To borrow from Willie Shakespear’s Hamelet…
So Sat, I bought a whistle…
Now I have always believed, but where I got the info, I have not a cooking clue, or for the purposes of this post, that a whistle WITH a pea, is better (from an audibility point of view) than a comparable whistle WITHOUT a pea…
Truth be told, I actually don’t know… ehere were a few variants of whistles thatwhere available, which threw me into a right-royal twizz
Never mind the COVED thing, I kinda felt that they might get just a wee tad annoyed if I started to blow a dozen different whistles in the shop, at the top of my lung power just to try and establish "which was the loudest / most audible… 
I settled for a bog standard metal cased whistle with a pea, over a plastic one that had what looked like 2 whistles combined into one but no pea…
Anybody actually know which is the more audible one???
, but I sure enjoyed reading your post!