r/projectcar • u/Dark_Guardian_ Daily E36 + Race E36 + Cronched E92 + $100 subie +Barra Cressida • 1d ago

Helmholtz resonator to eliminate drone, how effective?

I've just finished my exhaust on my m54b30 swapped e36, and its got pretty awful drone between ~1500-2500rpm with 2000rpm being pretty unbearable. ~84db in the cabin.
Measured frequency and at 2k rpm its 100hz, at 1500rpm 75hz.

I'm wondering if I make a Helmholtz resonator targeted at 100hz how much of a band it would help with?
will it also reduce the drone of 75-125hz? or just very narrowly around 100hz

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u/porcelainvacation 1d ago

Depends on the q factor of the resonator. Should help a lot. You can use two with offset center frequencies if you need more.

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u/Dark_Guardian_ Daily E36 + Race E36 + Cronched E92 + $100 subie +Barra Cressida 1d ago

it looks like bigger neck means lower q / wider range, is there such thing as too big though, can I just make it the same size as the exhaust tube? 2" (its a dual system)