IMO — Two or more Whistles
1. If the whistles are fitted at a distance apart of more
than 100 metres, it shall be so arranged that they
are not sounded simultaneously.
2. Combined whistle systems are to be arranged when
one single whistle is likely to have a zone of greatly
reduced signal level, due to the presence of
obstructions.
3. If the whistles are to be used as a combined system
— fitted at a distance apart of not more than 100
metres — they shall be arranged to be sounded
simultaneously. The frequency of any of the whistles
shall differ from those of the others by at least 10
Hz.

Sound Distribution
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 risks to personnel.
The sound level of the vessel's own signal at listening
posts shall not exceed 110 dB(A) and as far as
practicable should not exceed 100 dB(A).

Extracts from COLREG. 1972 part D — Sound and Light Signals
Bell & Gong Signalling

Rule 33
Equipment for Sound Signals
(a) A vessel of 12 metres or more in length shall be
provided with a whistle and a bell and a vessel of
100 metres or more in length shall, in addition,
be provided with a gong, the tone and sound of
which cannot be confused with that of the bell.
The whistle, bell and gong shall comply with the
specifications in Annex III to these Regulations.
The bell or gong or both may be replaced by other
equipment having the same respective sound
characteristics, provided that manual sounding
of the required signals shall always be possible.

Rule 35
(f) A vessel at anchor shall at intervals of not more
than one minute ring the bell rapidly for about 5
seconds. In a vessel of 100 metres or more in
length the bell shall be sounded in the forepart of
the vessel and immediately after the ringing of
the bell the gong shall be sounded rapidly for
about 5 seconds in the after part of the vessel. A
vessel at anchor may in addition sound three
blasts in succession, namely one short, one prolonged
and one short blast, to give warning of
her position and of the possibility of collision to
an approaching vessel.

(g) A vessel aground shall give the bell signal and if
required the gong signal prescribed in paragraph

(f) of this Rule and shall, in addition, give three
separate and distinct strokes on the bell immediately
before and after the rapid ringing of the bell.
A vessel aground may in addition sound an appropriate
whistle signal.

Annex III
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 1 metre.

(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 more than 20
metres in length, and shall be not less than 200
mm for vessels of 12 to 20 metres 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.