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Nov 1975

Volume 58, Issue S1, pp. S2-S132

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back to top Session FFF. Underwater Acoustics VIII: Ambient Noise
Contributed Papers
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Statistical characteristics of underwater ambient noise fields (A)

W. J. Jobst and S. L. Adams

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S120-S120 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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A rigorous study of the statistical characteristics of ambient noise fields is required for the design of signal processing equipment. In this paper. we present preliminary data on the basic structure of underwater acoustic ambient noise fields. The behavior of noise power as a function of time, frequency, and space has been analyzed for several sets of field data at a broad range of acoustic frequencies. Results of a runs test for independence of samples, Kolmogorov‐Smirnov stationarity tests, and Kolmogorov‐Smirnov goodness of fit tests are presented for narrow‐band noise. New results describing the broad‐band behavior of the ambient noise process are presented. [Research supported by Office of Naval Research. Code 222.]
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Infrasonic sea noise at the Mid‐Atlantic Ridge near 37 °N (A)

J. R. McGrath

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S120-S121 (1975); (2 pages)

Online Publication Date: 11 Aug 2005

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Underwater ambient noise spectra, spectrum levels, and statistics are described for data continuously recorded in the 5–32‐Hz band for 176 h. These measurements were made using two ocean bottom seismographs located in 2478 m of water within the seismically active median valley of the Mid‐Atlantic Ridge near 37 °N. Laboratory tests of the H‐38 hydrophone show that flow noise levels developed by a 0.3 knot current—the maximum speed anticipated in the median valley—did not exceed the electronic noise level of the recording instrument. Over 500 underwater earthquakes were detected and epicenters were estimated to lie within about 20 km of the instruments. Seismic disturbances raised noise levels at least 20 dB across the acoustic band, varied in local magnitude from 1.2 to 2.5, persisted for as long as 2 min, and occurred on the average of 2.9 events/h. Sound pressure spectrum levels averaged over the entire recording period and compared to shipping noise curves proposed by Wenz and Urick infer that infrasonic and low‐frequency noise are dominated by distant shipping traffic noise. [This work was sponsored by Naval Electronics Systems Command, Code 320.]
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Seaquakes at the Mid‐Atlantic Ridge near 37 °N (A)

T. J. G. Francis

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S121-S121 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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Joint acoustic and seismic observations have been made at the intersection of a fracture zone and the Mid‐Atlantic Ridge near 37 °N over a period of several days. Over 500 seaquakes were detected; their epicenters were located by using S and P wave arrival times recorded by two ocean bottom seismometers. The analysis and discussion of seismic data, and their interpretation and application to a geophysical model of the region, indicates the rift valley to be an unusually low velocity region.
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Summertime acoustic conditions in Baffin Bay (A)

Harold M. Merklinger

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S121-S121 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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Noise measurements made in Baffin Bay during the summers of 1972 and 1973 indicated that noise levels were generally higher than in the North Atlantic. Two distinct types of ice‐related noise were identified. Loud crashes having a roughly RC‐shaped spectrum were observed to result from the rolling and breakup of icebergs. A steady, white noise was observed in the presence of ice debris. Attempts to relate steady noises with stable icebergs failed. During both years seismic survey operations contributed significantly to the average noise levels at low frequencies.
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Measurement of the ambient noise horizontal directionality in the ocean with a circular superdirective array (A)

J. S. Bird

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S121-S121 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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This paper describes the degree to which the horizontal directivity of the ambient noise in the ocean may be characterized using a six‐element circular superdirective array and a pressure‐gradient beamform technique. The approach used is to model the noise field as a Fourier series and to consider the array as acting like a low‐pass filter in the spatial frequency domain. The effect of the beam pattern on the noise field is calculated exactly and a method developed to remove this effect, except for bandlimiting in the spatial frequency domain. A method to reconstruct a time‐averaged representation of the noise field that is continuous in the horizontal plane even though the array used a discrete number of beams is developed. Using field data, it is shown that the magnitude and phase of the noise field coefficients may be used to make conclusions about noise mechanisms and to indicate the presence of discrete noise sources.
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Ambient noise vertical directionality—a useful empirical model (A)

J. H. Stockhausen

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S121-S121 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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Descriptions of the noise vertical directionality which transform into simple spatial correlation functions are particularly useful because directionality measurements are conveniently made in the spatial correlation domain. A useful model for the horizontally concentrated distant shipping noise is the function exp[−(cosθ/2ζ)2], where θ is the angle with the vertical and ζ is a parameter‐controlling beamwidth. For typical beamwidths this expression transforms with negligible error to exp[−(ζkd)2] for the spatial correlation function. Depression or elevation respect to the horizontal can be handled with simple elaboration. Several authors have described the directionality of the noise deriving from surface agitation in terms of cosθ and cos°θ (isotropic), both of which have closed‐form transforms In kd space. These three expressions are used in combination to characterize the noise field in the Labrador Sea as measured in three different wind conditions, from frequencies at which shipping noise dominates through the transition region to frequencies at which surface noise is the major factor.
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Ambient noise vertical directionality—a numerical model (A)

J. H. Stockhausen

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S121-S121 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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An interactive computer model for generating vertical noise directionality has been developed as an extension of Talham's method to allow unbottomed sensors. Essential features are: a sound speed profile consisting of three linear gradient segments, a selection of bottom types, and provision for volume attenuation and surface reflection loss. The noise directionality is first computed and then numerically transformed into the spatial correlation function applicable to a vertical array. A uniform surface distribution of sources is used, radiating power as cos2θ, where θ is the angle of incidence. It is shown that propagation conditions account for as much variation in noise directionality as there is between the fields resulting from cos2θ and cosθ sources in a much simpler propagation model.
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Ambient noise in the surface duct (A)

R. J. Urick

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S121-S121 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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On a number of occasions, and with a variety of instrumentation, the ambient noise level in an area north of St. Croix, VI, has been found to be higher, by amounts up to 8 dB, than at depths below the duct. The difference occurs at the frequencies greater than a few hundred hertz that are trapped in a 120–180‐ft duct. Other data in the form of time‐delay correlograms obtained with a vertical string of hydrophones indicate that the excess noise arrives at a receiver from near‐horizontal directions. It follows that a vertical array used in the surface duct would have a higher noise background and a lower array gain than when placed below the duct. A likely hypothesis is that the excess noise in the duct represents noise from distant near‐surface sources trapped by, and traveling within, the duct. This occurrence of duct noise may merely represent some peculiarity of the area where it was observed; on the other hand, it may well be a phenomen of widespread, though apparently hitherto unreported, occurrence wherever a well‐developed surface duct exists.
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The contribution of the sea urchin to ambient sea noise (A)

M. J. Castle and A. C. Kibblewhite

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S122-S122 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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An unusual component of the ambient sea noise field of coastal New Zealand waters has been recognized for some years. The phenomenon involves an increase in noise level at sunset and sunrise, predominantly in the range of 1 to 2 kHz, with seasonal trends varying the pattern. The effect was reported first in 1962 and tentatively attributed to the seaurchin Evechinus chloroticus Valenciennes (1846). A continuous study in the shallow waters of the Hauraki Gulf throughout 1973 has confirmed this hypothesis. The results of this study will be presented and the characteristics of the effect reviewed.
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World shipping distributions (A)

Donald Ross, John Mahler, and Louis Solomon

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S122-S122 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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Ocean noise in the lower frequencies (25–250 Hz) is due principally to surface shipping. In the absence of extremely close sources, this is due to many distant ships scattered throughout the acoustic basin. Merchant and fishing vessel distribution is presented for the North Atlantic and North Pacific Ocean between the equator and 70 °N. The information is presented for 5° squares with method for achieving distribution in 1° squares. The distribution applies to the time period 1972–1973 but may also be used for periods within two to three years of these dates, requiring only scaling of the total numbers. The shipping distribution presented herein is a coordinated product of three independent studies and represents the most current information available.
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