What do I need to know about dampening exhaust resonances?
 

On my new 98 E300D I had to get some exhaust work done. Specifically, the entire section of pipe from the cat to the muffler was replaced. However, one thing I noticed is that there is a resonance at 900 rpm that was not there before. (The exhaust guy did not know the word "resonance," but he understood when I used the word "vibration." haha)

They are happy to do whatever they need to do to make it right, but claim that I probably need new engine mounts. Personally, I doubt that this is the case, but I will know for certain when my indie looks at the car next week.

Working on the assumption that the mounts are fine, how does one go about eliminating/dampening exhaust resonances and vibrations? The exhaust shop said they could install a flex pipe but that I shouldn't need it since that's not how the car was designed.

Thanks.

I would say that vibration resonance, as distinct from sound resonance, could be fixed by either a flex connection or a different stiffness of mount or perhaps a mass clamped to the pipe somewhere.

A noise resonance can only be fixed by changing the length or diameter of the pipe, basically. In industrial applications there are duct resonators and other tricks to dampen resonance.

Sometimes with a new muffler, the muffler will "break in" as whatever sound damping material inside (if any) settles in, etc.

Rgds,
Chris W.
'99 E300D, 407K

It also makes me wonder what changed that accounts for the resonance. The material? How it's suspended? Where the bends in the pipe occur?

Well that was simple. Turns out that the heat shield was bent in such a way that it was touching the muffler....and vibrating at 900 rpm. Bent it out of the way, problem solved.