Ambient Noise Spectrum Level Research Articles

Analysis of Ambient Noise Spectrum Level Correlation Characteristics in the China Sea

The ambient noise field in the ocean is formed by a mixture of many source types, such as seismic noise, marine noise, shipping noise, and wind-generated noise. The correlation matrices of the ambient noise spectrum levels offer a promising approach for identifying and isolating the active source mechanisms in ambient noise datasets. This study attempts to use frequency correlation matrices to examine inter-frequency relationships and identify frequency bands that are dominated by specific sources present in samples of ambient noise. Deep-ocean ambient noise recorded by hydroacoustic monitoring systems in the East China Sea and South China Sea was used to analyze the frequency correlation characteristics over a broadband frequency range, with a focus on the contribution of wind-related and shipping-related source mechanisms to the overall ambient noise field at various frequencies. The primary sources observed in both the South China Sea and East China Sea were shipping sources and wind sources. During the observation window, wind was responsible for a significant amount of the ambient noise energy above 477 Hz in the East China Sea, while the corresponding frequency threshold was shifted up to 575 Hz in the South China Sea. Another interesting feature was also observed in the East China Sea: a higher degree of correlation was found between the levels at the wind noise dominated frequencies and mixture noise (mainly consisting of wind noise and shipping noise) dominated frequencies at the upper receiver than at the lower receiver. Data analysis results show that the cause of this effect is that shipping activity contributes more to the soundscape at the lower receiver than at the upper receiver.

Wind dependence of ambient noise in shallow water of Bay of Bengal

This work reports on investigations into the wind dependence of ambient noise in the Bay of Bengal. Ambient noise measurements were made in the shallow water of Bay of Bengal using a portable broadband, high frequency data acquisition system together with a sensitive hydrophone suspended from the measuring platform at a depth of 5 m from the surface where the ocean depth was 25 m. Periodic measurements were carried out for one year corresponding to a wind speed range between 2 m/s and 9 m/s during summer, monsoon and winter seasons. The proportionality of the noise level with wind speed for frequencies ranging from 500 Hz to 6 kHz for each season was studied. The analysis reveals that the correlation between the wind speed and the ambient noise spectrum level was higher at lower frequencies. The results of empirical fitting based on analysis were used for noise level prediction and the model predictions compare well with the measured noise level. Further it was observed that the wind generated noise level measured during summer was approximately 8 dB less than that in other seasons. On the other hand the proportionality between the noise level and the wind speed was less during winter.

Acoustic pressure-vector sensor array

Pressure-vector sensors measure both scalar and vector components of the acoustic field. December 2003 measurements at the NUWC Seneca Lake test facility verify previous observations that acoustic ambient noise spectrum levels measured by acoustic intensity sensors are reduced relative to either acoustic pressure or acoustic vector sensor spectrum levels. The Seneca measurements indicate a reduction by as much as 15 dB at the upper measurement frequency of 2500 Hz. A nonlinear array synthesis theory for pressure-vector sensors will be introduced that allows smaller apertures to achieve narrow beams. The significantly reduced ambient noise of individual pressure-vector elements observed in the ocean by others, and now at Seneca Lake, should allow a nonlinearly combined array to detect significantly lower levels than has been observed in previous multiplicative processing of pressure sensors alone. Nonlinear array synthesis of pressure-vector sensors differs from conventional super-directive algorithms that linearly combine pressure elements with positive and negative weights, thereby reducing the sensitivity of conventional super-directive arrays. The much smaller aperture of acoustic pressure-vector sensor arrays will be attractive for acoustic systems on underwater vehicles, as well as for other applications that require narrow beam acoustic receivers. [The authors gratefully acknowledge the support of ONR and NUWC.]

Correlations between Ambient Noise and the Ocean Surface Wave Field

Abstract Measurements of the ambient noise spectrum level N with simultaneous, coincident wind and wave measurements were made from RP FLIP in fall 1991. The measurements were designed to investigate the correlation between the ambient noise and relevant surface wave parameters. The results suggest that wave parameters related to the incidence of wave breaking correlated well with the ambient noise level. The correlation between N and the rms wave amplitude a was found to he poor but that between N and the rms amplitude of the local wind sea aw was comparable to that between wind speed U and N. Similar good correlations were found between the rms wave slope s and N, and the higher frequency surface wave spectral levels and N. Correlations between the surface wave dissipation estimates D based on the Hasselmann and Phillips models and the ambient noise were comparable to those between the wind speed and the ambient noise. The mean square acoustic pressure was found to be proportional to Dn with n in the ra...

The correlation between ocean wave energy dissipation and ambient noise

An experiment that measured the NSL between 2 to 25 kHz, the wind speed U, and the directional wave spectrum was conducted off the coast of Oregon to examine the correlation between the ambient noise spectrum level (NSL) and surface wave parameters. These measurements show that although the correlation between the ambient NSL and the rms wave height is poor, the NSL and wave height spectrum energy levels at frequencies somewhat higher than the spectral peak give correlation coefficients that are comparable to that between the well-known correlation between U and NSL. Estimates of wave energy dissipation due to breaking waves using models proposed by Komen etal. [J. Phys. Ocean. 14, 1271–1285 (1984)] and Phillips [J. Fluid Mech. 156, 505–531 (1985)] likewise give comparable correlation coefficients. These results are consistent with the laboratory experiments [M. R. Loewen and W. K. Melville, J. Fluid Mech. 224, 601–623 (1991)] which show that the energy dissipated by wave breaking scales with the acoustic energy radiated. The implications of these results for acoustic remote sensing of breaking waves and associated processes of air–sea interaction are discussed.

Low‐frequency ambient noise mechanisms: A comparison of open‐ocean and under‐ice data

A large increase in ambient noise spectrum level is observed in the open ocean at low frequencies. Most investigators have suggested wind and wave reduced (surface coupled) mechanisms. [Kibblewhite et al., J. Acoust. Soc. Am. Suppl. 1 74, S121 (1983)]. Our analysis of recent under‐ice ambient noise data shows no large increase at low frequencies thus indirectly supporting the hypothesis of a surface coupled mechanism in the open ocean. The general level and frequency dependence of the spectrum suggests, however, the existence of a locally generated noise threshold which may be due to internal waves or seismic activity. [Work is funded by ONR Code 425‐AR.]