Eliminating Drone

[Evoking]

Since some aftermarket exhausts can drone does anyone know if this drone elimination approach would work on the Z4M? Simple process of separating the pipes 1/4 of an inch so they don't reverberate off each other thus magnifying sound.

http://www.m3forum.net/m3forum/showt...07#post3313807

[iamwiz82]

I don't want to register for another forum. Can you re-host the pics?

[Evoking]

Here is the fix on M3. Looks like nothing was done but apparently just a bit more separation makes a big difference.

Wonder if we could do this at the megaphone looking part before the x pipe. This is where it seems the pipes are the closest...

[ZStig]

interesting

[Dammmittt]

It's not quite that simple. What you're doing here is changing the natural frequency by adding the piece of metal in there. It's my guess that the small piece of metal there greatly increases the lateral stiffness of the pipes making the natural frequency higher (above what your engine can produce through RPMs).

The 1/4 inch of separation isn't going to make that much of a difference as far as acoustics are concerned. It's like saying that if you move your head 1/4 of an inch from a loud speaker you it would much quieter.

As an example to this, it would be interesting to see how different materials between the pipe change the natural frequency. For example, if you put a spunge between the pipes, then rubber, plastic, wood, aluminum, steel, etc. Each time you put something stiffer between the pipes, you should raise the resonant frequency, so the drone occurs at higher and higher RPMs, until you don't hear it at any RPMs.