1/4 wave side branch exhaust resonator for 99-134hz cancellation

[fastline12]

Auto exhaust system producing resonant droning in the cabin from 99-134hz, up 8db from other frequencies. Exhaust system does have a resonator muffler, then 4ft of exhaust pipe before exit.

Testing exhaust stream from the exit of the pipe shows 60C temp. Research test method was simply to place PVC piping over the exhaust with a T, then another 6" length of pipe. Idle frequency was 99hz and was calculated to need about 36" for a 1/4 wave tube. It proved to cancel at 100hz, and/or moved the frequency to 80hz.

Droning was completely removed at the targeted 1800-2200rpm with a branch length of 12" with the PVC but now wondering we we were also getting a further damping effect by the extensions on the pipe and maybe to totally from the branch resonator.

Question
1. Where is the optimal place to locate the side branch in the exhaust system?
2. Can frequency be accurately tested with an accelerometer on the side of the piping and to further find the max amplitude possibly for branch placement?
3. If permitting space dictates branch resonator can only be added near the exit of the piping, will this largely affect the usefulness of the design?

 

[fastline12]

I should further add that frequency at max amplitude is 122hz

 

[GregLocock]

1) at a pressure maximum for teh frequency in question.

2a) yes
2b) probably

3) suck it and see, the tailpipe exit might well be a pressure maximum.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[fastline12]

We did some testing with this system and found a max amplitude very near the exit of the pipes. Even though there were concerns of the branches affecting sound quality being near the exit, we decided to try it. It was determined that though the lower frequencies could be effectively canceled, there were new, annoying higher frequency noises similar to what 6 cyl engines will produce. We have not tested the frequency range as of yet and considering abandoning the project.

One thing we might try though is to fiberglass fill the tubes which should, in theory, do a decent job of attenuation of the higher frequencies yet allow the lower frequencies to move right through. It is kind of a last resort but we are kind of puzzled with the new noises. There is also back pop in the exhaust which was not there before.