It took 8 or 10 tries to get dialed in, but I finally got the suckers nailed. Picture a 5" square piece of plywood with a 1.5" hole in the middle, then a 1" slice of 4" PVC pipe, then another piece of ply with hole. Exhaust hose runs in one side, another comes out the other side - with a 44dB drop at the target frequency! Magic. One minute you've got an annoying high pitch scream, the next minute there's just a quiet Woosh of air.
Now to come up with a shop-ready version that looks decent, then make 44 of them.
Thanks Wayne, that article provided a good launch.
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