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Dec 1977

Volume 62, Issue S1, pp. S1-S102

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back to top Session Q. Underwater Acoustics III: Ambient Noise (Precis‐Poster Session)
Precis‐Poster Papers
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Wind‐induced low‐frequency ambient sea noise: Mechanism and modeling (A)

N.‐C. Yen and A. J. Perrone

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S38-S38 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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A study has been made on the wind‐induced ambient sea noise in the frequency range of 1–10 Hz. The noise generating mechanisms due to wind turbulence, surface wave, and wave turbulence are analyzed as the possible causes for the wind‐induced ambient sea noise observed in the deep ocean. Based on the assumption of a stationary ocean surface disturbance, the noise power spectra due to the aforementioned noise‐generating mechanisms are derived; furthermore, with the incorporation of propagation condition and bottom loss, a relationship between wind speed and ambient noise level in a certain water depth is formulated. The ambient noise level predicted by the theory agrees in order of magnitude with the reported experimental data obtained in Bermuda and Grand Bank areas. Analysis of noise spectrum level and wind speed variation has also been made in the determination of the dominant parameters affecting the wind induced noise. As a result of this study, a semiempirical wind noise model is suggested for the purpose of data comparison and as a guideline in wind noise level prediction. [Work supported by ONR.]
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Ocean ambient noise measurements near Lanai: A quiet location (A)

F. P. Armogida and T. E. Stixrud

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S38-S39 (1977); (2 pages)

Online Publication Date: 11 Aug 2005

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Ambient noise measurements were conducted off the south‐west coast of Lanai in the Hawaiian Islands. Recordings were made for approximately 8 h periods on both a SSQ‐57A sonobuoy with the hydrophone at 300 ft and with a special VLF buoy with the hydrophone at 850 ft. Temperature profiles were taken throughout the 24 h period on location and local ship traffic observed. Spectrum levels recorded at 850 ft with no ship traffic present were essentially flat at approximately 68 dB re 1 μ Pa from 25 to 200 Hz. Ambient noise at 300 ft ranged from 74 dB re 1 μ Pa at 25 Hz to 65 dB at 200 Hz.
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Vertical directionality of low‐frequency ambient noise in the ocean (A)

G. B. Morris

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S39-S39 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Using a 532‐m‐long vertical array suspended near the sound channel axis, the vertical directionality of ambient noise in the 10–50‐Hz frequency band was measured at a location about 200 miles southwest of San Diego. The outputs from 20 hydrophones, configured as a nonuniform spaced array, were digitized for later beamforming. The frequency domain beamforming produced beam noise level versus direction for 200 discrete frequencies, separated by about 0.2 Hz, covering the band from about 10–50 Hz. At times when there were no ships in the immediate vicinity of the array, the noise in the 20–50‐Hz interval was concentrated in a near horizontal direction with most of the noise included in the angular segment from −14° to +14° with respect to the horizontal. Below about 20 Hz this strong, near‐horizontal lobe of high noise appears to diminish with decreasing frequency. In many instances in the 10–15‐Hz band, the noise no longer shows the strong concentration near the horizontal but instead appears to be almost isotropic. [Work supported by ONR.]
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Standard deviation of ambient noise in the South Pacific Ocean (A)

R. W. Bannister, R. N. Denham, K. M. Guthrie, and D. G. Browning

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S39-S39 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Ambient noise data collected during Project SPAN 3 in the South Fiji Basin, an area of relatively low shipping density, have been analyzed to investigate the standard deviation of the time series. In general it has been found that the standard deviation decreases with frequency from about 6 dB at 10 Hz to 1 dB at frequencies above 200 Hz. This is consistent with a transition from distantly generated sound sources at low frequencies to local sources such as wind‐generated wave action at higher frequencies. This interpretation is supported by data samples which are separately dominated by weather noise under different wind conditions and by identified local shipping noise. A comparison is made with other published results. [Work supported by NUSC and DSE.]
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Statistics of quasistationary ambient noise due to merchant shipping in restricted spatial sectors (A)

J. C. Heine

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S39-S39 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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The received noise from a restricted spatial sector containing a small but random number of transiting merchant ships is locally (in time) stationary and Gaussian but with fluctuating time‐averaged power. The statistics of these fluctuations are estimated by considering the ensemble statistics of two processes. The first is the short time averaged (minutes) power received from a fixed set of N ships in the sector, where the contribution from each ship fluctuates due to multipath interference. The second process is the long time‐averaged (hours) power, which is the mean value of the first process. Fluctuations here are due to variations in the number and radiated power of the ships in the observed sector. The fluctuation statistics of the former process are shown to be well represented by a gamma density function for which the mean and variance are determined by those for the received power of the individual components. The statistics of the latter are shown also to be well represented by a gamma function for which the mean and the variance are determined by the mean and variance of the ensemble of source powers and by the average number of surface ships in the observed sector.
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Temporal world shipping distributions: Introduction (A)

L. P. Solomon, A. E. Barnes, and C. R. Lunsford

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S39-S39 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Shipping distributions serve as primary inputs to ambient noise models. The variation of ambient noise with season is in part caused by the nonstationarity of ship distribution throughout the world for time scales of approximately three months. The distributions vary primarily due to two factors: (1) changing routes due to weather, and (2) variation in port traffic caused by economic and other reasons. The determination of temporal distributions demands both factors be known. The seasonal route envelopes were generated by reported positions at sea for a 24‐month period. The port traffic is available from various sources. Both routes and port traffic are functions of ship type. [Supported by ONR.]
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Temporal world shipping distributions: Techniques (A)

L. P. Solomon, A. E. Barnes, and C. R. Lunsford

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S39-S39 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Ship distributions are a function of route envelopes and port traffic levels. Data for seasonal envelopes and pert traffic was available. Lloyds lists 2425 ports, this allows approximately 3 000 000 possible routes. Canonical ports and routes were generated: 50 ports and 75 routes. Modeling of the routes for commercial traffic between any two points, subject to constraints (land avoidance, strait passages) based upon economic reality results in generally simply connected envelopes on a sphere. Multiply connected routes may be analyzed though the union of a set of simply connected envelopes. The use of the real data coupled with the mathematical formulation consisting of a series of bilateral one to one mapping functions is presented.
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Interpolation and smoothing of periodic data: Applied to shipping fields (A)

A. E. Barnes, V. J. Hill, and L. P. Solomon

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S39-S40 (1977); (2 pages)

Online Publication Date: 11 Aug 2005

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Shipping fields are being generated on seasonal and monthly bases, in addition to the standard yearly averages. These fields are necessary for computer ambient noise models. However, to use only the set of discrete fields would result in discontinuous behavior of the noise model results as the computer code proceeds from one time interval to the next. To preclude this, an interpolation of the discrete data sets must be done in the time domain. A number of interpolatory functions are considered for either seasonal (4 data points) or monthly (12 data points) periodic fields. Linear, quadratic, and cubic splines were actively investigated. The linear spline does not model seasonal fluctuations in the data well; cubic splines do, and are easy to compute. Linear splines are wholly adequate for monthly fluctuations. [Work supported by ONR.]
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Effects of quasistationary noise on the detection of quasistationary signals (A)

J. C. Heine and J. R. Nitsche

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S40-S40 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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Receiver operator characteristics derived for post detection integration receivers are significantly perturbed if the signal power fluctuates with a characteristic time roughly two or more times the receiver integration time. In this paper, the effects on detection performance of slow fluctuations in both signal power and in noise power, such as occur in the 10–200‐Hz region due to merchant shipping, are explicitly considered and are compared to results for signal fluctuations alone. Further degradation in detection performance in the region of PD ⩾ 0.5 is shown to occur and the sensitivity of the degradation to the characteristic times of the fluctuations and to the statistics of the noise power fluctuations is demonstrated. The effects of the noise statistics on experimental determination of sonar system performance is discussed.
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Bubble noise measurement facility (A)

T. C. Mathews, D. J. Paladino, and David W. Taylor

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S40-S40 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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A facility has been constructed at the David W. Taylor Naval Ship R & D Center for the measurement of bubble splitting noise. The facility is patterned after the bubble splitting apparatus of Sevik and Park of Pennsylvania State University [M. Sevik and S. H. Park, J. Basic Eng. Trans. ASME, Paper No. 72‐WA/FE‐32, 1–8 (1972)]. The difference in the new facility is the provision for measurement of bubble splitting noise. In addition to the measurement of sound pressure levels associated with controlled bubble splitting the acoustic capabilities of the facility provide a means for measuring the bubble size distribution under different flow conditions. Flow characteristics of the facility, background noise spectra and bubble noise data are presented.
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Point sensor for vertical directional noise environments (A)

A. J. Friedman, E. H. Hyams, and D. Jaarsma

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S40-S40 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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A new point‐sensor concept is defined, evaluated, and shown to have enhanced performance, relative to other simple configurations, in ocean environments which exhibit noise vertical directivity. This property is characteristic of deep ocean areas at low frequencies, and the successful exploitation of the acoustics of such an environment imply special sensor and processing design factors. The concept treated in this paper involves three orthogonal, directional elements and a single omnidirectional element, with all elements colocated. The receiver utilizes cross correlation between one directional element, oriented along the vertical, and the other elements as well as individual element power spectra. The outputs are suitably combined to produce log‐likelihood statistics and maximize detection probability for weak signals arriving along angles at or near the noise peak. Analytical results are presented for detection performance of this concept relative to other concepts in representative signal and noise environments. [Work supported by Naval Electronics Systems Command, Code 320.]
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Single hydrophone technique for obtaining spectral source levels of marine mammals in coastal waters (A)

L. C. D. R. R. Bostian and H. Medwin

J. Acoust. Soc. Am. Volume 62, Issue S1, pp. S40-S40 (1977); (1 page)

Online Publication Date: 11 Aug 2005

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During the Gray Whale migration from the Aleutians to Baja California the mammal travels in coastal waters, thereby presenting an opportunity for the study of its sound spectral source levels. Using the theory of rough surface scattering, the knowledge of the bottom impedance, and correlation techniques, it should be possible to decompose the shallow water reverberation into the contributions from different paths. From this, the range, the depth, and the spectral source levels of the sounds of the mammal can be determined by use of only one hydrophone rather than the conventional three or four. Results are presented of an experimental study in the NPS Ocean Acoustic Wave Facility using models of the whale's pulsed radiation and of the coastal environment. [Work supported by ONR.]
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