Clause 1 Whistles

(1) Frequencies and range of audibility

(a) The fundamental frequency of the signal must lie within the range 70-700Hz;

(b) The range of audibility of the signal from a whistle must be determined by those frequencies, which may include the fundamental and/or one or more higher frequencies, that provide the sound pressure levels specified in Appendix 3.1(3), and that lie within the range—

(i) 180-700 Hz (± 1%), for a vessel of 20 metres or more in length; or

(ii) 180-2100 Hz (± 1%), for a vessel of less than 20 metres in length.

(2) Limits of fundamental frequencies

To ensure a wide variety of whistle characteristics, the fundamental frequency of a whistle must be between the following limits—

(a) 70-200 Hz, for a vessel 200 metres or more in length; and

(b) 130-350 Hz for a vessel 75 metres but less than 200 metres in length; and

(c) 250-700 Hz, for a vessel less than 75 metres in length.

(3) Sound signal intensity and range of audibility

A whistle fitted in a vessel must 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 on 1/3-octave band—

(a) within the range of frequencies—

(i) 180-700 Hz (± 1%), for a vessel of 20 metres or more in length; or

(ii) 180-2100 Hz (± 1%), for a vessel of less than 20 metres in length;

(b) of not less than the appropriate figure given in the table below—

less than 200

less than 75

115+

111∇

The range of audibility in the table isfor information only. It is approximately the range at which a whistle may be heard on its forward axis, with 90% probability in conditions of still air, on board a vessel having average background noise level at the listening posts (taken to be 68 dB in the octave band centred on 250 Hz and 63 dB in the octave band centred on 500 Hz). In practice, the range at which a whistle may be heard is extremely variable and depends critically on weather conditions. The values given can be regarded as typical, but under conditions of strong wind or high ambient noise level at the listening post, the range may be much reduced.

(4) Directional properties

The sound pressure level of a directional whistle must be not more than 4 dB below the prescribed sound pressure level on the axis at any direction in the horizontal plane within ± 45 degrees of the axis. The sound pressure level at any other direction in the horizontal plane must be not more than 10 dB below the prescribed sound pressure level on the axis, so that the range in any direction will be at least half the range on the forward axis. The sound pressure level must be measured in the 1/3rd-octave band which determines the audibility range.

(5) Positioning of whistles

When a directional whistle is to be used as the only whistle on a vessel, it must be installed with its maximum intensity directed straight ahead.

A whistle must be placed as high as practicable on a vessel, in order to reduce interception of the emitted sound by obstructions and also to minimise hearing damage risk to personnel. The sound pressure level of the vessel’s own signal at listening posts must not exceed 110 dB (A) and, as far as practicable, should not exceed 100 dB (A).

(6) Fitting of more than one whistle

If whistles are fitted at a distance apart of more than 100 metres, it must be so arranged that they are not sounded simultaneously.

(7) Combined whistle systems

If, due to the presence of obstructions, the sound field of a single whistle or one of the whistles referred to in Appendix 3.1(6) is likely to have a zone of greatly reduced signal level, a combined whistle system should be fitted so as to overcome this reduction in signal.

For the purpose of this rule, a combined whistle system is to be regarded as a single whistle.

The whistles of a combined system must be located at a distance apart of not more than 100 metres and arranged to be sounded simultaneously. The frequency of any one whistle must differ from those of the others by at least 10 Hz.