FLUCTUATION AND VARIABILITY OF SHALLOW WATER AMBIENT NOISE FROM TIME SERIES MEASUREMENTS

Abstract

Medium period variations of ambient noise from time series measurements at two sites in shallow Bay of Bengal is reported. The noise at site1 is contributed by local shipping as well as wind, while at site2 wind is the dominant noise source. The data is analyzed for noise properties in the 0.1–7.5 kHz band. An increase in average noise level of up to 10 dB is noted for site1 owing to ship traffic contribution. The statistics of noise samples is reported in terms of standard deviation σ of the measured levels relative to the mean. Standard deviation values exhibit a wide range and decreases from 5 dB at low frequencies to 1.5 dB at 7.5 kHz. For site2, the standard deviation varies from 4.0 dB at low frequencies to 2 dB at 7.5 kHz and consistency is seen in the frequency band 2.0–6 kHz thus ascertaining strong influence of uniform noise source wind in this band. Wind speed dependence on noise variability is checked and it shows a decrease in standard deviation for increase in wind speeds and drops down to 1–2 dB for 5 beau fort. For higher wind speeds highly variable local shipping noise is better masked by wind/sea state noise which is uniform in nature.