摘要:

An integral expression for the spatial coherence function of pure‐tone sound fields is formulated under hypotheses that the component fields of individual paths or modes are locally plane waves and mutually incoherent in the average. The total field is not assumed to be a Gaussian random variable. The resulting coherence is quasi or locally homogeneous, varying slowly with average position as the strength of the component fields change. Statistical estimates for the contributions of these components are taken from existing literature on average transmission loss. The expression for coherence has an explicit dependence on the environmental parameters, the range, and the depths of source and receivers. Numerical results and approximate expressions are developed for channels characteristic of shallow and deep water.Subject Classification: [43]20.15; [43]60.20.

ISSN:0001-4966    

DOI:10.1121/1.381106

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Acoustic parametric convolvers operate by spatially integrating a term produced by the nonlinear interaction of two propagating strain waves. The integrand will have an additional spatially dependent weighting factor and inaccuracies in the convolution output will result if the product term is not uniformly produced and summed. Under appropriate assumptions, the expansion of the acoustic field into an angular spectrum of plane waves is used to show that no erroneous weighting is introduced by diffraction, although diffraction does contribute a uniform loss to the convolver output. Under additional assumptions, the diffraction loss for the convolved output is shown to equal that which occurs in transmitting from one transducer to the other.Subject Classification: [43]25.30, [43]25.35; [43]35.54; [43]60.30.

ISSN:0001-4966    

DOI:10.1121/1.381107

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Analysis of the ambient‐noise horizontal directionality data from a recent environmental acoustic exercise indicates that wind‐generated noise at 10 Hz is spatially correlated over large apertures. The noise levels measured by a large aperture, linear towed array were highest near broadside during high‐wind conditions. Spatially correlated wind‐generated noise arriving from the sea surface directly above the array is offered as a possible explanation and is consistent with results of previous wind‐noise experiments. In the limiting case of very high wind speeds (?35 knots), the array response measured for this nearfield effect closely approximated the farfield broadside beam pattern at 10 Hz. At lower wind speed, the array response was wider near broadside. This implies that the spatial correlation of wind‐generated noise at 10 Hz becomes greater as the wind speed increases.Subject Classification: [43]28.20, [43]28.65; [43]50.80.

ISSN:0001-4966    

DOI:10.1121/1.381108

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Ambient sea noise has been measured at 40 positions in mainly tropical waters near Australia: in the Indian Ocean; the Arafura and Timor Seas; and in the Coral and Tasman Seas (Pacific Ocean). Wind‐speed dependence was observed at all frequencies of measurement from 22 to 5000 Hz. At hydrophones suspended at depths between 8 and 25 m, the wind‐ dependent noise appeared to be independent of bottom depth which varied from 26 to 6700 m. The rate at which wind‐dependent noise increased with increasing wind speed was found to be significantly less than has been observed in the North Atlantic Ocean. Non‐wind‐dependent noise, observed at frequencies below 250 Hz, varied by about 10 dB within a particular sea and by up to 20 dB between seas, and could be broadly related to the relative shipping densities and propagation conditions in the individual seas. In the shallow Arafura and Timor Seas north of Australia, there is evidence that noise of distant shipping is not a significant component of the ambient noise. A model of traffic noise in the Tasman Sea predicts levels in agreement with observed noise levels.Subject Classification: [43]30.70.

ISSN:0001-4966    

DOI:10.1121/1.381109

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The use of phase measurement for sinusoidal signal arrival‐time extraction of overlapping signals subjected to multipath conditions in shallow water is considered. The phase‐measurement method is applied to signal data from a research cruise. The method does not give convincing results. For distances greater than 10 000 m the results are negative. The results obtained are strongly dependent on the sound velocity profile. The signal pulse length was a very important parameter in the results. The optimal pulse length was shown to be greater than 10 msec and less than 100 msec regardless of the hydrophone depth for the two depths investigated.Subject Classification: [43]30.20; [43]60.30.

ISSN:0001-4966    

DOI:10.1121/1.381110

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The objectives of this report are to review selected published measurements of the attenuation, or energy damping, of low‐strain shear waves in surficial water‐saturated sands and silt clays (mud) which might occur as marine sediments. In various computations, a linear viscoelastic model is favored in which velocity dispersion is negligible, the linear attenuation is proportional to the first power of frequency, and the specific dissipation function, 1/Q, and the logarithmic decrement are independent of frequency. The logarithmic decrement is favored as a measure of energy damping because of research in soil mechanics. The very sparse data indicate that in water‐saturated sands and silt clays, the logarithmic decrements are mostly between 0.1 and 0.6. If approximate values of shear wave energy losses are required for generalized computations, it is suggested that a value for the logarithmic decrement of 0.30±0.15 be assumed for sands, and 0.2±0.1 for silt clays. Measured logarithmic decrements of compressional waves in sands average about 0.10±0.03; in silt clays about 0.02±0.01. The average values of the ratio of compressional‐to‐shear‐wave logarithmic decrements, using the above average values, would be 0.3 for sands and 0.1 for silt clays.Subject Classification: [43]30.20; [43]40.50, [43]30.30.

ISSN:0001-4966    

DOI:10.1121/1.381111

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The problem of the transmission of sound from an airborne source into the ocean, through the rough ocean surface, is considered. A high‐frequency approximation is adopted, requiring for its validity that the sound wavelength in air be (much) less than typical radii of curvature and than typical displacements of the ocean surface. This condition is approximately met in many applications. Attention is focused on observation directions with small grazing angles. The result is approximately unaffected by surface roughness when the grazing angle is greater than the rms surface slope, and approaches a limit (proportional to the variance of the surface slope) when the grazing angle is less than the slope. The result is independent of the source height, under the stated approximations, as is also the case for the transmission through a plane surface. The enhancement due to surface roughness, even at moderate wind speed, ranges up to 10 dB.Subject Classification: [43]30.25.

ISSN:0001-4966    

DOI:10.1121/1.381112

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

We discuss two aspects of the general problem of sound propagation in an ocean characterized by a range‐dependent profile. We first consider a normal‐mode formalism for describing a situation in which the eikonal approximation can be applied in the horizontal plane. The difficulty with this approach is shown to be an ordering in the geometric optics path‐length parameter due to mode coupling. By example we indicate how this difficulty can be overcome. We then use the formalism to discuss the approximations which make up the parabolic equation method. The parabolic equation is derived by assuming the eikonal approximation in the horizontal plane and a degenerate spectrum approximation. The degeneracy approximation is reflected in a stationary phase approximation to an integral representation for the pressure field. We then derive improved equations by relaxing these assumptions. We relax the degeneracy assumption by considering an improved stationary‐phase approximation and we relax the eikonal assumption by considering the ’’supereikonal’’ approximation.Subject Classification: [43]30.20, [43]30.25, [43]30.50.

ISSN:0001-4966    

DOI:10.1121/1.381113

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

A hydrodynamic model for flows in the deep ocean is developed in order to determine the velocity field and sound‐speed distribution for use in acoustic transmission problems. A scaling of the governing equations is constructed that explicitly includes sound speed. A subsequent perturbation expansion yields a set of approximate equations for motions nearly in geostrophic and hydrostatic balance, such as large‐scale, quasisteady currents and Rossby waves. The quasigeostrophic potential vorticity equation or a simpler limiting case of this equation arises from the perturbation scheme to govern higher‐order dynamics of the stream function for these flows. The results of the analysis are used to obtain a significant simplification of the ray equations of geometrical acoustics for moving media. For the particular class of flows considered here, the model equations are applicable if the ocean depth is about 1 km or greater and if the spatial and temporal scales of variation of the motions are of the order of 100 km and 10 days, respectively. A solution for a flow such as the Antilles current is derived. Isospeed curves for this solution are shown in a plane perpendicular to the current, and specific features of the curves are discussed and interpreted.Subject Classification: [43]30.25, [43]30.20; [43]28.60.

ISSN:0001-4966    

DOI:10.1121/1.381090

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Two types of electromagnetic ultrasonic transducers were made and tested on mild steel, stainless steel, and aluminum. One of the transducers generates and detects a relatively large amount of longitudinal wave and simultaneously a small amount of radially polarized transverse wave in nonmagnetic metal. The other is for a large amount of radially polarized transverse wave and a small amount of longitudinal wave, also in nonmagnetic metal. Both transducers showed different characteristics for ferromagnetic metal, giving greater efficiency to the radially polarized transverse wave. It was also shown experimentally with an electromagnetic transducer that the mode conversion between longitudinal waves and radially polarized transverse waves occurs fairly efficiently at a vertical reflection.Subject Classification: [43]35.60, [43]35.80; [43]85.40, [43]85.48.

ISSN:0001-4966    

DOI:10.1121/1.381091

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP