(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 (+/-1 %) for a vessel of
20 metres or more in length, or 180 – 2100 Hz (+/-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 (+/-1 %) for a vessel of 20 metres or more in length, or 180 – 2100 Hz (+/-1 %) for a vessel of
less than 20 metres in length, of not less than the appropriate figure given in the table below.
Length of vessel in metres
1/3rd-octave band level at
1 metre in dB referred to
200 or more
75 but less than 200
20 but less than 75
Less than 20
Audibility range in
* When the measured frequencies lie within the range 180 – 450 Hz
t When the measured frequencies lie within the range 450 – 800 Hz
t When the measured frequencies lie within the range 800 – 2100 Hz
The range of audibility in the table above is for information and is approximately the range at which a whistle may
be heard on its forward axis with 90 per cent 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.
(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 ±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, so that the range in any direction will be at least half the
range on the forward axis. The sound pressure level shall be measured in that 1/3rd-octave band which
determines the audibility range.
(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, in order to
reduce interception of the emitted sound by obstructions and also to minimize hearing damage risk to
personnel. The sound pressure level of the vessel’s own signal at listening posts shall not exceed 110 dB (A)
and so far as practicable should not exceed 100 dB (A).
(f) Fitting of more than one whistle
If whistles are fitted at a distance apart of more than 100 metres, it shall be so arranged that they are not
(g) 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 paragraph (f) above is likely to have a zone of greatly reduced signal level, it is recommended that
a combined whistle system be fitted so as to overcome this reduction. For the purposes of the Rules a
combined whistle system is to be regarded as a single whistle. 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.
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 300mm 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.
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.