Just to add a bit of info about other whistles to this conversation, Pat O'Riordan's whistles had flat bottom/arched top windways, as did David Boisvert's Greenwood whistles.

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"Those who can make you believe absurdities
can make you commit atrocities." - Voltaire

Add another; my Humphrey also has flat bottom/curved top. Don't rmember the model name.


@VioPhilist.... right off the printer, how smooth are the interior surfaces of the windway? Don't know if it would affect the issues you are having, but if it is fairly rough, have you thought about printing the opening a bit on the small side, and opening it up with needle files etc. to refine it?

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In Cape Breton, Nova Scotia