COLREGs trainerAnnexes
Annex III
Whistle frequencies and ranges, bell and gong specifications.
Wording follows the International Regulations as printed in the US Coast Guard Navigation Rules handbook (2024), with British spelling, and is shortened in places. Download the rules
The fundamental frequency of the signal shall lie within 70–700 Hz. The range of audibility shall be determined by those frequencies (which may include the fundamental and/or higher frequencies) lying within 180–700 Hz (±1%) for a vessel of 20 m or more, or 180–2100 Hz (±1%) for a vessel of less than 20 m, that provide the sound pressure levels in paragraph 1(c).
To ensure a wide variety of whistle characteristics, the fundamental frequency shall be: (i) 70–200 Hz for a vessel 200 m or more in length; (ii) 130–350 Hz for a vessel 75 m to under 200 m; (iii) 250–700 Hz for a vessel less than 75 m.
A whistle shall provide, in the direction of maximum intensity at 1 m, a 1/3-octave-band sound pressure level (within 180–700 Hz, ±1%, for a vessel of 20 m or more; 180–2100 Hz, ±1%, under 20 m) of at least: • ≥ 200 m: 143 dB (≈ 2 NM) • 75 to < 200 m: 138 dB (≈ 1.5 NM) • 20 to < 75 m: 130 dB (≈ 1 NM) • < 20 m: 120 dB at 180–450 Hz, 115 dB at 450–800 Hz, 111 dB at 800–2100 Hz (≈ 0.5 NM) (referred to 2 × 10⁻⁵ N/m²). Audibility ranges are approximate, 90% probability in still air with average background noise, and may be much reduced in strong wind or high ambient noise.
The sound pressure level of a directional whistle shall be not more than 4 dB below the on-axis level within ±45° of the axis, and not more than 10 dB below at any other horizontal direction, so the range in any direction is at least half that on the forward axis. Measured in the 1/3-octave band that determines the audibility range.
A directional whistle used as the only whistle shall be installed with maximum intensity straight ahead. A whistle shall be placed as high as practicable to reduce interception by obstructions and minimise hearing damage. The level of the vessel’s own signal at listening posts shall not exceed 110 dB(A) and, so far as practicable, not exceed 100 dB(A).
If whistles are fitted more than 100 m apart, they shall be arranged so that they are not sounded simultaneously.
Where obstructions are likely to create a zone of greatly reduced signal level, a combined whistle system is recommended. For the Rules it counts as a single whistle. Its whistles shall be no more than 100 m apart, sounded simultaneously, and the frequency of any one shall differ from the others by at least 10 Hz.
(a) Intensity: a bell, gong, or similar device shall produce a sound pressure level of not less than 110 dB at 1 m. (b) Construction: made of corrosion-resistant material giving a clear tone. The mouth of the bell shall be not less than 300 mm in diameter for vessels of 20 m or more in length. A power-driven striker is recommended (manual operation must remain possible); the striker mass shall be at least 3% of the bell’s mass.
The construction, performance and installation of sound signal appliances shall be to the satisfaction of the appropriate authority of the State whose flag the vessel is entitled to fly.
A training aid, not the official text. The COLREGs are an IMO convention, and your national authority publishes the version that applies in its waters. Check the rules themselves before you rely on them at sea. Download the rules.