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post #3 of (permalink) Old 16-08-2016, 00:20
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Join Date: Dec 2009
Car: MG TF
Posts: 1,447
My experience with an aftermarket exhaust (an MGOC Standard S/S backbox) was that it was susceptible to a little drone at around 3000rpm. After doing some basic maths around the speed of sound, the wavelength of sound at 3000rpm (50Hz) which is 6.6m, I reckoned it was about 1/4 wavelength between exhaust pipes, if you take the measurement via the exhaust system.

So, that could form a resonator - with a reflection from the open end of either pipe reverberating back and forth between each pipe opening (oddly enough, sound reflects back both from the closed end of a pipe, and the open end of a pipe).

So, I stuffed a sock up one exhaust pipe and lo! The drone disappeared. Until the sock blew out.

All of which is by way of saying, I think you actually could have a worthwhile effect on any resonant drone, although I'm not sure if a hard surface on your valve would work. It might - it will reflect sound too, but in the opposite phase to an open pipe, which may have a cancelling effect. Worth a try.
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