摘要:

The acoustic radiation produced by baffled flat strip‐shaped sources of infinite length is treated in detail. The nearfield intensity distribution, nearfield to farfield transition, and cylindrical farfield radiation is calculated for homogeneous radiator surface excitation; such beams are shown to partially focus at their last maximum of intensity. Farfield radiation is calculated for radiators with a Gaussian surface velocity distribution, for shaded transducers, and for arrays of strip‐shaped sources. The use of such radiators in Doppler flow measuring systems is discussed.

ISSN:0001-4966    

DOI:10.1121/1.382493

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Continuing our previous work on viscoelastic scattering ofpwaves from fluid‐filled spherical cavities in sound‐absorbing materials, we now perform the theoretical analysis needed for the complete study of the monopole mode of vibration. Our approach is based on the Breit–Wigner formulation of nuclear scattering theory, and from it we show how the scattering amplitudes consist of two parts, the smooth background of the evacuated cavity and the resonance contribution of the filler fluid. We present a complete analytical study of the giant fundamental resonance and its overtones in the monopole case. The results are then particularized to the Rayleigh region and also to the case of evacuated cavities in lossless rubber and to gas bubbles in water, recovering and extending previous results available in the literature. Studying the sound pressure level at the center of air‐filled cavities in lossy materials, we quantitatively determine how absorption shifts the resonance peaks and broadens their width. Using our theory we find that the width of the resonance peaks depends linearly on the resonance frequencies, and we show how this important result can be used to determine the shear absorption parameter of viscoelastic materials. We also find that thegiantmonopoleresonance for air‐filled cavities in rubber contains a contribution from the shear waves in the cavity wall and another from the resonance oscillations of the air‐filling in a 15:1 ratio, for this particular material combination. We display numerous plots, some obtained from the analytic monopole formulas developed here, and some numerically obtained by computer for dipoles and quadrupoles. Finally, we show graphs of the summed scattering amplitudesfppandfpsaccounting for the first twenty multipole contributions, and the dominance of the monopole term emerges again from this result. We have assumed ’’weak’’ absorption to simplify the analytical treatment, but any viscosity level can be handled by computer using this method.

ISSN:0001-4966    

DOI:10.1121/1.382494

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

High‐frequency propagation in waveguides or ducts can be formulated in terms of a guided mode or a ray expansion. When the duct is very wide, the excessively large number of modes or rays often necessitates the truncation of the resulting mode or ray series. This paper examines the truncation problem and shows that a hybrid formulation, in terms of a properly chosen number of rays and guided modes, can account for the remainder field in either case. The hybrid field representation has appealing physical content in that a few of the lowest modes account for the collective behavior of rays with many reflections, while a few of the lower‐order rays account for the collective behavior of the higher‐order guided modes. Other field representations involving rays, modes, a canonical integral, a continuous spectrum, and nearfield perturbation are also examined. Numerical comparisons for a specific example show the utility and range of validity of these alternative representations.

ISSN:0001-4966    

DOI:10.1121/1.382495

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Those results of the theory of the baffled, uniform‐piston radiator that can be calculated exactly are extended to some other cases, especially the simplest case of a simply supported radiator, the simplest case of a clamped‐edge radiator and a Gaussian radiator. It is also shown that from the solution to a problem with boundary conditions framed in terms of velocity, the solution to a corresponding problem, having boundary conditons framed in terms of pressure, can be obtained very easily.

ISSN:0001-4966    

DOI:10.1121/1.382496

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Theoretical estimates of space, time, and frequency coherence are obtained for a sound field generated by a moving source and observed at a stationary array of sensors. It is shown that spatial coherence depends principally upon the spread of ray‐path‐arrival angles at the receiving sensors and on the orientation of the receiving array to the sound field. Temporal coherence depends upon the ray‐path angle spread at the source and on source motion. Oscillatory source motion introduces frequency modulation of the transmitted signal with sideband levels related to projector displacement amplitude. Frequency coherence depends upon the time spread between arrivals which propagate over different acoustic paths. Examples of theoretical coherence functions are presented for multipath channels of interest in underwater acoustics.

ISSN:0001-4966    

DOI:10.1121/1.382497

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

A general analytical method for determining the radiated sound fields from axisymmetric surfaces of arbitrary cross section with general boundary conditions is developed. The method is based on an integral representation for the external solutions of the Helmholtz equation. An integral equation is developed governing the surface potential distribution which gives unique solutions at all wavenumbers. The axisymmetric formulation of the problem reduces its solution to the numerical evaluation of line integrals by Gaussian quadrature. The applicability of the solution approach for both a sphere and finite cylinder is demontrated by comparing the numerical results with exact analytical solutions for both discontinuous and continuous boundary conditions. The method is then applied to a jet‐engine‐inlet configuration and the computed results are in good agreement with exact values.

ISSN:0001-4966    

DOI:10.1121/1.382498

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

The velocity potential impulse response function is computed for a cylindrical piston in a compliant baffle. It is shown that, in general, the impulse response is that of a piston in a noncompliant baffle, plus a small wake, or reverberation, which dies out in a time the order of the transit time of the wave to the point in space. The slow decay of the wake means that the energy in the wake from a short pulse will be considerable fraction of the total energy. The solution is obtained in closed from from a baffle which matches the fluid impedance. Second sound heat pulse propagation in helium II obeys the scalar wave equation and it will have a second sound heat pulse impulse response which is isomorphous with that of the pressure wave in a fluid. It is pointed out that the analyses of the transient electromagnetic field of a vertical dipole antenna above a conducting earth, and the seismic transients in a layered earth can be of use in discussing the acoustic problem explored here.

ISSN:0001-4966    

DOI:10.1121/1.382499

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

Asymptotic results are obtained for the first compressional head wave arrival in a fluid‐filled borehole with a modified asymptotic technique. The results are verified with direct numerical integrations and shown to be quite different from those obtained with the ordinary asymptotic techniques. The modified asymptotic technique accounts for the effect of an extraneous pole, which is neglected in the ordinary asymptotic method. The basic assumption in the modified asymptotic method is that the observation point must be far away from the source point, whereas in the ordinary asymptotic technique, the frequency is assumed to be large. The pressure responses of the first compressional arrival are illustrated and compared. It is found that the amplitudes of the compressional arrivals are strongly dependent on the Poisson’s ratio and the compressional velocity of the formation.

ISSN:0001-4966    

DOI:10.1121/1.382500

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

We analyze the two‐dimensional problem of multiple scattering by randomly distributed and oriented scatterers, and compare the results with those for the aligned scatterers. Waterman’sT‐matrix approach in conjunction with the statistical averaging for both position and orientation are employed to obtain the phase velocity and attenuation due to geometric dispersion for a wide range of frequencies. Analytical expressions for the dispersion relation are also derived at low frequencies for both randomly distributed and oriented inclusions and cracks.

ISSN:0001-4966    

DOI:10.1121/1.382419

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP

摘要:

This paper describes a study of the absorption of sound by pine trees. Swept frequency measurements were made with small boughs in a reverberant box. Tests for branch and needle resonances were also made. The observed absorption is comparable to the expected thermoviscous absorption in the boundary layer of air that surrounds the individual needles.

ISSN:0001-4966    

DOI:10.1121/1.382475

出版商:Acoustical Society of America    

年代:1979

数据来源: AIP