From Brother Steve’s (aka StevieJ on the chiff) Tin-Whistle pages:
How to get useful “foldback”
This tip comes from an excellent Montreal fiddler, Dave Clark. He might have got it from observing Paddy Keenan!
In loud sessions, with a quiet whistle, sometimes you can’t hear yourself very well, even though you’re projecting fine to people on the other side of the room. Try wearing a wide-brimmed hat. It’s like having monitor speakers mounted on your temples.
I’ve heard / read that the Reyburn whistles (Esp the older ones) can be fairly loud. If one comes up for sale second hand on the site, it might be worth a look?
I’ve got three high Ds and they have different volume levels:
c1980 Feadog: very soft, only can be heard if I’m playing with one or two other people.
Burke Session Bore: medium volume, can be heard in a medium sized session
Susato: very loud, can be heard even if there are 30 people, some playing accordions and banjos.
About a whistle being louder than it seems, a strange thing happened at a session many years ago. It was a smallish session with two or three fiddlers and maybe a guitar and somebody playing a Generation whistle. In person the fiddles dominated the sound and the whistle was soft. I taped several of the tunes, and when I listened to the tape later, all I could hear was the whistle! Unfortunately the whistler didn’t know the tunes and was noodling around trying to figure them out… so all I ended up with was a tape of a whistle noodling! (Let that be a warning to noodling whistlers… )