摘要:

Sonobuoy data in a number of shelf areas have yielded travel time curves which are relatively smooth and which result in velocity–depth profiles that are nearly straight lines to depths of several kilometers. Moreover, many of the records contain several multiples of the first refracted arrival which are well separated from the first arrival and free of excessive noise. Such data are ideal for comparing the amplitude of the first arrival and the amplitude at the corresponding point on the multiple at double distance and double time. By taking the ratio of these amplitudes, the effects of geometrical spreading and the angular dependence of the source are removed so that estimates of total attenuation as a function of depth and frequency are much more easily studied. In this paper we present an example of such a study using sonobuoy data from the Sarawak basin which is located in the South China Sea (4 °N–110 °E). Measurements made in the frequency range of 10 to 60 Hz suggest that the logarithmic decrement δ or the quality factorQare not constants but depend rather strongly on frequency in the diving waves that were studied.

ISSN:0001-4966    

DOI:10.1121/1.388834

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The effect of a range‐dependent environment, typified by fronts or eddies, or convergence zone intensities is investigated. An analytical approach is taken to examine vertical redistribution of acoustic energy resulting from explicit changes in sound channel structure and from implicit effects of horizontal sound‐speed gradients. Under certain oceanographic conditions, such a redistribution is shown to lead to catastrophic signal losses at all ranges for receivers at fixed depths. A generalized Snell’s law in a range‐dependent environment is derived and then utilized to identify regions where such losses may occur. It is shown that large changes in transmission result only when horizontal sound‐speed changes evaluated over a ray path are not small compared with the amount of sound‐speed depth excess. Several examples are presented illustrating the theory using parabolic equation numerical simulations. A method is also presented to predict expected signal losses as a function of source position in an eddy or front.

ISSN:0001-4966    

DOI:10.1121/1.388835

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The facet‐ensemble method is used to compute the complex field scattered by a corrugated surface with large roughness. The method employs in part the frequency transform of an asymptotic approximation to the exact impulse solution for diffraction from a rigid (or pressure release, if desired) ridge or trough [M. A. Biot and I. Tolstoy, J. Acoust. Soc. Am.29, 381–391 (1957); A. D. Pierce,Acoustics(McGraw–Hill, New York, 1981), pp. 489–490.] In the method, the scattering surface is approximated by joining edge to edge long plane strips (facets). Each adjacent pair of facets makes up a ridge or trough. Theoretical scattered acoustic pressure amplitude values are then obtained by superposing the diffracted and reflected contributions from individual ridges and troughs. A similar method was introduced by Novarini and Medwin [J. C. Novarini and H. Medwin, J. Acosut. Soc. Am.64, 260–268 (1978); H. Medwin and J. C. Novarini, J. Acoust. Soc. Am.69,108–111 (1981)]. A comparison is provided here between amplitude values measured in a water‐tank, scattering experiment [G. A. Sandness, Ph.D. thesis, Univ. of Wisconsin, Madison (1971)]and values predicted using the facet‐ensemble method. Agreement is good in general and remains so when the number of approximating facets per spatial wavelength of the surface is changed. Also provided is a comparison (for the experimental geometry) between the facet‐ensemble method and numerical evaluation of the Helmholtz–Kirchhoff integral. Most of the scattering occurred near the normal direction, and the Helmholtz–Kirchhoff integral is accurate for that direction. Agreement in this case between the integral solution and the facet‐ensemble method, therefore, is good. The facet‐ensemble method shows promise for estimating accurately the complex field scattered from a rough rigid (or pressure release) surface for any number of receivers at any number of locations.

ISSN:0001-4966    

DOI:10.1121/1.388836

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Selected surface reverberation and bottom reverberation returns were used to compute estimates of the probability density function of the instantaneous reverberation. To estimate the densities, 6500 samples of surface reverberation and 3078 samples of bottom reverberation were used. The collections of samples were tested for randomness, independence, homogeneity, and normality. Both the surface and bottom reverberation were found to be non‐Gaussian. Kernel density estimation techniques were applied to the collections of samples to provide univariate estimates of the densities. The densities were seen to be nearly Gaussian, but with heavier tails. Heavier tailed densities generally result in higher false alarm rates for detectors designed for a Gaussian noise process.

ISSN:0001-4966    

DOI:10.1121/1.388837

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Recordings of ambient sea noise from four deep water sites between Australia and New Zealand have been examined for correlation between the narrow‐band spectrum level and the observed local wind speed. Significant wind dependence is observed at all frequencies between 37 and 800 Hz. When the results are reprocessed to examine only locally generated noise (horizontally arriving sound from distant sources is rejected by making use of the directional properties of a steerable vertical hydrophone array), it is found that the wind dependence of the observed noise at low frequencies is significantly enhanced. The rate of increase of locally generated noise with wind speed is very similar to published values for Australian waters, except for frequencies below 100 Hz, and much lower than most reported measurements in northern hemisphere waters. By steering upward‐looking beams, the levels of sound radiated (per steradian) from the surface, at various wind speeds, were measured. The measurements were adjusted, where necessary, to account for variations in bottom loss at different sites, using measured normal incidence bottom reflection losses. At the higher wind speeds the spectrum of the estimated surface source level appears to be almost flat across the band of frequencies observed, which is a feature of recent published results from other oceans. At lower wind speeds the results rise with decreasing frequency below 100 Hz suggesting the addition of noise from a different source which has not been identified.

ISSN:0001-4966    

DOI:10.1121/1.388840

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The Wilson wind‐generated source level densities and the ASTRAL propagation loss model are combined to form a wind‐generated noise model. The model treats storms as a geographically distributed source and is capable of modeling both distant and local storm noise. Model estimates are compared with omnidirectional measurements from the northeastern Pacific Ocean at five frequencies and three receiver depths. The results are very good and indicate the necessity of modeling both shipping and wind‐generated noise in comparisons with measured data.

ISSN:0001-4966    

DOI:10.1121/1.388841

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A modified Sell‐type ultrasonic transducer, utilizing Teflon materials and the hard‐anodization process for the aluminum back plates, shows significant improvement in operating characteristics over earlier models. The vibrational self‐relaxation times of ClCN ( pτ=29 ns bar) and BrCN ( pτ=15 ns bar) are measured for the first time. The results are discussed in relation to the empirical Lambert–Salter plot for V–R,T energy transfer.

ISSN:0001-4966    

DOI:10.1121/1.388851

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The interaction of pulsed carbon dioxide laser radiation with the free surface of water gives rise to shock waves in the water and in the air above the surface. In addition a train of high‐frequency, ∼20 MHz, oscillations in the water have been observed using ultra‐high‐speed photography by means of a nitrogen laser with a pulse duration of ∼1 ns. Confirmation that the oscillations are not due to diffraction effects in the shadowgraphs has been obtained by direct pressure transducer measurements in the water.

ISSN:0001-4966    

DOI:10.1121/1.388852

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

When subjected to alternating quiet nights (32 dB) and noise nights (equivalent levels of 47 dB) a group of 14 subjects showed an average increase in the fraction of deep sleep of about 2.5% resulting from the traffic noise. Another group of 12 subjects whose noise nights were at 60 dB had an average of 4.6% increase in deep sleep during these nights. The number of wakings also increased for both groups but, as was found before, this adapted rapidly with the number of nights. The average latency of sleep onset does not appear to be affected by the traffice noise but individual differences are great and may be of opposite sign. Latency of sleep onset and waking both show appreciable ‘‘laboratory effect’’ which takes longer to disappear than the one or two nights usually assumed.

ISSN:0001-4966    

DOI:10.1121/1.388853

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Certain techniques of adaptive filtration, time delay estimation, and minimum variance parameter estimation were synthesized into a hybrid adaptive filtration technique and applied to a problem in seismic event location. Processing seismic wave data generated by strong impulsive source and recorded by a geophone array, we succeeded in doubling the range of detection previously achieved by conventional beamforming methods. The degree of accuracy of bearing estimation was also improved significantly. Additionally a method of observing the nature of the roots of the AR process can replace the conventional power spectrum method for observing wave fronts. The root migration was found to be very sensitive to signal structure.

ISSN:0001-4966    

DOI:10.1121/1.388854

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP