Originally Posted By: dougwalkabout
The ACR seems interesting; I'd like to try one, since the form factor is very handy.

The ACR is very thin at 1/4 inch (1.0" x 2.0" x .25") - I keep the one I tested in my EDC neck pouch under my shirt. Here's the spec page for the ACR WW-3/Survival Res-Q Whistle. In an earlier post, Blast said he liked his (though he misspelled ACR in that post). The ACR is widely listed online in the $1.35-$3 price range.
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I have a bunch of the Fox 40 minis and they're okay as backup whistles, but aren't quite as loud as I'd like, and I prefer a lower pitch. I carry a Storm when I really need to make noise.
Given your cited criteria, you might also look at the whistle from TOPS knives. Click on the photo here to see the TOPS in top, bottom, and side views, or
view this Youtube video to see it from all angles.
The TOPS Knives whistle page includes a pretty reasonable direct order price for qty 5 (shipping included),

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I'm surprised at the good reviews of the JetScream. I bought one and it's not loud at all -- very disappointed. Maybe I got a dud?
I have no personal experience with them. The JetScream was measured at 109.3/112 dB adult/child in the 2008 whistle test, Part 1, pretty much at par with the ACR, though the JetScream was rated a bit hard to blow.
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)