33 CFR §67.10-10

1. (a)Sound signals required by §§ 67.20-10, 67.25-10, and 67.30-10 must be operated continuously, regardless of visibility, unless the sound signal is controlled:
   1. (1)By an attendant on the structure;
   2. (2)Remotely by an attendant on a nearby structure; or
   3. (3)By a fog detection device capable of activating the sound signal when the visibility in any direction is reduced to the rated range at which sound signal operation is required by this part.
2. (b)During construction and until such time as a sound signal is installed and operating on a platform, the whistle of an attending vessel moored alongside the platform may be used to sound the signal required for the structure by this part.