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Here you go Alabama, just for you: this explains formula and based on the frequency you need to eliminate determines the length of pipe. "Simply put, the formula for determining the length of a Helmholtz chamber used to knock out a drone frequency is:
1100 feet/second (roughly the speed of sound) divided by the target frequency you're trying to eliminate (in cycles/second) gives a full wavelength in feet per cycle. Divide this number by 4 to get a quarter wavelength (a much more convenient length to work with). Finally, multiply this wavelength by 12 to get the chamber length in inches.
Tap this length of pipe into the exhaust tube, making sure the other end of the pipe is capped off, and you have a Helmholtz chamber that takes the annoying frequency and bounces it back into the flow 180 degrees out of phase, knocking it out. The results: no more drone"
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