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      10-11-2012, 01:45 PM   #13
MikeE92M3's Avatar

Drives: 08 E92 M3
Join Date: May 2012
Location: Richmond, VA

iTrader: (5)

Originally Posted by DB View Post
How did you determine the necessary length of the helmholtz chambers? Thanks for sharing all of this.
I used a formula that I had seen on another thread for a different car (350z) Formula is V/WV (Velocity/Wavelength) standard.

V=1187 Ft/Sec for soundwaves (standard wavelength speed)

You have to find out what frequency (Hz) your drone is prevalent at. I measured anywhere from 160-180 hz, depending on what gear, in my car. So Lets say 165-175 Average. I will do 175 for the purpose of this solution.

1187(wavelength)/175 (hz, drone frequency) = 6.78

Take 6.78, divide it by 4 (conversion) (6.78/4) and you have 1.69 feet (14 inches)

The pipe needs to be at least 14 inches to cancel out the drone.

Hope this makes sense. It took me a few minutes to grasp it but once you measure the drone frequency with a microphone or app (I used a HZ app on my iphone), it is easy to do the math from there.
2008 (07/08) E92 M3 BPM Stage II Tuned by Mike Benvo, 6MT, Turner Test Pipes, Magnaflow Custom Catback Exhaust (Magnaflow X Pipe, secondary cat delete, Aero Resonators and Helmholtz Chambers),Vorsteiner CF Lip, Hamann Style CF rear diffuser, CF Rear Decklid Spoiler, BMW Performance Kidney Grills, CF Side Grills, OEM GTS Wheels, K&N Drop-In Filter, GP Thunder 8k Angel Eyes, Painted Reflectors.