Ambient Sea Noise Research Articles

Monitoring the Miramare Marine Reserve: Assessment of protection efficiency

The Miramare Reserve, in the Gulf of Trieste (North Adriatic‐Italy), is a small marine protected area (121 hectares) for research, education and conservation and is managed by WWF Italy. Different techniques for taking a qualitative and quantitative census of the fish species, for mapping the nests of Gobius cruentatus, Tripterygion delaisi and Cbromis chromis, for studying the reproductive behaviour of C. cbromis and for measuring the sea ambient noise were used. Visual census techniques were applied in the comparison of two areas which were similar from a morphological point of view, but differed in human activity.

Ambient sea noise dependence on local, regional and geostrophic wind speeds: Implications for forecasting noise

Measurements of ambient sea noise in two regions near Australia are presented as a function of wind speed (a) measured locally (at a buoy, distance about 120 m), (b) measured at weather stations (distances up to 51 km) and (c) predicted from the geostrophic air flow. Estimates of geostrophic winds from atmospheric pressure contours are used in forecasting wind speeds. Good correlation of noise with local wind speed was observed. Poorer correlations were observed between noise and regional wind speeds or those predicted from geostrophic wind speeds: trends were similar but the spread of the data points was greater. Correlations were better when the wind field had minimal influence by the presence of land upwind. The variation of geostrophic wind speed with time generally followed that of the local wind speed but peaks and troughs were sometimes displaced in time by a few hours, accounting for some of the spread of the data. These results suggest that geostrophic wind, and thus forecasts of wind speeds, would be useful in forecasting sea noise.

Ambient noise in shallow seas: Propagation issues

Whatever the source of ambient noise in a shallow sea, propagation to a distant receiver shapes in a major way its spectral character, its temporal properties, and its coherence or spatial distribution functions. Any one ambient noise mechanism is generally an incoherent superposition of a large number of similar elemental sources. Thus propagation modeling for ambient noise must address the multiple source question, and as well the modeling can take advantage of simplifications often afforded by multiple path averaging. In this context, the main confounding elements of shallow sea propagation including, but not limited to, downward refraction, sloping bottoms, sediment interaction, rough bottom scattering, and water column fluctuations are reviewed. In such complicated propagation channels the noise is modified in important detail, although much of its character remains broadly recognizable.

A ray-model theory on spatial correlation and vertical directivity of surface-generated ambient noise in shallow water

The ambient noise in shallow water is more complicated than that in the deep sea, and it has greater geographical and seasonal variations since there is a strong bottom interaction. A theoretical model of ambient sea noise including surface-noise sources and a stratified medium ocean is discussed. The noise sources are assumed to be statistically independent directional point sources distributed over the sea surface. The effects of the surface–source feature, velocity profile, and bottom property on ambient noise are studied. In general, the ray theory is suitable for calculating the contribution of near-field sources, whereas the normal-mode theory is appropriate for far-field sources. In this paper, a ray-mode theory suitable for near- and far-field sources is developed. The concise formulas for calculating the intensities, spatial correlation, and vertical directivity of ambient noise are present and some numerical simulations are given.

Underwater acoustic ambient noise levels on the eastern Canadian continental shelf

This paper reports the analysis of shallow-water ambient noise levels collected by Defence Research Establishment Atlantic during 14 cruises over the period 1972 to 1985. A weighted average is formed to de-bias the samples, with the aim of answering the question: ‘‘If one were to pick a site randomly on the eastern Canadian continental shelf at a random time, and perform an ambient noise measurement, what would be the expected noise level?’’ The samples are also grouped according to whether they were taken on the Scotian Shelf, the Grand Banks, or the Flemish Cap, and according to season. The frequency range covered is 30 to 900 Hz. The weighted mean and standard deviation of the noise levels are presented, as well as the correlation coefficient between the noise levels and wind speed. The results show that the eastern Canadian continental shelf as a whole presents levels that are characteristic of areas with high shipping density and good acoustic propagation, with the Scotian Shelf showing generally higher ambient noise levels. Finally, a comparison with Piggott’s ambient noise measurements [C. L. Piggott, ‘‘Ambient sea noise at low frequency in shallow water of the Scotian Shelf,’’ J. Acoust. Soc. Am. 36, 2152–2163 (1964)] on the Scotian Shelf is made, and an explanation for the observed difference is suggested.

Acoustic properties of ice edge noise in the Greenland Sea

A towed array experiment was performed by NRL in 1982 in the vicinity of the ice edge in the Greenland Sea. The array was towed normal and parallel to and at ranges between 4–50 km from the ice edge; the received data were beamformed to study the horizontal directionality of the noise. The results indicated that a few localized ‘‘hot spots’’ (with dimensions <5 km) in the vicinity of the ice edge were the dominant noise sources. These sources persisted for nearly 1 day. The nominal separation between the hot spots was about 50 km. The ice edge noise spectrum measured at these ranges decreased 11–12 dB per octave with frequency between 100–500 Hz, as compared with the 6 dB per octave spectrum of ambient sea noise. Possible sources are hypothesized in the context of known eddy, wind, and wave effects in the vicinity of the ice edge.

Using eigenvalue analysis to identify interference in ambient sea noise vertical directionality measurements

Ambient noise vertical directionality measurements at low frequencies have shown that noise arrives from long distances at near horizontal angles, from moderate distances at larger angles, and from local sources at high angles. Therefore levels of noise from different sources can be measured by examining vertical linear array beam powers at the appropriate angles. However, when measurements are made, it is important to be able to distinguish between acoustic signals, ambient noise, and data contamination. Some experimental results are examined, using conventional and adaptive beamforming and eigenvalue–eigenvector decomposition, and used to establish criteria useful in deciding whether the measurements are ambient noise or otherwise.

Measurements of wind‐generated noise levels in the Eastern Indian Ocean

Recordings of ambient sea noise made with a vertical linear hydrophone array (VLA) at five sites in the Eastern Indian Ocean have been analyzed for wind‐speed dependence. Use is made of the vertical directional discrimination of the VLA to resolve noise generated at a distance and locally generated wind noise. The results are compared with data from similar experiments off the east coast of Australia [A. S. Burgess and D. J. Kewley, J. Acoust. Soc. Am. 73, 201–210 (1983)].

Wind-generated surface noise source levels in deep water east of Australia

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.

Ambient sea noise elimination method and apparatus

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Low-frequency ambient ocean noise and sound transmission over a thinly sedimented rock bottom

Measurements of ambient sea noise and sound transmission loss were made at a coastal site with a hard rock bottom, in 20 fathoms of water. A bottomed hydrophone was used; data were taken in the range from 50 to 800 Hz. A series of intermittent measurements over a period of 18 months showed the noise levels to be unusually low; spectrum levels of 40 to 78 dB re 1 μPa at 100 Hz were observed. Close correlation of noise level with wind velocity was found over the entire frequency band. CW transmission measurements at 97, 360, and 580 Hz disclosed exceptionally high loss in the area; between 1 yd and 2 mi the loss was about 100 dB. A combination of sparsity of shipping and poor transmission is believed responsible for the low noise levels. An exploratory test with a vibration sensor on the bottom indicated results similar to those of the water-borne sound experiments.

Mechanisms of wind-induced low-frequency ambient sea noise in the deep ocean

Theories of wave interaction and turbulence have been extended to the study of noise-generating mechanisms for the interpretation of the reported experimental low-frequency ambient noise data in the frequency range of 1 to 10 Hz. The problem is formulated by assuming a stationary disturbed ocean surface which also serves as the interface of wind turbulence and wave turbulence. The noise spectrum level at deep depths in the ocean is hypothesized as the result of the convolution of surface wave, wind turbulence, and wave turbulence actions which satisfy the radiation condition. Due to the scaling difference between acoustic waves and flow disturbance, the physical arguments have been applied to obtain a simplified mathematical derivation. With incorporation of propagation conditions and the ocean bottom, the noise level received by a sensor in the water column is predicted from the assumed noise-generating mechanisms and appears to agree in order of magnitude with the experimental data. The theoretical approach to ambient noise problems from this study suggests some important parameters for noise modeling and also provides a reference for planning experiment and data comparison. [Work supported by ONR.]

Variability of low-frequency ambient sea noise

In March 1976, measurements were made of the ambient sea noise in the South Fiji Basin to the northeast of New Zealand. As part of the analysis, the standard deviation of the noise in each 1/3- octave band between 10–500 Hz was computed. The effect of receiver depth, wind speed, and frequency was examined. There is not a marked dependence of standard deviation on receiver depth. When no shipping was present, the standard deviation was less than 2.5 dB at all frequencies and decreased to under 0.1 dB above 100 Hz. When shipping dominated, the standard deviation was much higher below 100 Hz, being 5.5–7 dB at 10 Hz but decreasing to less than 2 dB above 200 Hz. This behavior is explained by a model in which a single ship dominates. A comparison with the results from other ocean areas indicates three regimes for ship dominated noise: (1) single-ship dominated areas where the standard deviation decreases with frequency; (2) distant multiple-ship dominated areas where the standard deviation increases with frequency; and (3) local multiple-ship dominated areas where the standard deviation is almost independent of frequency.

Variability of low-frequency ambient sea noise : Bannister, R. W., R. N. Denham, K. M. Guthrie, D. G. Browing and A. J. Perrone, 1979. J. acoust. Soc. Am., 65(5): 1156–1163

Variability of low-frequency ambient sea noise : Bannister, R. W., R. N. Denham, K. M. Guthrie, D. G. Browing and A. J. Perrone, 1979. J. acoust. Soc. Am., 65(5): 1156–1163

Wind-induced low-frequency ambient sea noise: Mechanism and modeling

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.]

Economical computation of array gain of large lattice acoustic arrays in anisotropic sea noise

The performance of an underwater acoustic array is dependent upon its gain in the presence of noise. While isotropic noise is often utilized for simplistic considerations of system performance, such analyses are of limited validity because ambient sea noise is significantly directional. Thus, it is necessary to evaluate the behavior of arrays having many elements and multi-dimensional configurations in realistic anisotropic sea noise. Such computations have been very expensive using very large computers and impossible or impractical with other computers. This paper describes a technique comparable to a multi-dimensional FFT for radically reduced cost for the computation of array gain of lattice-shaped arrays of one, two, or three dimensions. The computational aspects are discussed. Typical results are presented for various horizontal and vertical arrays and compared with the author's prior work reported at EASCON '75 and the Ninth International Congress on Acoustics, 1977.

Models for the amplitude fluctuations of narrow-band signals and noise in the sea

The sound from a steady distant sinusoidal source in the sea fluctuates in amplitude because of multipath interferences, surface motion, internal waves, microstructure, and other propagation effects. Such fluctuations are found to follow a Rician or modified-Rayleigh distribution having as a parameter the fraction of random power in the received signal. At the output of conventional sonar processor—namely, a narrow-band filter, a squarer, and an integrator—the fluctuation statistics are determined by the propagation processes occurring in the sea between source and receiver. On the other hand, ambient sea noise is found, from analyses of field recordings, to have fluctuation statistics determined by the processor itself; ambient noise samples at the processor output obey a chi-square distribution having a number of degrees of freedom equal to twice the bandwidth-time product of the processor, as would be expected from a Gaussian input. The two distributions—Rician power for signals and chi-square for noise—while formally different, have remarkable similarities in the limits. In short, the output fluctuation statistics of narrow-band signals and Gaussian ambient noise can apparently be predicted from estimates of the degree of randomness introduced by the prevailing propagation conditions, and from a knowledge of the processor, respectively, provided these statistics remain stationary during the analysis period.

Amplitude fluctuation statistics of narrow-band signals and noise in the sea

The sound from a steady distant sinusoidal source in the sea fluctuates in amplitude because of multipath interferences, surface motion, internal waves, microstructure, and other propagation effects. Such fluctuations are found to follow a Rician or modified-Rayleigh distribution having as parameter the fraction of random power in the received signal. At the output of a conventional sonar processor—namely, a narrow-band filter, a squarer, and an integrator—the fluctuation statistics are accordingly determined by the propagation processes occurring in the sea between source and receiver. On the other hand, ambient sea noise is found, from analyses of field recordings, to have fluctuation statistics determined by the processor itself; ambient noise samples at the processor output obey a chi-square distribution having a number of degrees of freedom equal to twice the BT product of the processor, as would be expected from a Gaussian input. The two distributions—Rician-power for signals and chi-square for noise—while formally different, have remarkable similarities in the limits. In short, the output fluctuations statistics of narrow-band signals and Gaussian ambient noise can apparently be predicted from estimates of the degree of randomness introduced by the prevailing propagation conditions, and from a knowledge of the processor, respectively, provided these statistics remain stationary during the analysis period. [Work supported by the Navy ASW Systems Project Office, Code ASW-13.]

Ambient sea noise in waters near Australia : CATO D. H., 1976. J. acoust. Soc. Am., 60 (2): 320–328

Ambient sea noise in waters near Australia : CATO D. H., 1976. J. acoust. Soc. Am., 60 (2): 320–328

Detection of trawling noise by Atlantic cod(Gadus morhuaL.)

Ambient sea noise and the noise generated by a small wooden trawler were recorded in Passamaquoddy Bay in summer and in winter. The recordings were analysed in “real‐time” in third‐octave bands. The measurements were combined with published data on masking effects on hearing of cod to calculate a minimum range at which cod can first detect trawling noise. The range is 10,500 ft (3.2 km) in summer, and 8,200 ft (2.5 km) in winter.