Years ago when I worked on outdoor centres a few of the lads got the flat sort of half moon shaped shepherds whistles and used them on sessions. They where great and very loud but try as I might I could never make them work (I have never been able to whistle through my fingers either). I carried my acme thunderer but rearly used it as I am blessed with a shouting voice that would put a drill instructor to shame. I always carry a whistle while mountaineering or if out on the water canoeing, sailing etc but have never had to use one in anger, neither have I ever heard one used as a distress signal and had cause to investigate (not saying they aren't, just that I've never used or heard one). Think I first learnt the 6 blasts, 3 blasts signal back in cubs but have been reminded of it all thought scouts, cadets, My outdoor pursuits career and my time in the TA.
Nowadays I carry a small orange plastic whistle clipped the the ear defenders on my chainsaw helmet every day at work....it gets used several times a week to attract the attention of people on site, climbers up trees and to be heard over the sound of chainsaws working. Just last week I used it to stop a young climber working for me cutting off a large section of a tall pine pole that he'd left his top anchor attached too (meaning when he cut it, it would drop but effectively be tied to him and despite his second anchor may well have pulled him out of the tree....at best it would have been really painful). I watched him put the gob in and thought he'd drop the anchor next...he didn't, he started the back cut. With his saw on full revs he would never have heard me shouting....he did hear the whistle and thankfully stopped and look down. Some of the boys used to laugh at my whistle jangling about on my helmet....now most of them have them
The most times I've ever used a whistle (again my trusty acme thunderer) was back in the early to mid 90's while attending raves
I still think an acorn cup makes the best and most rustic whistle and can be painfully loud if you get the right cup
Cheers,
Bam.
(well impressed a chat about whistles can run into 5 pages lol)