NoiseModelAnalysisandEstimationofEffectdueto...
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Hindawi Publishing Corporation International Journal of Oceanography Volume 2011, Article ID 950838, 4 pages doi:10.1155/2011/950838 Research Article Noise Model Analysis and Estimation of Effect due to Wind Driven Ambient Noise in Shallow Water S. Sakthivel Murugan, 1 V. Natarajan, 2 and R. Rajesh Kumar 1 1 Department of ECE, SSN College of Engineering, Tamilnadu, Chennai 603110, India 2 Department of Instrumentation Engineering, MIT Campus, Anna University, Chennai 600044, India Correspondence should be addressed to S. Sakthivel Murugan, [email protected] Received 1 June 2011; Revised 27 September 2011; Accepted 28 September 2011 Academic Editor: Robert Frouin Copyright © 2011 S. Sakthivel Murugan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Signal transmission in ocean using water as a channel is a challenging process due to attenuation, spreading, reverberation, absorp- tion, and so forth, apart from the contribution of acoustic signals due to ambient noises. Ambient noises in sea are of two types: manmade (shipping, aircraft over the sea, motor on boat, etc.) and natural (rain, wind, seismic, etc.), apart from marine mammals and phytoplanktons. Since wind exists in all places and at all time: its effect plays a major role. Hence, in this paper, we concentrate on estimating the effects of wind. Seven sets of data with various wind speeds ranging from 2.11 m/s to 6.57 m/s were used. The analysis is performed for frequencies ranging from 100 Hz to 8 kHz. It is found that a linear relationship between noise spectrum and wind speed exists for the entire frequency range. Further, we developed a noise model for analyzing the noise level. The results of the empirical data are found to fit with results obtained with the aid of noise model. 1. Introduction Ocean ambient noise is an inherent characteristic of the medium having no specific point source. It is the residual noise background in the absence of individual identifiable sources that may be considered as the natural noise envi- ronment for hydrophone sensors. It comprises of number of components that contribute to the noise level (NL) in vary- ing degrees depending on the location of measurements [1]. The sources include geological disturbances, nonlinear wave interaction, turbulent wind stress on the sea surface, ship- ping, distant storms, seismic prospecting, marine animals, breaking waves, spray, rain, hail impacts, and turbulence [2]. In general, over a broad frequency range, the ambient noise spectrum characteristic varies depending on the sources and conditions prevailing at the measurement location. A direct connection between wind force and the level of ambient noise is observed for a frequency range of 500 Hz to 25 kHz. Noise level spectrum is summarized in [3]. Knudsen spectra [4], show the strong dependence of spectral power level with wind speed and sea states. The work on spectra and sources of ambient noise in the ocean observed a decrease in wind/sea state dependency of underwater ambient noise below 500 Hz. Further, it was shown that there was no dependency below 100 Hz, and the wind-generated noise measurement shows self-similar spec- tra between 100 Hz to 10 kHz [3, 4]. It is inferred that the effect of wind speed is dominant in the frequency band of 22 Hz to 5 kHz by experimenting at 40 different locations [5]. Further high frequency noise is said to be highly correlated to wind speed from 4–15 m/sec. The optimal frequency range for estimating wind is from 2–6 kHz [6]. Ambient noise is the sustained unwanted background noise prevailing at any location. This masks the signals from underwater acoustic instruments. So the detection of back- ground noise is essential to enhance the signal-to-Noise ratio (SNR) of acoustic-based underwater instruments. Hence, measurement and characterization of ambient noise forms a significant part in any underwater activity. The interest in ambient noise characterization has grown and presently is of much interest to engineers working in the fields of active and passive sonars, underwater sensors, and signal processing.
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Hindawi Publishing CorporationInternational Journal of OceanographyVolume 2011, Article ID 950838, 4 pagesdoi:10.1155/2011/950838
Research Article
Noise Model Analysis and Estimation of Effect due toWind Driven Ambient Noise in Shallow Water
S. Sakthivel Murugan,1 V. Natarajan,2 and R. Rajesh Kumar1
1 Department of ECE, SSN College of Engineering, Tamilnadu, Chennai 603110, India2 Department of Instrumentation Engineering, MIT Campus, Anna University, Chennai 600044, India
Correspondence should be addressed to S. Sakthivel Murugan, [email protected]
Received 1 June 2011; Revised 27 September 2011; Accepted 28 September 2011
Academic Editor: Robert Frouin
Copyright © 2011 S. Sakthivel Murugan et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.
Signal transmission in ocean using water as a channel is a challenging process due to attenuation, spreading, reverberation, absorp-tion, and so forth, apart from the contribution of acoustic signals due to ambient noises. Ambient noises in sea are of two types:manmade (shipping, aircraft over the sea, motor on boat, etc.) and natural (rain, wind, seismic, etc.), apart from marine mammalsand phytoplanktons. Since wind exists in all places and at all time: its effect plays a major role. Hence, in this paper, we concentrateon estimating the effects of wind. Seven sets of data with various wind speeds ranging from 2.11 m/s to 6.57 m/s were used. Theanalysis is performed for frequencies ranging from 100 Hz to 8 kHz. It is found that a linear relationship between noise spectrumand wind speed exists for the entire frequency range. Further, we developed a noise model for analyzing the noise level. The resultsof the empirical data are found to fit with results obtained with the aid of noise model.
1. Introduction
Ocean ambient noise is an inherent characteristic of themedium having no specific point source. It is the residualnoise background in the absence of individual identifiablesources that may be considered as the natural noise envi-ronment for hydrophone sensors. It comprises of number ofcomponents that contribute to the noise level (NL) in vary-ing degrees depending on the location of measurements [1].The sources include geological disturbances, nonlinear waveinteraction, turbulent wind stress on the sea surface, ship-ping, distant storms, seismic prospecting, marine animals,breaking waves, spray, rain, hail impacts, and turbulence [2].In general, over a broad frequency range, the ambient noisespectrum characteristic varies depending on the sources andconditions prevailing at the measurement location.
A direct connection between wind force and the level ofambient noise is observed for a frequency range of 500 Hz to25 kHz. Noise level spectrum is summarized in [3]. Knudsenspectra [4], show the strong dependence of spectral powerlevel with wind speed and sea states.
The work on spectra and sources of ambient noise inthe ocean observed a decrease in wind/sea state dependencyof underwater ambient noise below 500 Hz. Further, it wasshown that there was no dependency below 100 Hz, and thewind-generated noise measurement shows self-similar spec-tra between 100 Hz to 10 kHz [3, 4].
It is inferred that the effect of wind speed is dominant inthe frequency band of 22 Hz to 5 kHz by experimenting at 40different locations [5]. Further high frequency noise is saidto be highly correlated to wind speed from 4–15 m/sec. Theoptimal frequency range for estimating wind is from 2–6 kHz[6].
Ambient noise is the sustained unwanted backgroundnoise prevailing at any location. This masks the signals fromunderwater acoustic instruments. So the detection of back-ground noise is essential to enhance the signal-to-Noise ratio(SNR) of acoustic-based underwater instruments. Hence,measurement and characterization of ambient noise formsa significant part in any underwater activity. The interest inambient noise characterization has grown and presently is ofmuch interest to engineers working in the fields of active andpassive sonars, underwater sensors, and signal processing.
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2 International Journal of Oceanography
2. Data Collection and Algorithm
2.1. Data Collection. Noise measurements were made usingtwo calibrated omnidirectional hydrophones mounted in avertical array at 5 m and 15 m depths. The hydrophones weresuspended from the measurement platform using the ropeand mounting arrangement that links to the rope. The hyd-rophones have a receiving sensitivity of −170 dB over afrequency range between 0.1 Hz and 120 kHz. The data wereacquired at a rate of 50 kHz and 500 kHz, filtered and digi-tized with portable data acquisition system with 12-bit res-olution. During the period of data collection, all machineryon the boat/ship was switched off and the recording systemwas powered by battery. The wind speed was simultaneouslymeasured during each sampling. The measurement consistsof 7 sets of data. The wind speeds of collected data range from2.11 m/s to 6.57 m/s. Measurements showing the evidence ofnoise from non-wind-dependent sources such as rain, dol-phin, and ship were not included in the analysis.
2.2. Algorithm. Theoretically the relationship between thenoise levels is assumed to be linear to the logarithm of thewind speed [7], and this can be expressed as
NL = B + 20n log(U), (1)
where NL andU stand for noise level and wind speed, respec-tively. The constants B and n were determined by fittingthe experimental data to the model at different frequencies.n is obtained from 1/20th slope of regression line and theordinate intercept of the line gives B for each empiricalfit. The spectral analysis was carried out in MATLAB usingWelch method of averaging periodogram. The frequencyrange of interest for the current study was from 500 Hz to8 kHz where the best correlation between the wind speed andthe noise level has been observed.
3. Results and Discussion
Analysis has been carried out to study wind-dependentambient noise spectrum level in the frequency range between500 Hz and 8 kHz. Noise spectrum for seven different windspeeds [2.11, 3.32, 5.92, 6.03, 6.06, 6.16, and 6.57 m/s] isshown in Figure 1. It is observed that the noise level increaseswith increase in wind speed.
Figure 2 is also known as regression plot. It is observedfrom Figure 2 that there is a steep increase in the slope andnoise level as wind speed increases. Table 1 shows the valuesof B and n obtained from regression plots. The value of slopeis maximum at 500 Hz and decreases as frequency increases.The value of n and B obtained from the empirical fitting isused for validation with measured noise level.
Figures 3, 4, 5, and 6 show the comparison of predictedand measured noise levels for wind speed of 2.11 m/s,3.32 m/s, 5.92 m/s, and 6.57 m/s, respectively. It is seen thatthe predicted noise level is in good agreement with the meas-ured noise levels. As the wind speed increases, the predictednoise model deviates from the measured noise level.
0 1000 2000 3000 4000 5000 6000 7000 800060
65
70
75
80
85
NSL
(dB
)
2.11 m/s3.32 m/s5.92 m/s6.03 m/s
6.06 m/s6.16 m/s6.57 m/s
Frequency (Hz)
Figure 1: Noise spectrum at different wind speeds.
0 1 2 3 4 5 6 7 8 9 1060
65
70
75
80
85
90
Wind speed (m/s)
NSL
(dB
)
500 Hz1000 Hz2000 Hz4000 Hz
5000 Hz6000 Hz8000 Hz
Figure 2: Noise level at different frequencies for varying windspeeds.
4. Conclusion
In this paper, an estimation of power spectral density of am-bient noise due to wind at various speeds ranging from2.11 m/s to 6.59 m/s is analysed and observed that the effectof wind is dominating at lower frequencies from 100 Hz to5 KHz. A noise model for estimation of effect of wind atdifferent wind speeds for various frequencies was developedand found that it best suits with the practical data. The ana-lysis shows that noise level increases as wind speed increases.
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International Journal of Oceanography 3
Table 1: Values of B and n from regression plots.
Frequency (Hz) B n Frequency (Hz) B n
500 67.14 0.13 4500 63.19 0.01
1000 66.57 0.08 5000 63.56 0.03
1500 68.16 0.05 5500 61.04 0.08
2000 65.98 0.06 6000 62.5 0.02
2500 64.28 0.05 6500 64.06 0.003
3000 66.93 0.02 7000 62.56 0.01
3500 62.29 0.04 7500 63.41 0.02
4000 65.04 0.04 8000 63.04 0.006
0 1000 2000 3000 4000 5000 6000 7000 8000 900050
55
60
65
70
75
80
85
90
95
100
NSL
(dB
)
Predicted noiseReal time noise
Frequency (Hz)
Figure 3: Comparison of predicted and real time noise for windspeed of 2.11 m/s.
0 1000 2000 3000 4000 5000 6000 7000 8000 900050
55
60
65
70
75
80
85
90
95
100
NSL
(dB
)
Predicted noiseReal time noise
Frequency (Hz)
Figure 4: Comparison of predicted and real time noise for windspeed of 3.32 m/s.
0 1000 2000 3000 4000 5000 6000 7000 8000 900050
55
60
65
70
75
80
85
90
95
100
NSL
(dB
)
Predicted noiseReal time noise
Frequency (Hz)
Figure 5: Comparison of predicted and real time noise for windspeed of 5.92 m/s.
0 1000 2000 3000 4000 5000 6000 7000 8000 900050
55
60
65
70
75
80
85
90
95
100
NSL
(dB
)
Predicted noiseReal time noise
Frequency (Hz)
Figure 6: Comparison of predicted and real time noise for windspeed of 6.57 m/s.
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4 International Journal of Oceanography
There was good correlation between wind speed and noiselevel in the frequency range between 500 Hz to 8000 Hz.
Acknowledgments
The data for the above analysis is provided by National Insti-tute of Ocean Technology, Chennai. The authors wish toacknowledge the valuable support given by Dr. G. Latha, Pro-ject Director, Ocean Observation and Acoustic System, NIOTfor the excellent contribution of the relevant data for pro-cessing. The authors also wish to thank Dr. S. Radha HOD,ECE Department for her support.
References
[1] R. J. Urick, Principles of Underwater Sound, McGraw-Hill, NewYork, NY, USA, 3rd edition, 1983.
[2] R. A. Wagstaff, “An ambient noise model for the northeast paci-fic ocean basin,” IEEE Journal of Oceanic Engineering, vol. 30,no. 2, pp. 286–294, 2005.
[3] G. M. Wenz, “Acoustic ambient noise in the ocean: spectra andsources,” Journal of Acoustic Society of America, vol. 30, pp.1936–1956, 1962.
[4] V. O. Knudsen, R. S. Alford, and J. W Emling, “Underwater am-bient noise,” Journal of Marine Research, vol. 7, pp. 410–429,1948.
[5] D. H. Cato, “Ambient sea noise in water near Australia,” Journalof the Acoustical Society of America, vol. 60, no. 2, pp. 320–328,1976.
[6] L. Zedel, L. Gordon, and S. Osterhus, “Ocean Ambient SoundInstrument System: acoustic estimation of wind speed and di-rection from a subsurface package,” Journal of Atmospheric andOceanic Technology, vol. 16, no. 8, pp. 1118–1126, 1999.
[7] C. L. Piggott, “Ambient Sea noise at low frequencies in shallowwater of the Scotian Shelf,” Journal of Acoustic Society of Ame-rica, vol. 36, pp. 2152–2163, 1964.
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