摘要:

In a previous paper, we solved the problem of propagation of a plane wave in an anisotropic random medium. The characteristic correlation distances in the horizontal propagation direction were assumed to be large compared to the radiation wavelength divided by 2π, (λ̄/2π), while in the vertical direction, these distances were, roughly speaking, assumed to be of the same order or smaller than λ̄/2π. For this medium, we derive here the equation governing the propagation of the coherence function for a finite beam or source of characteristic dimensiona. This equation is then averaged over the vertical direction and solved. The solution is given in terms of a definite integral over the horizontal spectrum of the index‐of‐refraction field. The point source solution (a→ 0) and the plane‐wave case (a→ ∞) are given as special cases. Explicit solutions are given for the propagation of an acoustic field in the ocean under conditions for which the spectrum satisfies a minus two power law.

ISSN:0001-4966    

DOI:10.1121/1.1903496

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Ray propagation has been analyzed coherently for a moving‐source‐fixed‐receiver system by taking into account source motion from the results of a fixed model in which spreading loss, travel time, boundary losses, and boundary phase shifts are calculated. Specific examples of the amplitude and phase time series are given for a source moving at 5 knots from 500 to 520 km radially from the receiver. These time series are given for sampling intervals of 1 min and in shorter sections for 0.2 sec, indicating the effects on phase of the source motion. The data clearly indicate the effect of multipath and the usefulness of this technique in the study of spatial variations in the medium. It is demonstrated that multipath interference due to source motion in the medium is a major contributor to transmission fluctuations in both phase and amplitude.

ISSN:0001-4966    

DOI:10.1121/1.1903497

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Elastodynamic stress intensity factors are computed for diffraction of antiplane shear waves by a crack in a layered composite. The crack is normal to and bisected by the midplane of the layer. Both cases of a partially broken layer and a completely broken layer are studied. Integral transform techniques are used to formulate the problem as a singular integral equation. The propagation of symmetric modes (Love waves) is studied numerically, and stress intensity factors are obtained for several values of frequency, geometrical parameters, and material constants. The amplitude ratios of the incident waves to incident plus scattered waves at a large distance from the crack are also calculated.

ISSN:0001-4966    

DOI:10.1121/1.1903498

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The problem of sound scattering from a time‐varying irregular free surface is treated anew. The wave equation with its complicated surface boundary condition is transformed to another partial differential equation with a simple, perfectly reflecting, boundary condition. An integral equation for the Green's functionG′ of this latter equation is found which admits a Neumann series solution. The Green's functionGfor the original wave equation is determined fromG′ via a simple transformation. The Green's functionGis explicitly calculated to second order in the surface wave height. A plane‐wave scattering formalism is developed in which scattered‐wave amplitudes are expressed in terms ofG′. The differential cross section for surface scattering is expressed in terms of the scattered‐wave amplitudes and is explicitly calculated to second order in the wave height and expressed in terms of the space‐time covariance of the irregular surface. With suitable additional approximations, the autocovariance and power spectrum are computed to second order for an acoustic signal generated from a fixed sinusoidal source. The results are expressed in terms of previously derived plane‐wave quantities, the differential cross section, and forwardscattering amplitude. The perturbation calculations are valid provided the rms wave height is small compared to the principal wavelength of sound used.

ISSN:0001-4966    

DOI:10.1121/1.1903499

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Integral equations describing the multiple‐scattering effects of waves propagating in a discrete random medium are solved using Fourier transform techniques. The average intensity in the medium for a given source radiation is expressed in terms of a Green's function which consists of residue contribution at a pole representing diffusion and a branch cut integration pertaining to nearfield. It is shown that for a plane wave, multiple scattering becomes significant when the particles are mostly scattering (albedo,b>0.5) for all distances greater than a fraction of an optical depthZ. When the particles are mostly absorbing, multiple scattering will not take effect until at leastZ>2. For a spherical wave, the multiple‐scattering influence does not become significant until several optical depths, except when the scatterers are extremely low loss (b→ 1).

ISSN:0001-4966    

DOI:10.1121/1.1903500

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The general three‐dimensional high‐frequency oscillatory flow near a stagnation point is investigated through the method of matched asymptotic expansions. The nonlinear Reynolds's stresses create a steady streaming motion in the fluid. It is found that this streaming is minimized when the surface near the stagnation point is axisymmetric and is maximized when the surface is a saddle.

ISSN:0001-4966    

DOI:10.1121/1.1903501

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The cross section for the scattering of low‐frequency (∼ 100 Hz) sound by an aircraft trailing vortex is computed using the Born approximation. The predicted cross section is large enough to permit size and strength measurements. Although precise location of a vortex is better done with high‐frequency refraction, a monitoring system based on low‐frequency acoustic radar is possible, and a system combining the best features of each is also feasible.

ISSN:0001-4966    

DOI:10.1121/1.1903502

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

An experiment was conducted on a full‐scale jet engine to investigate the aerodynamic noise generating regions in the free jet. Cross‐correlation measurements were made between the static pressure fluctuations and the farfield radiated sound. These measurements were made for two different static pressure probe positions and a large number of farfield positions (at various angles). In addition, each test geometry was run for four different jet exit velocities. The measured, normalized cross‐correlation functions varied between 0.004 and 0.155. A newQ‐function, based on the above normalized cross correlation is defined and plotted. This function represents the source strength per unit volume within the jet region. ThisQ‐function shows dependence on the probe position, the angular position of the farfield microphone, and the jet exit Mach number. Third‐octave analyses of both the probe signal and the farfield radiated sound were made. The results show that cross‐correlation techniques are a valuable tool in the investigation of the aerodynamic noise generating regions of an actual jet engine.

ISSN:0001-4966    

DOI:10.1121/1.1903503

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The acoustic properties of deep scattering layers were examined using explosive sources at a few hundred sites in the main basins of the North and South Atlantic and the North and South Pacific, in the Labrador, Norwegian, Mediterranean, and Caribbean Seas, and in Baffin Bay. Representative day and night spectra of column strength are presented for these areas in the frequency band 0.8–25.6 kHz along with estimates of the effective radii of the swimbladders of the dominant scatterers. The spectra tend to fall into homogeneous groups, with the most pronounced changes in type taking place near known faunal boundaries. In mid‐ocean, in the North and South Atlantic and the South Pacific, scattering is high near the equator and at mid‐latitudes, with considerably lower values in between. Very low scattering strengths are also found in the cold waters at high latitudes in both hemispheres. In a number of cold water areas in the northern hemisphere there is a pronounced peak in sound scattering at low frequencies, caused by scatterers with effective radii of swimbladders of 1 cm or greater. As some of these relatively large scatterers are probably commercial fish, it appears that a measurement technique similar to that used in this investigation would be a useful tool in resource management of commercial fisheries.

ISSN:0001-4966    

DOI:10.1121/1.1903504

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The USRD type E8 transducer was operated as an acoustic parametric source, and its performance as such was measured in two separate laboratory experiments and at sea. The parametric source was driven by a primary signal that consisted either of two high‐frequency components or of an amplitude‐modulated high‐frequency carrier. The primary signal was centered at the transducer resonance frequency near 1.4 MHz, and the observed difference frequencies ranged from 20 to 200 kHz. During typical operation the primary electrical signal had a total power of approximately 6 W, and a difference‐frequency signal at 50 kHz had a source level of about 154 dBre1 μPa m. This paper describes the design and performance of the parametric source. It also presents a sequence of radiation patterns and values of beamwidth that illustrate difference‐frequency beam formation in the nearfield of the parametric source.

ISSN:0001-4966    

DOI:10.1121/1.1903505

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP