Decibel re 1 micropascal

Underwater sound pressure level is referenced to 1 micropascal (uPa), unlike airborne sound which uses 20 uPa, so the two scales are not interchangeable. Source level is quoted as dB re 1 uPa at 1 m. Shipping noise is a regulated concern: large merchant ships radiate broadband levels well above 180 dB re 1 uPa at 1 m at low frequency, and the IMO issued voluntary guidelines (MEPC.1/Circ.906, 2023) to reduce underwater radiated noise affecting marine life.