CFR > Title 33 - Navigation and Navigable Waters > CHAPTER I—COAST GUARD, DEPARTMENT OF HOMELAND SECURITY > SUBCHAPTER E—INLAND NAVIGATION RULES (parts 83 to 90) > PART 86—ANNEX III: TECHNICAL DETAILS OF SOUND SIGNAL APPLIANCES > SUBPART A—Whistles (§86.01 to §86.15) > 86.13—Combined whistle systems.

86.13—Combined whistle systems.

(a) A combined whistle system is a number of whistles (sound emitting sources) operated together. For the purposes of the Rules a combined whistle system is to be regarded as a single whistle.
(b) The whistles of a combined system shall:
(1) Be located at a distance apart of not more than 100 meters,
(2) Be sounded simultaneously,
(3) Each have a fundamental frequency different from those of the others by at least 10 Hz, and
(4) Have a tonal characteristic appropriate for the length of vessel which shall be evidenced by at least two-thirds of the whistles in the combined system having fundamental frequencies falling within the limits prescribed in § 86.03, or if there are only two whistles in the combined system, by the higher fundamental frequency falling within the limits prescribed in § 86.03.

Code of Federal Regulations

Note: If due to the presence of obstructions the sound field of a single whistle or of one of the whistles referred to in § 86.11 is likely to have a zone of greatly reduced signal level, a combined whistle system should be fitted so as to overcome this reduction.