Noise emission to water (URN)

Underwater radiated noise (URN) is the sound a ship emits into the water, dominated at most speeds by propeller cavitation and at low speed by machinery and hull-borne noise. It is quantified as a radiated-noise source level in dB referenced to 1 micropascal at 1 m, measured by hydrophones as the ship passes on a controlled range. Class notations such as DNV SILENT set limit curves by ship function, SILENT-E for general merchant ships through stricter tiers like SILENT-R for research vessels and SILENT-F for fishing. The IMO issued revised guidelines for reducing URN (MEPC.1/Circ.906) to limit harm to marine mammals. Design levers are a less-cavitating propeller, wake conditioning, and resilient machinery mounts.