"This is probably the loudest whistle I have ever heard, although I can say for sure since I plug my ears with my fingers when I blow. I carry it in reserve as a last resort for aggressive bears and cougars when all other measures fail..."

"I teach skydiving, and while Hollywood would have you believe that entire conversations can easily be held while in freefall, this just isn't the case- 150mph winds tend to be quite loud. Just for fun, I began buying every "miracle whistle" I came across, and testing them in freefall with friends, thinking that we could use a simple code for a few things, just to give us a line of communication without requiring line of sight.
I had tried everything- fox 40, storm, etc. I hadn't tried the Hyperwhistle yet because it looked too large and ungainly. Finally I was out of alternatives so I ordered one.
OK...this thing is loud. Crazy loud. Any reviewer who claims otherwise or compares its volume unfavorably to any other whistle on Amazon as of the date of this writing is lying. No doubt about it.
In fact, simply angle the mouthpiece into the relative wind during freefall and I think they can probably hear it on the ISS. LOUD. Seriously."

"This whistle was the only one of its kind with 5 of us back packing the gates of the Artic National Park in Alaska, and it was truly fantastic. At times we needed to send one of us around a bend to scout parts of a river or make a climb and signal back....and this whistle overcame the river noises or mountain curves. Outstanding piece of equipment. I wish it was a little less bulky but trust me, well worth having it."