Underwater Radiated Noise (URN) generated from shipping activity is known to have significant environmental impacts to some marine life.1 Quantifying URN from ships is an inherently difficult undertaking because vessels are equipped with many and various types of machinery that each contribute to different frequencies of noise and vibration levels.2 Many studies show that, at higher operational speeds, the main source of URN is due to cavitation of the propeller.2,3,4,5 At low speeds, the main source of URN is generally associated with machinery noise propagating into the water column. Once a ship reaches its cavitation inception speed, the URN level and pressure pulses are dominated by the cavitating propeller.2 Cavitating propellers have more complex noise spectra with considerably higher sound pressure levels than the noise spectra for non-cavitating propellers.
The overall URN level of surface ships can be associated with vessel speed and displacement up to 100 Hz for commercial vessels.3 Propeller cavitation noise is known to start at 50-150 Hz but can extend to 10,000 Hz. Low-frequency sounds are of more concern because of their transmissibility over long distances, and they often match the frequency of sounds of importance to marine mammals. Research was conducted to investigate the noise from different types of ships. Two dominant features that show predominant low-frequency noise at the low-frequency range result from propeller blade cavitation and their harmonics. For container ships and vehicle carriers there is some evidence for an increase in radiated noise with an increase in speed.3