Interesting to hear about the Acme whistles. I need to find one of those and try it out for "keychain" duty.

I have a couple of the Fox 40 Sharx whistles. They are VERY loud and effective (the original Fox 40s are nowhere near as loud) but too big for my keychain. The Storm whistle is a bit louder still, and the sound is more directed forward (thus easier on the ears of the user). But a Sharx, tethered to a pocket compass, is standard pocket carry for me when I'm anywhere off pavement.