Rules for Marine Horns (72 COLREGS)

How do you choose the right horn (ship’s whistle) for your vessel?    What are the regulations?

Before 1972, selection of a ship's whistle was largely a subjective decision; as was approval by regulatory agencies. Generally speaking, an "adequate" sound signal was selected and/or accepted without much concern as to tone or specific loudness at the source.

In the interest of maritime safety and to aid uniform enforcement, the Inter­governmental Maritime Consultative Organization (IMCO) formulated a set of standards regarding Technical Details of Sound Signal Appliances and, in 1972, adopted them as ANNEX III to the International Regulations for Preventing Collisions at Sea which are commonly referred to as the '72 COLREGS. These regulations became effective on July 15, 1977, adopted by the United States as the Navigational Rules Act of 1977. The vessels of all flag states having ratified the ’72 COLREGS are bound to these rules.

IMCO was renamed in 1982 to the International Maritime Organization (IMO). Following the signing of the '72 COLREGS, the United States renewed former efforts to update and unify the various inland navigation rules. This effort resulted in the enactment of the Inland Navigational Rules Act of 1980 (INRA '80) which very closely parallels the '72 COLREGS. As with the '72 COLREGS, the Inland Navigational Rules Act has an ANNEX III regarding technical details for sound signal appliances. The INRA '80 ANNEX III differs only slightly from the '72 COLREGS ANNEX III in that it provides slightly more latitude regarding sound pressure level and audibility ranges at different frequencies. The effective date for the Inland Navigational Rules was December 24, 1981 except for the Great Lakes which was March 1, 1983.

The 1972 International Regulations for Preventing Collisions at Sea ('72 COLREGS), ANNEX III, provides the technical details required of sound signal appliances. This specifies the fundamental sound frequency range of the whistles to be fitted to various classes of ships, the class being determined by the vessel's length. Within each class the minimum sound pressure level intensity at a distance of one meter from the horn is specified. This determines the theoretical range of the whistle, although the actual range will, under many conditions, be much greater than this.

This information is displayed in the following table:

By Length of Vessel, Db readings in a 1/3 Octave Band @ 1 Meter

200 Meters (656 ft.) or more: 70-200 Hz, 143 dB

75 to 200 Meters (246 to 656 ft.): 130-350 Hz, 138 dB

20 to 75 Meters (65 to 246 ft.): 250-700 Hz, 130 dB

12 to 20 Meters (40 to 65 ft.): 180-450 Hz, 120 dB


12-20 Meters: (40 to 65 ft.): 450-800 Hz, 115 dB


12-20 Meters: (40 to 65 ft.): 800-2100 Hz, 111 dB .5

In a nutshell, the larger the vessel, the louder and lower tone the ship’s whistle must be.

This makes intuitive sense: if you can’t see due to fog or restricted visibility, the sound of the ship’s horn gives you an idea of the size and maneuverability of the vessel. So… do you know if the horn on your vessel is appropriate and complies with the ’72 COLREGS?

In the United States, the U.S. Coast Guard is the flag state authority that has responsibility for determining whether or not the horn on your vessel is compliant.

In the past, the U.S.C.G. would issue Certificates of Compliance to manufacturers for each horn model they made as evidence it met the rules. Today, the U.S.C.G. no longer provides these certificates, but relies on ABS, or other classification societies Type Approval certificates to verify compliance to 72 COLREGS.

Therefore, if you have a vessel over 12 meters in length, you must have a Type Approval certificate for your horn to verify it meets the rules. For pleasure craft between 12 and 20 meters, the NMMA (National Marine Manufacturers Association) also acts as an independent certification authority for horns to the A-23 ABYC standard.

In order to achieve certifications such as Type Approvals from various class societies such as ABS or DNV etc., manufacturers of sound signal appliances (A.K.A. horns, bells, gongs) must make accurate and repeatable measurements of the sound characteristics of these devices. These measurements must be made in an environment free of reflecting surfaces and other disturbing influences such as background noise, wind, etc. Generally speaking, the best environment for measuring sound level output is inside a specially designed and built chamber referred to as an anechoic chamber.

To ensure that sound level data submitted to customers, regulatory agencies, and independent certifying authorities is accurate, Kahlenberg Industries has such a chamber for testing and collecting sound data on their line of sound signal appliances. This anechoic chamber has been found to be suitable by a number of certifying authorities. The instruments Kahlenberg uses to collect dB and frequency data on their sound signals are certified with accuracy traceable to the National Bureau of Standards.

Type Approval Certificates Certificates for all Kahlenberg air and electric horns (as well as ship’s bells and gongs) have been issued by ABS confirming their compliance with the IMO '72 COLREGS.

Products for vessels 12 to 20 meters are also certified by the NMMA, and listed on their website accordingly.


Generally, the horn on a vessel should be installed facing directly forward, as high as possible, in an area with a minimum of equipment or structure near the horn which could disrupt the travel of sound. The '72 COLREGS give additional recommendations relating to the installation of navigation horns (ship’s whistles).

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