A few months ago I posted about a new exhaust that I built for my 2GR swapped MR2.  It sounds great, it looks awesome (especially in combination with the rear diffuser that I built around it, write-up on that coming soon), but I very quickly found that it had one major drawback.  TERRIBLE drone at about 2500rpm.  This wouldn't be a big deal if I never drove my car on the street, but I do, and so something had to be done!

Before we go any further, here's a picture of the exhaust with the diffuser, and a flyby video of the exhaust on the track.  You can see why I wanted to fix this exhaust, rather than building something new that might not fit or might change the sound.
MR2 with custom rear diffuser and center exit exhaust

 


Google "exhaust drone" or some variation of that term and two things will come up repeatedly: quarter wave resonators, and Helmholtz resonators.  People tend to use the the terms interchangeable, but they are in fact different things, although they function in similar ways.  A quarter wave resonator (also sometimes called a "J-pipe") is a length of pipe, often J shaped, T'eed into the exhaust at one end with the other end capped.  A Helmholtz resonator on the other hand is a closed canister attached to the exhaust by a length of smaller pipe.  The quarter wave resonator (as it's name implies) is tuned by adjusting the length of pipe such that it is equal to 1/4 of the wavelength of the sound frequency you are trying to eliminate.  This way the sound waves bounce off the end of the pipe and return to the exhaust a half wavelength out of phase, canceling out the sound.  The Helmholtz resonator is tuned by adjusting the diameter of the connecting pipe, the length of the connecting pipe, and the volume of the canister.  This makes it a little more complicated to calculate, but it is also has the potential to be much more compact.  For example, to cancel the drone in my system the quarter wave resonator would have be about 28" long.  You can read more about the Helmholtz resonator and find the equations I used in my calculator here.

The first step in building either of these devices is knowing the frequency that is causing the problem.  For this I turned to a cell phone app.  It may not be 100% accurate (and I did not bother to calibrate it, so the db numbers are likely not right), but it's good enough.  Cruising at 2500rpm produced the following:

Drone frequency in Sound Analyzer App

Clearly, we have a problem at 100hz to 125hz!


I built two spread sheets to calculate the required dimensions for a Helmholtz or quarter wave resonator, they can be downloaded below:
Helmholtz Resonator Calculator
Quarter Wave Resonator

Note that since these types of resonators depend on the speed of sound, they are temperature dependent.  However, because the exhaust does not actually flow through the resonator, they don't operate at the full exhaust gas temperature.  In my testing my resonator tends to run around 90-110° Fahrenheit while cruising on a warm day.

I ended up building several iterations of my resonator, targeting 125hz.  The first used 3" tube for the canister and 1" tube for the connecting pipe, and while it helped a lot it was (I think) just a little too small to really do the job.  So I built another using 1.25" tube for the connector, and 4" for the canister.  As the diameter of the connector goes up, so does the required volume of the canister.  Similarly, as the length of the connector goes up the required volume goes down.  So there are a lot of ways you can adjust the three key dimensions to get the same effect.  My calculator includes fields to calculate three separate options so they can be viewed and compared side by side.

Center exit Borla exhaust with Helmholtz resonator

The larger resonator seemed to help more than the small one, but still didn't seem quite as effective as I was hoping.  However, at this point I discovered an error in my spread sheet (two missing sets of parentheses).  Interestingly, the error had only effected my calculation for the small resonator by a few percent, but had effected the calculation for the large one by about 30%!  With that corrected I modified the resonator one more time, taking almost 3" out of the length of the canister.  The end result on the frequency analysis was pretty profound:


Reduced drone in Sound Analyzer App

Driving, the results are even more impressive.  It went from almost instant headache to a little drone but not too bad.  Interestingly, there are a couple of areas where the frequency analysis appears to be slightly louder with the resonator, but keep in mind this is a crappy uncalibrated cell phone mic we are talking about!  Here's the final resonator installed under the car:

Helmholtz resonator

At this point, the drone is mostly gone, but it still could be just a little quieter overall for long drives.  So for temporary use I picked up a pair of 3" silencers to fit into the exhaust tips.  I had to borrow an exhaust pipe expander from the auto parts store to make them fit, and also grind the edges of the silencers down slightly with a belt sander, but the end result is they tap into place easily with a hammer, stay there without any fasteners, and can be removed easily with a slide hammer when I get to the track.  I'm sure they cost a few hp, but it's a hardly noticeable amount driving around town or cruising on the highway, and they pretty much eliminate the remaining drone and drop the overall sound level by a couple of db.
Exhaust silencers

8 comments

  1. Thursday, 27 January 2022 20:46
    Fascinating process and design to solve a problem.  Do you have any pre-post dyno numbers to show the hp difference?  Do you foresee it making a difference being turbo vs NA?
    1. Friday, 28 January 2022 08:29
      I don't have any 100% back to back dynos on the resonators effect.  But I do have back to back dynos of the y-pipe alone, and with the Berk muffler, and the power was exactly the same.  I also have back to back dynos of this exhaust vs the open y-pipe where it made an extra 5hp with the muffler / resonator / catalytic converter installed.  So it appears that the resonator may be worth a few HP.  More details in my 2GR Power Gains article.
  2. Wednesday, 16 March 2022 04:14
    Hello, i would be very grateful if you can help me...I have an Invidia Q300 Axleback on my Lexus RC200T and the drone is awful...I put a helmotzh resonator yesterday but there is still too much bass tone...My drone frequency is 70hz...What will be the exact size of the chamber and neck?    Please reply ASAP  Thanku  Odysseas Koromias 
    1. Saturday, 26 March 2022 10:46
      There is no one size fits all resonator, even for a specific frequency.  The volume of the canister and the diameter and length of the neck all effect the frequency response.  That is why I provide the calculator.  That said, the larger the resonator the more sound it will be able to cancel.  And increasing the volume of the resonator will require adjusting the diameter and or length of the neck to maintain the same frequency response.
  3. Friday, 06 May 2022 07:33
    Would you be willing to build the same one for me lol?  What's it worth to you?  Where did you get the parts?  My drone appears to be 120-140hz @90 decibels, depending on the rpm. Thanks,George
    1. Friday, 06 May 2022 09:20
      At the moment I'm not building these, sorry.  I don't remember exactly where I got the parts, probably a variety of sources.  Tube from ebay probably, pretty sure I found a seller there with 4" diameter stainless disks as well, for the caps.  The elbow, might have been Ace Race parts, or Verocious Motorsports.
  4. Thursday, 26 May 2022 10:55
    Fantastic writeup, the spreadsheet is very helpful. Just a thought, if I wanted to remove two frequencies (I have drone at 106hz and at 144hz) and I have a dual exhaust setup, would it be feasible if I built a resonator for each frequency between the two exhaust? Say, 107hz resonator for the left exhaust pipe and 144hz on the right? I mean either way I will probably build and weld one on first and see that impact and if it reduces it significantly enough I might end up building one for the other frequency.
    1. Tuesday, 31 May 2022 14:49
      You can certainly target multiple frequencies with multiple resonators.  Doing one of each on either side of a dual exhaust would probably not be ideal however, as each half of the exhaust would still have one of those frequencies at full volume, so it may not reduce the drone by enough.  You might be better off targeting a mid point like 125hz on both, but I don't know for certain.

Questions or comments?

In reply to Some User