International Rules for Sound Signaling, IMO, (72 COLREGS)

International Sound Signaling Rules (IMO, ’72 COLREGS)

Choosing the Right Ship’s Whistle

Before 1972, horn selection was subjective and loosely regulated. To improve safety and consistency, the IMO (formerly IMCO) established technical standards for sound signaling devices in ANNEX III of the 1972 COLREGS, effective July 15, 1977. These rules apply to all vessels under flag states that ratified the treaty, including the U.S. via the Navigational Rules Act of 1977.

Horn Requirements by Vessel Length

The COLREGS specify frequency and minimum sound pressure levels (measured at 1 meter) based on vessel length:

Vessel Length	Frequency Range (Hz)	Min SPL (dB)
≥ 200 m	70–200	143
75–200 m	130–350	138
20–75 m	250–700	130
12–20 m (Option 1)	180–450	120
12–20 m (Option 2)	450–800	115
12–20 m (Option 3)	800–2100	111.5

Summary: Larger vessels require louder, lower-frequency horns.

Certification and Compliance

In the U.S., the Coast Guard enforces compliance. Horns must have Type Approval from ABS or similar classification societies. For vessels 12–20 meters, NMMA certification to ABYC A-23 is also accepted.

Manufacturers must test horns in anechoic chambers to ensure accurate, repeatable sound measurements. Kahlenberg Industries uses such a chamber, with instruments traceable to the National Bureau of Standards. Their products are certified by ABS and NMMA.

Installation Guidelines

Horns should be mounted forward-facing, as high as possible, and away from obstructions that could block sound. The ’72 COLREGS provide further installation recommendations.