Over the years, I have purchased several whistles. All, or most of them, are fairly soft sounding and such. Now I know louder whistles are out there, and I just need to find them [preferably in the range of…lets say…a soprano C]. Any suggestions???
Your question reminds me of one reportedly put to a famous French chef: What is a good low-fat cheese?
His answer: “Madame, there isn’t one.” 
(Just kidding.)
I’m a big fan of Susato whistles, which have quite a bit more sound than a lot of whistles. I originally bought one because I attend a rather large session and I was lost in a mass of fiddlers and guitarists with my little Oak. Turned out I also loved the sound of them.
~Crysania
Susato is moderately loud.
If you want --really-- loud, call up Michael Cronnolly of M&E flutes and see if he’ll make you a whistle.
I have a high D he made which is louder than a clarinet, very much louder than a Susato.
Do be aware his whistles aren’t easy blowers, though–you’ll need a well-supported column of air, and you’ll need a lot of it.
–James
If you’re talking D whistles, I think the best combination of loud with good sound is Thin Weasel. If you want moderately priced, Water Weasel, in the inexpensive range, Susato. If you can bear a C whistle, the Walton’s is a fantastic, pretty loud, C. Some other good loud whistles are Silkstones, but the sound donesn’t quite have the character of Weasels.
Sweetheart Profession D. It’s the one I use for Morris Dancers. 'Nuff said.
Redwolf
The Desi Seery whistle is definitely loud too!
Older Chieftain - unbeatable for volume!
Dunno about the new ones.
HTH
Aren’t Copeland’s loud?
This is funny. I think most good whistles are too loud. It’s hard to find a good not-so-loud whistle, but EASY to find a good loud whistle! In no particular order:
New Abell
Bleazey
Seery
Swayne
Overton
Copeland
Sweet
Thin Weasel
Some (not all) Water Weasels
Syn
Boisvert
O’Riordan
Silkstone
Have fun.
I would second Redwolf…the sweetheart pro is a loud whistle. Les.
I kind of hate Susato whistles but some like them so that’s a matter of personal preference.
Syn’s are wonderful and pretty loud. I have a set of Syn whistles that get more play time than any other whistles around here.
My Bleazey is louder than my O’Riordan but not by very much. The Bleazey is fairly breathy and the O’Riordan is pretty pure sounding. My two Syns (aluminum and lancewood) are next, pretty close, but not quite as loud. The aluminum is fairly breathy and the lancewood is nicely pure sounding. Then comes my Overton, Weston Sapele and lastly (surprise!) my Hoover redwood.
The Boisvert I had probably falls in with the Overton/Syn group. Same for the Thin Weasel. The Lon Dubh I had for a while was quieter than the Overton, probably similar to the Weston.
It is easier to distinguish volume differences on low whistles, at least to me.
Sweetheart Pro model is loud on its own, but in a crowd it doesn’t cut through the noise. It’s perfect alone or in a small group.
The Bleazey (high d) I had was not at all breathy.
I’ve got an A/G combination (one mouthpiece, two tubes) made by Gary Humphrey that I want to sell. The reason: they are gorgeous sounding BUT TOO LOUD for my needs. Let me know if you’re interested!
I have a Thin Weasel, a couple Water Weasels, and a Reyburn set. The Reyburn set is louder than either Weasel, right in the middle as far as price is concerned, and has a beautiful voice.
A Reyburn would be the ticket in my opinion.
Review, pics, & sound sample: http://chiffboard.mati.ca/viewtopic.php?t=30213
I ordered a loud Overton for outdoor and indoor with wifes drum kit whistle. It has a very narrow windway. Just deep enough to get a cotton thread through for cleaning. It has lots of back pressure and took some getting used to, but it is my favourite. It doesn’t take a lot of air just a thin high pressure stream.
Brian
I have quite a lot of D whistles. The loudest would be a Thin Weasel. My Copeland D is loud at the top of the second octave but not as well balanced. After that there are a lot of whistles with good volume but not quite as loud. Overton, Sindt, Busman, Abell … take your pick.
Susatos are consistently loud. IMO, you shouldn’t get a Susato if you are looking to move from a beginner’s whistle to a more expensive one and don’t have much experience. They differ a bit from key to key but can be quite nice whistles but take a bit of experience to control well.