ANNEX III - Technical Details of Sound Signal Appliances

1. Whistles

  • (a) Frequencies and range of audibility: The fundamental frequency of the signal shall lie within the range 70 - 700 Hz. The range of audibility of the signal from a whistle shall be determined by those frequencies, which may include the fundamental and/or one or more higher frequencies, which lie within the range 180 - 700 Hz ($\pm 1\%$) for a vessel of 20 metres or more in length, or 180-2100 Hz ($\pm 1\%$) for a vessel of less than 20 metres in length and which provide the sound pressure levels specified in paragraph 1(c) below.
  • (b) Limits of fundamental frequencies: To ensure a wide variety of whistle characteristics, the fundamental frequency of a whistle shall be between the following limits:
    • (i) 70 - 200 Hz, for a vessel 200 metres or more in length;
    • (ii) 130 - 350 Hz, for a vessel 75 metres but less than 200 metres in length;
    • (iii) 250 - 700 Hz, for a vessel less than 75 metres in length.
  • (c) Sound signal intensity and range of audibility: A whistle fitted in a vessel shall provide, in the direction of maximum intensity of the whistle and at a distance of 1 metre from it, a sound pressure level in at least one 1/3rd-octave band within the range of frequencies 180 - 700 Hz ($\pm 1\%$) for a vessel of 20 metres or more in length, or 180-2100 Hz ($\pm 1\%$) for a vessel of less than 20 metres in length, of not less than the appropriate figure given in the table below.
  • (d) Directional Properties: The sound pressure level of a directional whistle shall be not more than 4 dB below the prescribed sound pressure level on the axis at any direction in the horizontal plane within $\pm 45$ degrees of the axis. The sound pressure level at any other direction in the horizontal plane shall be not more than 10 dB below the prescribed sound pressure level on the axis.
  • (e) Positioning of whistles: When a directional whistle is to be used as the only whistle on a vessel, it shall be installed with its maximum intensity directed straight ahead. A whistle shall be placed as high as practicable on a vessel to reduce interception of the emitted sound by obstructions and minimize hearing damage risk to personnel.
  • (f) Fitting of more than one whistle: If whistles are fitted at a distance apart of more than 100 metres, they shall not be sounded simultaneously.
  • (g) Combined whistle systems: If due to obstructions the sound field of a single whistle or one of the whistles referred to in paragraph 1(f) above is likely to have a zone of greatly reduced signal level, a combined whistle system should be fitted. The whistles of a combined system shall be located at a distance apart of not more than 100 metres and arranged to be sounded simultaneously. The frequency of any one whistle shall differ from those of the others by at least 10 Hz.

2. Bell or Gong

  • (a) Intensity of signal: A bell or gong, or other device having similar sound characteristics shall produce a sound pressure level of not less than 110 dB at a distance of 1 metre from it.
  • (b) Construction: Bells and gongs shall be made of corrosion-resistant material and designed to give a clear tone. The diameter of the mouth of the bell shall be not less than 300 mm for vessels of 20 metres or more in length. Where practicable, a power-driven bell striker is recommended to ensure constant force, but manual operation shall be possible. The mass of the striker shall be not less than 3 per cent of the mass of the bell.

3. Approval

  • The construction of sound signal appliances, their performance, and their installation on board the vessel shall be to the satisfaction of the appropriate authority of the State whose flag the vessel is entitled to fly.