Abstract
During the past decades, ship-building and engine-manufacturing have developed very quickly. The source levels of modern ships may have changed. The underwater radiated noise from many commercial ships of opportunity was measured by bottom-mounted self-contained hydrophone near the shipping lanes in the yellow sea of China one day in each month from June. 2014 to Mar. 2016. Twenty seven ships among them are selected whose peak value of 20-1000Hz passing curve is 10dB higher than background noise level. A Gaussian source distribution in depth direction is supposed and source levels normalized at different ranges are averaged in source levels acquisition to reduce the error of transmission loss calculation. A key result was the estimated results of RANDI-3 are much greater than measured results universally for ships whose length are larger than 200 meters, however, the two are close for ships whose length are less than 150 meters. Furthermore, the samples were divided into two categories to analyze. The broad-band source levels of modern commercial ships are not highly correlated to ship length but closely related to ship speed. The decay slope is less than RANDI-3.
Highlights
At this stage, the detecting frequency band of sonar has gradually extended to low frequency or very low frequency
The results show that the ship's broadband noise source level is closely related to the speed of the ship, and the correlation coefficient is above 0.85
For deriving the estimates of the source spectra from that data, we assumed that the source consists of a vertical distribution of incoherent point sources
Summary
The detecting frequency band of sonar has gradually extended to low frequency or very low frequency. Many empirical models have been proposed,The two earliest models were proposed by Ross [4] and Urick [5], who give the broad-band source level function of merchant ships or naval vessels expressed as a function of ship length, draft and speed. The individual source level can be calculated as a function of its speed and length These measurements were from older ships and narrower frequency bands. Megan and Ross [9], measured underwater radiated noise for seven types of modern commercial ships and pointed out simple models to predict source levels of modern merchant ships as a group from particular ship characteristics (e.g., length, gross tonnage, and speed) were not possible given individual ship-type differences.
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