From the Coast guards latest edition to the Rules:

ANNEX III, section 1, paragraphs (a) and (c) are amended to read as follows:

1. Whistles

(a) Frequencies and range audibility
The fundamental frequency of the signal shall lie within the range 70-700Hz. 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 180-700Hz (+/-1%) for a vessel of 20 meters or more in length, or 180-2100Hz (+/-1%) for a vessel of less than 20 meters in length and which provide the sound pressure levels specified in paragraph 19c0 below.

(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 meter from it, a sound pressure level in at least one 1/3rd-octave band within the range of frequencies 180-700Hz (+/-1%) for a vessel of 20 meters or more in length, or 180-2100Hz (+/-1%) for a vessel of less than 20 meters in length, of not less than the appropriate figure given in the table below.

Length of vessel in meters
1/3rd-octave band level at 1 meter in dB referred to 2X10-5N/m2 Audibility range in nautical miles
200 or more
75 but less than 200
20 but less than 75 143
138
130 2
1.5
1
Less than 20 120*1
115*2
111*3 0.5

*1 When the measured frequencies lie within the range 180-450Hz
*2 When the measured frequencies lie within the range 450-800Hz
*3 When the measured frequencies lie within the range 800-2100Hz

(It was a table in the original page, sorry)

A