receiver depth, wind speed, and frequency was examined. There is not a marked dependence of standard deviation on receiver depth. When no shipping was present, the standard deviation was less than 2.5 dB at all frequencies and decreased to under 0.1 dB above 100 Hz. When shipping dominated, the standard deviation was much higher below 100 Hz, being 5.5–7 dB at 10 Hz but decreasing to less than 2 dB above 200 Hz. This behavior is explained by a model in which a single ship dominates. A comparison with the results from other ocean areas indicates three regimes for ship dominated noise: (1) single‐ship dominated areas where the standard deviation decreases with frequency; (2) distant multiple‐ship dominated areas where the standard deviation increases with frequency; and (3) local multiple‐ship dominated areas where the standard deviation is almost independent of frequency.

ISSN:0001-4966    

DOI:10.1121/1.382782

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP