This is the Fox 40 Classic Whistle for Scuba Diving and Water Sport Safety Features: Self-clearing chamber when submerged No moving parts to freeze, jam, or deteriorate Louder than any other whistle Used by Coast Guard and Search & Rescue The Fox 40 Classic is designed to be heard above ambient noise, the roar of engines, breaking waves and thundering gale-force winds. The chambers are designed to self-clear when submerged in water. There are no moving parts to freeze, jam or deteriorate.