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Acoustic propagation in shallow coastal waters is determined by the physical properties of the ocean medium, and the gradient in properties along with the boundary conditions (upper and lower). Apart from this, other factors of crucial influence are site-specific source nature, bathymetry, sediment properties and sound speed profiles. Different modeling approaches such as numerical Ray tracing, Parabolic Equation approaches have been used in shallow waters of Indian Ocean [Sridevi etal, 2010; Sanjana et al., 2014] where as very limited observational experiment is conducted in the East and West coastal waters [Hareesh Kumar et al, 2006]. An acoustic transmission and reception experiment is carried out at the shallow waters of Chennai with in 20 m depth contour and across a range of 9 km to understand the acoustic environment impact on acoustic propagation. Signal with a centre frequency of 1 kHz is transmitted from a mid depth source and received by a vertical array of 3 hydrophones (KE08,KE07,RESON) spaced 1 m apart in the middle of the water column. The signals are sampled at 10 kHz for duration of 10 minute at each range. The distance between transmitter and receiver array is maintained within 2-9 km, the ranges being 2 km, 5 km, 7 km and 9 km respectively. Source level is calculated by recording signals with a hydrophone of sampling frequency 96 kHz for duration of around 3 minutes. Apart from these, Sound speed profiles and CTD observations are also carried at each location. A propagation loss modeling has been attempted for the shallow Bay of Bengal considering a range independent approach. A Normal mode approach model KRAKEN with range independent approach has been used and results have been compared with observational output. Model has incorporated bathymetry and sediment characteristics of experiment location. Real oceanic wave guide experiences a lot of acoustic propagation influencing factors than model considered wave guide and so it is expected to have differences in transmission loss. Estimated transmission loss by modeling approach shows an overall increasing trend in nature with range. Sound speed, geometry of wave guide, frequency of interest, bottom characteristics, water column depth, number and position of source, density of water column and bottom layers, absorption in medium and wave guide boundaries etc strongly influence shallow water sound propagation. Such influencing factors were effectively incorporated in this modeling approach. KRAKEN normal mode range independent approach derived transmission loss for mid depth source frequency band (850-1050 Hz) has been validated with acoustic field observation conducted in same location. Transmission loss experience strong agreement for short range than long ranges between field and model data. This type of validation approach increases the reliability and accuracy of model. Also such comparative study provides some quantitative support for sound propagation study in shallow waters and help to design and develop location specific acoustic devices for both active and passive experiments. Key word: Transmission loss, Normal mode References Sridevi B., T.V Ramana Murty, Y. Sadhuram, M.M.M Rao, K. Maneesha, S.Sujith Kumar, P.L Prasanna, 2010: Impact of internal waves on sound propagation off Bhimilipantam, East coast of India. Estuarine and Coastal Shelf, Vol.88(2), 249-259. Sanjana M. C., G. Latha, A. Thirunavukkarasu, G. Raguraman, 2014: Acoustic propagation affected by environmental parameters in coastal waters. Indian Journal of Geo-Marine Sciences, 43(1), 17-21. Hareesh Kumar P.V., K.V Sanilkumar, V.N Panchalai, 2006: Shallow water internal waves and associated acoustic intensity fluctuations. Defence Science Journal, Vol.56, No. 4, 485-493.
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