![]() The noise levels in the spectrum generated from two sources, geophony and biophony, have been observed to vary between diurnal and lunar phases and also between seasons. In this study, the soundscape in shallow waters of the eastern Arabian Sea is explored through passive acoustic measurements in a time series between October 2015 and January 2016. The study of soundscapes is an emerging field of research that attempts to provide important baseline information on environmental conditions and biological compositions. Thermal noise, the result of Brownian motion of water molecules near the hydrophone, is the dominant noise source above about 60 kHz. At high frequencies, acoustic attenuation becomes extreme so that all noise sources are confined to an area close to the receiver. military and mapping), as well as small vessels, contribute anthropogenic noise at mid-frequencies. ![]() Ambient noise in the mid-frequency band is primarily due to sea-surface agitation: breaking waves, spray, bubble formation and collapse, and rainfall. Medium frequency sound cannot propagate over long ranges, owing to greater attenuation, and only local or regional (10s of km distant) sound sources contribute to the ambient noise field. During this time, oil exploration and construction activities along continental margins have moved into deeper water, and the long-range propagation of seismic signals has increased. Over the past few decades the shipping contribution to ambient noise has increased by as much as 12 dB, coincident with a significant increase in the number and size of vessels comprising the world's commercial shipping fleet. Shipping and seismic sources contribute to ambient noise across ocean basins, since low-frequency sound experiences little attenuation, allowing for long-range propagation. The low-frequency band is dominated by anthropogenic sources: primarily, commercial shipping and, secondarily, seismic exploration. Different noise sources are dominant in each of 3 frequency bands: low (10 to 500 Hz), medium (500 Hz to 25 kHz) and high (>25 kHz). Ocean ambient noise results from both anthropogenic and natural sources. The quiet nature of this site in the absence of local ships allows correlation of wind speed to wave noise across the 10-220 Hz spectral band of this study. The median noise levels at this site are low for an open water site due to the poor acoustic propagation and low average wind speeds. Biological sounds and the sound of wind driven waves controlled ambient noise levels in the absence of local ships. Median levels including local ships are 6-9 dB higher than median levels chosen from times when local ship noise was absent. Local ship noise, however, was present in 31% of the recordings in 1963 but was present in 89% of the recordings in 2005-2006. When noise from local ships was excluded from the 2005-2006 recordings, median sound levels were essentially the same as were observed in 19. Navy reports document ambient noise levels at this site in 64 and a seafloor recorder documents noise during 2005-2006. Repeated ocean ambient noise measurements at a shallow water (110 m) site near San Clemente Island reveal little increase in noise levels in the absence of local ships. 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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