Midfrequency Ambient Noise Notch using Time-Series Measurements in Bay of Bengal

Vertical directionality pattern of ambient noise in shallow waters of Bay of Bengal has been investigated in the 0.5-4-kHz frequency band using time-series measurements carried out for 25 days using an automated noise measurement system. The pattern seen is in general anisotropic with a well-developed lower lobe characteristic of hard bottom, and the pattern varies with time depending on sources contributing to the noise field and variation in environmental conditions such as sound-speed profile. Noise in the midfrequency band is dominated by wind-driven wave activity and, under suitable oceanographic conditions, shows a notch in the horizontal for the downward refracting environment. The evolution of the directionality pattern and the notch width and depth have been investigated in the 2-4-kHz band because understanding the notch characteristics would be useful in the operation of receiving sonar systems. Sound-speed profile of the water column and sediment grab data at the experimental site is used to characterize the environment. Directionality pattern is studied with respect to the sound propagation in a reflecting environment and the critical angle of the seabed has been estimated theoretically as well as from measurements. This paper shows that the noise notch is well pronounced for high wind speeds and notch depth increases with wind speed.