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For my first print I tried PLA with 25% infill and only one chamber worked. I also found that that the whistle leaked when I covered both exit holes and blew into the whistle.

For my second print I used 100% infill and everything worked. But I thought it was pitched too high.

For my third print I scaled it up to 115% all around (Simplify3D) and I like the lower pitch better. However, you can see that the
"ceilings" in the air passages are pretty rough looking and I don't know if the print job could take much more of an increase. I might try another 115% overall, and then an additional increase on the Y axis to make the chambers longer.