LS1GTO Forums banner

Hemholtz Resonator (1/4 wave resonator)

42K views 63 replies 18 participants last post by  smitty2919 
#1 · (Edited)
Anyone heard of this? The idea is to find what RPM your car drones. Then after some simple multiplication and division, you can find the length of tube required to add to your exhaust to kill drone at that RPM.

It's been around for a long time being used in intake tract silencers. There is a 13pg thread on a mustang forum where people have done this and came out with amazing results. This goes deeper than the mustang crowd too. Since researching, I have found it on Beemr forums,Audi forums and other "sensitive hearing" oriented car enthusiaites.

Quote from post in Mustang forum from the guy who started it:

"My magnaflow had the usual 2000 rpm drone, so I tried something a little different (I think) to get rid of it. The drone is now not just better, it is completely gone - the exhaust is downright quiet cruising at 2000 rpm. I read an article on the internet about "tailpipe resonance", and decided that this has to be the problem with aftermarket catbacks. They probably all have the same 6ft tailpipe mine has, which if the exhaust is at about 500 degrees, will resonate at 134 Hertz, which is the engine firing rate at 2000RPM. So I added a 134 Hz, 1/4 wave resonator to both tailpipes. Its just a section of pipe "T"ed into the tailpipes right in front of the gas tank. They are about 26" long each, which maybe a little shorter than ideal, but that's what fits in the available space. 26" must be long enough, because it sure as heck works. One end of each pipe is welded shut, and the other is T'ed into the tailpipes. If anyone is interested, I can try and post a picture."

Corsa of course promotes a RSC Technology in their mufflers which goes beyond this since they are able to tune the drone withing the confines of a muffler.

You would simply "T" off your exhaust with a centerline length of tube that you calculated and cap the end....done. My drone comes in at 1700 and stops around 2000, so I calculated based on 1750rpm giving me 29" of pipe needed.

How I plan to redo mine...mimic the Corsa single 3" but with a pipe "t'd" off. (not to scale)



On a Mustang:





On a GTO(was posted in mustang forum)


Cut away from a New Camaro muffler that suffers from drone:



Ohh, look...on a G8 lol:




Other exhaust companies have been doing this as well it seems:





I had a plan to do this to my Mach 1 when I had it, but then I threw in Dynomax VT mufflers and they worked well enough I was done with the "anti drone" quest.

My L/T's, catless mids and dual 3" SW catback with no mufflers drone BAAAAD, so it would be sweet if I could make a catless/muffler-less system not drone. LOL


Thought? Opinions? Ideas? I would link the Mustang forum thread so people can read for themselves, but I don't know how mods like that.


Sorry this isn't another "what exhaust is the best to put on my (enter year GTO here)??" lol Hopefully this will get a little technical.
 
See less See more
9
#40 · (Edited)
Thanks Mike. I didnt see locarion as a big issue since some mustang guys put them in the midpipes and some put them in the tailpipes. Not comparing to a mustang, but thats just an example of two very different locations.

I agree with the plunger idea. I was thinking to use more of a trombone idea. The resonator pipe will be maybe a foot behind the x pipe in my current dual catback then tune it from there.

Tuesday nigbt I am going to the exhaust shop to get a quote. I will drive on the single 3" and see if it changes the drone rpm. Then head back to get resonator pipe added with trombone/plunger idea. Whatever is easier for the shop to fabricate.

Good news is they are interested in this idea and willing to tackle it since they deal with a good amount of cars whose owners put mufflers ontheir cars that resemble a sound that comes out your rear.
 
#41 ·
3" is pretty big. The larger the diameter, the longer the pipe will be for a particular frequency. Too small, and it won't attenuate the stop band as well.
 
#42 ·
Only reason for the 3" is my dual 3" is overkill on a DD stock car, so to "simplify" the exhaust I was going to single 3". If 3" will make the resonator pipe required to be longer, all the more reason to do like you said about the adjustable part.

I will have it "t" off as far forward ad I can to allow the pipe as much room.
 
#44 ·
I ended up downloading a "tone generator" on my Iphone. Since I have a aftermarket head unit with an AUX port, I was able to plug my Iphone into the head unit to play different frequencies.

It will play from 10 to 4000Hz. I calculated my drone at 116Hz. Theory is that I can play the same 116Hz through the speakers and see if my drone goes away. I will get out and do that today. If it is not 116Hz, I will play with the frequencies until I find it.
 
#45 · (Edited)
That won't work as expected. You can only achieve cancellation using pure sine waves under near-laboratory conditions when the emitters are precisely 1/2 wavelength apart, (148 cm @ 20C for 116 Hz) and are synchronized in phase and have equal amplitude. The exhaust sound is an asynchronous band of noise centered at 116 Hz, quite different from the sine wave coming from the Iphone, and the temperature in the exhaust pipe will differ substantially from ambient conditions for purposes of wavelength calculations. Your best bet without instrumentation is to simply vary the length of the resonator and evaluate the results subjectively.
 
#47 · (Edited)
Well if that is the case it will be interesting if the Iphone does ANYTHING. By LOOSE calculation it was 116Hz. Given the non lab environment, maybe it will turn into some other frequency as I scan through them. But like you said, it may do nothing at all.

The "end all" solution would be an adjustable pipe. From there Ill measure it and backtrack the calculation to see what frequency/ rpm I get and see if tbere is something you can do in the equation to compensate?


...or speed up...
Or I'd like to learn a little something about exhaust in particular exhaust drone and frequencies and what can be done to combat it. Yes, crossover and muffler selection can do this, but enough people experimented over the years that way to come up with the general rules of thumb we use now. Im not going to start some revolution in exhaust experiments, but this seems interesting.


Like I said, I planned on a single 3" anyways. Maybe adding a magnaflow 4"race muffler out back. But until then Ill play with this resonator to see what i can come up with.
 
#48 ·
hehe, i'm just jacking with ya! i've got a 1.8 mirage i'd like to put an exhaust on, but would actually like to tune OUT the upper fart can range, just give me a nice tone at idle/ off idle :)
my buddy had an AWESOME exhaust on his 2.8 jetta, but neither of us can remember who made it!
 
#49 · (Edited)
So that little drive was interesting. I set my phone at 116.6 Hz (my calculated frequency at 1750 rpm) and right off the bat I can see what MisterMike was referring to:

"That won't work as expected. You can only achieve cancellation using pure sine waves under near-laboratory conditions when the emitters are precisely 1/2 wavelength apart, (148 cm @ 20C for 116 Hz) and are synchronized in phase and have equal amplitude."

I first thought that I got my frequency wrong on the phone since the drone would get twice as bad, then better, then bad, then better. It would speed up then it would slow down going back and forth. I figure this is due to the fact that my phone and exhaust were both emitters. You would have to start the phone frequency at the exact right time for it not oscillate back and forth between doubling the drone and reducing it.Not to mention, it would be hard to keep the car at a constant 1750rpm. It will only be as close and precise as the cars CC system would allow.

When the frequencies were 180* out of alignment, I wouldn't say the drone was completely gone, but reduced yes.

What I have found out first hand is what MisterMike and others have been saying. There are many other factors in doing this. Most of them I (as a common chump) cannot account for or do anything about to "plan ahead". I tried leaving the phone tone playing but try to narrow in on the frequency by speeding up or slowing down. Still gave me the same drone amplification/reduction oscillation.

So, I took this away...1)it is possible to do this under a more controlled environment 2)those calculations (as I have read before) are for a very narrow frequency range 3) I'm kind of deaf now...lol

Most of the guys doing this mod have mufflers/cats/resonators somewhere in their system. Having a drone-free catless/muffler-less system I don't see happening just by the sheer noise that also gets generated that may be mistaken as drone. So my "done" may be actual drone PLUS a ton of noise (that I can't get rid of without a muffler).

Going forward I will have the 4" round magnaflow muffler installed in my single 3" system and go from there.
 
#50 ·
The other difficulty with this method is that a small device like a phone is not capable of generating a large amplitude signal at 116 Hz, a fairly low frequency in the grand scheme of things. To match amplitude with an automotive exhaust would require a decent woofer with a fair amount of power behind it.
 
#51 ·
This is what gets to me sometimes...I figure I'm doing the right thing in how to approach this with using the phone etc...cleaver ways to "verify" only to find out that it's all a bust lol.

It would be cool to be in a car and just "turn the radio on" and bye bye drone lol.
 
#52 ·
All I will say is:

My exhaust with cats: no drone
My exhaust without cats: drone
 
#53 ·
Well, I gave in and bought some SW mufflers off a member and took the cheap solution out had them welded in rather then redo the whole exhaust. Plus I had spoke with Svede1212 and MisterMike about the impact of a single 3.5"...specifically the Y design and how it impacts scavenging. I was impressed with Svede1212's "calculated" exhaust combo and how properlay placed components give good results.

Taking my car on anything more than a 30 min trip on the highway was getting old lol.

Interesting to know that I was misinterpreting "drone" with "exhaust noise" at my proposed 1750rpm spot. With the mufflers in it looks like I'm more around 1500rpm for drone.

I may or may not revisit this idea. With dual 3" pipes with mufflers, there is not much room for more pipe. I will drive it like this for a while since my ears are happier. I still believe this can/would work just on the simple fact that people with a WIDE range of cars/trucks are all using the same calculations/method and all are reporting great results.

Thanks for everyone suggestions/help.
Until next time, signing out for now...
 
#54 ·
So I'm going to bump this back up again...

I used the SW catback with SW mufflers for a while and my ears were happy. But then it seems like I got used to the that setup and I started hearing drone again. Not a lot but enough to get old when on a 1hr+ trip.

I quantify "no drone" as my L/T, catless and stock 04 catback (for any of you that have been in an 04 with this setup). Only issue is I would like to have a little more "bite" out of the exhaust note.

I picked up a welder, so now modification/plans have gotten one step closer. I still wouldn't mind making a single 3.5" system with a Dynomax VT muffler in the 05/06 location (buy mandrel bends and muffler and start welding), but I may be able to acheive my goal even cheaper.

I have since sold the SW catback and am back to the 04 catback. It dawned on me that there must me something in the stock mufflers/catback that eliminates drone. I know one is a "straight through" style and the other is "chambered"....also that there is no "crossover".

These are the only 2 pics I have found:




Anyone have pictures of the "skin" of the muffler removed to see a cutaway of them?
 
#58 ·
Hmmmm good read. Cool results at the end. I felt like I learned a lot in the exhaust section tonight..... Wow never expected that to happen.
That was my goal along with actually making this and having it work without breaking the bank. I doubt someone else will do this though.

Most people here will just buy a quieter exhaust or go Corsa. For any DIY'ers it's worth a try.

I'll say this though, exhaust work is tricky and we have a simple exhaust routing. Getting angles right etc. Gives a new found respect for the GOOD exhaust shops.
 
#60 ·
I have not. If i did, I would think to put it behind the rear seat, doors and over fender wells in the trunk.

While added weight is not a huge issue with me since it's no track car, how much weight did you think you added Norm?

Did you use Dynamat product? Or some other company?

I hear Second Skin has some great product with a butyl based and closed cell foam based materials that, when used together, work amazing.
 
#61 ·
Second skin is a good company. Go to sound deadening showdown to learn a lot of stuff.
Dynamat is not a sound deadener, however, so layering that stuff is a waste of time, money, and added weight.
Why is it a waste? Because its primary purpose is to stop vibrations, not block soundwaves. Use it on 25% of the panel, in the center, to prevent vibrations. Then add a combination of closed cell foam and possibly mass-loaded vinyl, and you will have much better results.
Adding tons of dynamat and get mediocre results.
 
#62 ·
^Exactly what I have read and seen online.

For now this resonator pipe has helped a lot. I'm sure down the road my ears will normalize to the reduced drone and it will "drone bad" again lol.

At which point I may look into Second Skin or equivalent to use a butyl/ccf/mvl material.

While I don't want to add a lot of weight, it IS a true DD so the added comfort would be nice.
 
#63 ·
Honestly, using some second skin and closed cell foam behind the rear seat should give good results.
Also, use it over any wheel wells and you'll have a huge drop in road noise, which will give you a nicer cabin sound.
If you want to go overboard, use it on the roof, the door panels, floor, and everywhere you can. You'll have pretty close to luxury car quietness, if not better.
 
#64 ·
Update:

Got a decibel meter in the car...70mph uphill only 73dB's!

For reference my 03 Mach 1 with stock manifolds, catted H pipe and Dynomax VT mufflers also registered 73dB's!
Same mustang with full size magnaflow mufflers was 83dB's.

I have 100sqft of Rattletrap and 4 yards of Ensolite showing up Thursday and going to install this weekend. I'm anxious to see how much quieter the car will get inside all while sounding like this outside :)



It's my poor mans Corsa "whale penis" exhaust :)
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top