![]() ![]() This is called passive SHM (Structural Health Monitoring). In civil engineering, we can apply this technique to monitor the state of health of structures. Passive identification of a system relies on the estimation of the parameters which describe that system only by using ambient sources. The relation is presented below for reference: The relation has been extensively validated by numerous researchers and adapted for varying sites. Piggott (1964), has reported a logarithm relation for the ambient noise and the wind speed based on his study at two depths in very shallow water (36 m and 51 m), where the non wind dependent factors were reported to be not significant and the contribution of the wind dominated source was observe to be dominant at nearly all wind speeds and all frequencies. The large spread of wind generated noise levels measured for the same wind speed and wave height in the absence of other noise sources is essentially attributed to the site dependence and the primary site dependent factor that influence the noise level being acoustic propagation which in turn are dependent on the season, ocean depth and the bottom composition. ![]() Similar work by Ingenito (1989) on site dependence of wind- dominated ambient noise in shallow water reported lower noise level of around 2 dB in the silt bottom at wind speed of 7 m/s when compared to sand bottom at wind speed 3-5 m/s and variability of 10 dB in the average noise spectrum level was observed in the essential fraction of the entire range of the spectrum. It was observed that the contradicting behavior of the summer and winter noise levels was due to wind stress at sea surface. Gayer & Wille (1984) in their work made measurements in North Sea and Baltic Sea and reported that the influence of propagation loss on the wind dependent shallow water noise appears to be marginal even at extremely different area. The ambient noise was also shown to be site specific in shallow water where speed of the sound and the bottom properties vary substantially with the location. Such study post extensive validation has the potential to develop predictive models for ambient noise estimation based on recorded data. The results of empirical fitting based on the analysis were used for noise level prediction and the model predictions were compared with the measured noise level. Linear relationship between the sea noise spectrum levels and the wind speed were found for the entire frequency range, but the slope were frequency dependent. The relative spectral energy distribution of sea noise is presented for a number of wind speeds. ![]() Ambient noise data were collected for January and March in the shallow water of Arabian Sea between the wind speed 0.0 m/s to 4.11 m/s. Real ambient noise recording at very shallow depths has been analyzed for two environmentally unique periods when the sound velocity profile and the wind conditions present distinct characteristics. Work presents a shallow water ambient noise analysis and modeling effort in the Arabian Sea. Second, except for a few summer calms, weather in the islands is severe, and strong winds and high sea states are sustained throughout the year. This in turn restricts the acoustic access at a bottomed hydrophone to a localized field. First, a combination of thermal structure and a very gradual bottom slope result in high propagation losses. The character of sea-noise in the area seems to be shaped to two factors. There was good correlation between wind velocity and ambient noise at 800 and at 1000 cps. Although no seasonal changes occurred in the upper spectrum, day-to-day variations in level at the higher frequencies generally followed patterns of change in local weather. The annual excursion in level from winter high to midsummer low was about 5 db at 50 cps and 3.5 db at 100 cps. Seasonal changes were observed in the spectrum below 400 cps. Also, the pressure levels at frequencies above 100 cps are considerably higher. The noise spectrum was found to be relatively flat compared with the spectrum measured in deep ocean areas of the western North Atlantic. Matching observations of wind and weather were coordinated with the noise data. Amplitude levels were measured at selected frequencies in the band 30 cps to 1 kc. Twenty minute samples of single hydrophone outputs were recorded during daylight hours about every 3 days over a 4-year period from the summer of 1957 to 1961. Shore-terminated, omnidirectional hydrophones bottomed in 240 and 350 fathoms, were used. Measurements have been made of ambient sea noise north of the Shetland Islands in the southern part of the Norwegian Sea. ![]()
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