REI Tri-Power Safety Whistle
Source: bought it new
Price Paid: $5
Very loud whistle.
Great loud whistle. It is orange so you will never lose it. However it is big, that might be a downside for some. My wife hates this thing because I am always testing it!
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Here's what other sites are saying:
The REI Tri-Power safety whistle is an essential piece of safety equipment to have on you during outdoor activities. 3-chamber design produces different pitches and blasts up to 120 decibels. Made of high-visibilty, non-corroding ABS plastic for durability. Whistle features a hands-free mouth grip. Metal clip with key ring allows you to attach the whistle to a PFD, pack or jacket. Safety whistle meets U.S. Coast Guard and Safety of Life at Sea (SOLAS) requirements; also meets anti-choking standards for children.