Equipment for Sound Signals.
(a) A vessel of 12 metres or more in length shall be provided with a whistle, a vessel of 20 metres or more in length shall be provided with a bell in addition to a whistle 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.
(b) A vessel of less than 12 metres in length shall not be obliged to carry the sound signalling appliances prescribed in paragraph (a) of this Rule but if she does not, she shall be provided with some other means of making an efficient signal.
The specifications for a whistle are given in Annex III, but in short, a vessel 200m or more in length produces a deep tone and vessels less than 75m in length produce a relatively shrill tone. An intermediate tone is produced for vessel between these sizes.
Paragraph (a) allows for the bell and gong to be replaced by other equipment having the same characteristics. However, it should be born in mind, that a bell and gong are still required to be carried.