1. Two whistles more than 100m apart must not be sounded
   simultaneously.
2. Combined system is recommended to overcome reduction
   of signal level due to obstructions.
3. Simultaneous sounding from combined system:
   - not more than 100m between whistles
   - at least 10 Hz frequency difference
4. Sound level at listening posts: not exceeding 110 dB(A)

Length of vessel in metres 1/3 octave Sound Pressure
Level at 1m, dB (minimum) 		Frequency Hz Audibility acc. to IMCO definition, Nautical miles
>200m
75m - 200m
20m - 75m
< 20m
143
138
130
120		 70-200
130-350
250-700
250-700

2,0
1,5
1,0
0,5

 

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