Hey peoples.

I have a aftermarket exhaust. New manifold, dump pipe, hi flow cat, single resonator and a dual tip straight through exhaust all connected by a 3" mandrel bent piping.

It works perfectly in many ways except one. Using EPA (Environment Protection Agency of Australia) measurement techniques it registers 83dB on Class A measurements (audible sounds) but 110dB on Class C measurements (Includes inaudble very low frequency soudns).

From a legal stand point, my car is a good 7dB below the legal requirement. (EPA only measures Class A) However, it annoys the crap out of me, because it makes the panels vibrate at certain RPMs and it interfere with the stereo.

I've sound proofed the car at a "cost" of about 30kg of mass (66lbs) and the sound has dropped somewhat, but I'd like to tackle it at the source, rather than add any more weight. I've already painted the inside of the panels that vibrate with a tar like substance, and laid a thick sound absorbent material under the carpet and boot.

Does anyone know if there's a mini-muffler or resonator, that specifically attenuates low frequency sounds, without restricting flow excessively?

 

i dunno if there's anything you can do about vibrating/rattling interior panels, i have a greddy ti catback and ever since i put it on it makes my panels rattle at certain speeds, i've heard from more people on here that their panels rattle too, its just how the evo is, it wont be a luxury car

 

I am currently researching the same issue for my car. I get a very annoying "drone" when I drive on the highway (between 3000 - 3500 rpms). I've been looking into various types of acousting foam, but one of the problems I've run into is that none of the foam I've found so far does a good job of stopping frequencies below 125Hz. Based on my calculations, these are the audible frequencies (includes harmonics) that need to be dampened:


RPM: Frequencies(Hz):
3000 24.99 49.99 _99.99 199.99 399.99
4000 33.33 66.66 133.33 266.66 533.33


So far, folding the trunk mat behind my back seat has made a noticeable difference, but it's still far from where I want it to be. IMO, the biggest noise generators are:

Passenger side rear wheel well (resonates)
trunk as a whole (resonates)
lack of rear seat sound proofing

I will be trying some better quality sound proofing material in the near future, and I will also see if I can line the underbody of the car, above the muffler with fireproof acoustic foam.

l8r)

 

I have the exact same prob with the Greddy Ti.

 

What exhaust is it? Check the front lower "tie bars" for rubbing.. also check around the drive shaft. What exhaust hangers do you have? stock?

 

Thanks for the feedback ludikraut and others. They're pretty well the same issues I've been having too. The boot resonation problem is lessened now that I've painted it with tar - of course it still resonates, nothing bar changing the volume of it is going to stop that, but because the panels are stiffer, the noise is "changed" to a higher frequency, then absorbed by other sound proofing. I'd recommend that. Very light weight mod. I think I put about 200gms of tar on it in total. It's only going to cost me about $100 mates rates to have the boot sprayed white again to cover the tar.

It's exactly the problem you describe that I want to go a "source" solution, rather than a "block" solution. (IE, none of them effective on LF sounds) You seem to know a fair bit about accoustics, so you'd know better than I, that low frequency sounds transfer very little energy to a material it's traveling through - therefore, the material needs to be very thick and very dense to stop very low frequency sounds. Something I'm not willing to live with, since I don't want to lose all the gains I got from getting a custom exhaust in the first place!

If you do find something that for some reason is very effective at stopping LF sounds, I'd love to hear about it, but in the mean time, I'm going to look for a centre expansion style minimuffler

Justchil; the problem is without doubt sound related, not vibration/rubbing related. But thanks for the advice anyway.

 

hmm I wonder if just exhaust wrap would help.. It at least made my old system a lot more tolerable when I wrapped the downpipe. I suppose you could wrap after the cat and it wouldn't add much weight or anything and might reduce the vibration.

 

You can combat this by installing a high quality resonator or an additional muffler.. www.burnsstainless.com sells high quality race mufflers that work really well for their size.

 

Now that you mention it, a resonator should cancel out low frequency sounds if designed specifically to do that. However, I was under the impression that resonators are generally designed to eliminate mid frequency sounds.

How could you tell which frequency a specific resonator was designed to tackle?

 

im looking at this now thinking the same thing, ive got a 70mm downpipe and 76mmflexi wrapped then it goes to 80mm then 90mmfrom the drivers seat to the rear where it splits into two 70mm side by side straight through muffs, and it really drones at highway cruise speed, at pressnt i wear earplugs for anything further than the shops lol, i can see lots of room in the rear for big truck like mufflers but i get the feeling this would not help the drone as i picture the drone comming from the length of 90mm pipe front to rear, the pipe is to close to the ground as is so i cant see fitting anything extra up front,,, the feds are starting to clamp down on noise here in nz too, looks like this will take a lot of research and coin to sort. maybe split at the front into 2,3,or 4 pipes haha wheres my calculator....

 

Add a second resonator and a big magnaflow muffler.

 

that vibration is normal .probly bc your evo is 10 years old

 

My exhaust utilizes factory catback (flapper valve removed), which include the two resonators, 3"DP and cat delete. Even though the car is very quiet and clean sounding, it does produce that low inaudible frequency that rattles the house as I pull up like a pipe organ.

There has got to be a way to get rid of it

 

Put a silencer in the muffler and use Foam.

 

I noticed a huge difference in low frequency that you can hear and feel from removing my Hi-flow cat and replacing it with a test pipe. My exhaust does have a large resonator built into it as well.