In 2014, IMO approved guidelines on reducing underwater noise from commercial shipping, to address adverse impacts on marine life. Given the complexities associated with ship design and construction, the Guidelines focus on primary sources of underwater noise, namely on propellers, hull form, on-board machinery, and various operational and maintenance recommendations such as hull cleaning.
Much, if not most, of the underwater noise is caused by propeller cavitation (the formation and implosion of water vapour cavities caused by the decrease and increase in pressure as water moves across a propeller blade - cavitation causes broadband noise and discrete peaks at harmonics of the blade passage frequency in the underwater noise spectrum). On-board machinery and operational modification issues are also relevant.
The Guidelines also include definitions and underwater noise measurement standards.
When adopting the Guidelines, it was noted that there were still significant knowledge gaps, and that sound levels in the marine environment and the contribution from various sources was a complex issue, so setting future targets for underwater sound levels emanating from ships was premature and more research was needed, in particular on the measurement and reporting of underwater sound radiating from ships. The Committee invited interested Member Governments to submit proposals to a future session.