Effects of Ship Speed and Propagation Distance on Pressure and Frequency of Underwater Noise Emitted from the Nagasaki Maru

摘要

Reduction of underwater noise emitted from ships is considered important for preserving and protecting marine mammals and for increasing the precision of ultrasonic instruments. However quantitative reports on underwater noise emitted by ships are scarce. In the present study, the characteristics of underwater noise emitted from the Nagasaki Maru were surveyed. The effects of ship speed and propagation distance on the pressure and frequency of the detected noise were assessed. Ship noise was recorded by a small stationary vessel equipped with digital hydrophone. The Nagasaki Maru sailed at 3 kt, 7 kt and 10 kt in a circle, at a constant distance from the recording vessel. The measured sound pressure level generated by the Nagasaki Maru was slightly higher than the desirable level, but similar to that of other newly built research ships. Sound pressure increased logarithmically with decreasing distance from the ship. Peak sound pressure was detected at approximately 300 Hz at 3 kt, 7 kt and 10 kt; however, the 500–600 Hz band also showedstronger sound pressure at 7 kt and 10 kt.