I haven't been able to find any good information on the Internet as to what whistle is best for a survival kit. I'll need to choose a whistle to use with clients when I "take to the woods". I picked up a whistle made by NRS for marine use at Eastern Mountain Sports for under $4. I needed a whistle for my whitewater training back in April and my kids had "borrowed" my other whistle at the time. The NRS whistle is flat and clips onto your clothing though there is a place to put a lanyard through.
I also picked up a Fox40 "Classic Whistle with Lanyard" at Walmart in the Sporting Goods section for about $6. My kids were very excited to help me "test" the whistles and we headed for the woods. Right away it was clear that the whistles were not audible over the babbling brook near my house.
However, it was amazing how far away we could hear each other with the whistles even in dense evergreens. My kids thought the Fox40 whistle was loudest. I thought the NRS whistle was loudest. I suspect this is because the FOX40 is a higher pitched whistle. I'm quite a bit older than the kids, so I am less able to hear the higher frequencies than my kids. Perhaps I'll get more scientific in the future, but for now I'd say either of the whistles is adequate, but I'll be buying more of the NRS whistles. The further away I can hear a whistle, the better. I'd be interested to hear someone else's opinion about different types of whistles.
On another note, I got my Maine Guide written exam results back yesterday. I did much better than I thought and was surprised at my score. I mailed in my $81 today and hope to get the actual license soon.