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Helmholtz Resonator Example

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MustardCat
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Helmholtz Resonator Example

#1

Post by MustardCat » 12 Jan 2019, 18:32

So that 2.25" exhaust I made for my daily driver Accord in post 11017 has been bugging me since I made it. It has had a loud drone that peaks around 3300rpm, which sucks because that is a typical cruise rpm.

I was on the verge of buying a more restrictive stock style muffler because there is no room under the car to fit more inline resonators, but I decided to do some googling first. I found this thread and I decided to try making a Helmholtz resonator before throwing more parts and money at it.

I used excess materials from when I built the exhaust so I didn't have to spend any extra money to experiment with this. Here it is:
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Sooo, results?

Fucking good!


The Helmholtz resonator completely transformed the exhaust! It worked way better than I was expecting. I thought it was only going to target around 3300rpm but I swear it also sounds better at idle as well.

It went from loud with an annoying drone, to a more aggressive version of the stock exhaust and no drone. All it took was a little more tube. Highly recommend trying one of these if you have a drone in your exhaust that you want to fix.
You can't rationalize cars like these. They're not something you buy with your head. You buy them with your heart because you love them, and who can explain love?
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User avatar
MustardCat
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Posts: 110
Joined: 16 Aug 2017, 20:34
Location: Mount Vernon, WA
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Re: Helmholtz resonator example

#2

Post by MustardCat » 12 Jan 2019, 18:34

Post from "Phr34k" of ClubRSX with Helmholtz calculations
hi! as many of us have aftermarket exhaust systems, i'm sure that exhaust drone is a common problem. i haven't seen another thread on CRSX showing how to eliminate drone, so hopefully this information will be useful for both current and future car enthusiasts. the thing i am referring to is called a "Helmholtz resonator," which is basically a dead-end pipe branching out from the exhaust pipe that bounces sound around to cancel itself out. i was able to completely get rid of a really bad drone that i had around 3-4k RPM, and since sharing is caring, here you go!

the concept of this resonator is quite complicated when trying to understand how sound and frequency works, but the steps involved are fairly simple. here's how:

1. you will need two variables: the drone RPM, and # of cylinders your engine has.
2. divide the RPM by 60 to get the RPS, or revolutions per second.
3. divide the # of cylinders by 2 to get # fired per revolution.
4. multiply RPS by # fired per rev to get the drone frequency in Hz.
5. divide the speed of sound by the Hz. the speed of sound is a constant 1,260 ft/s. the number is funky because sound travels faster in warm air, and the air coming from an exhaust is very hot.
6. take the result from #5 and divide by 4.
7. take the result from #6 and multiply by 12 to get the resonator length in inches needed to cancel the drone at that RPM.

here's my car's calculations as an example:

1. my car drones the worst at 3,300 RPM and has 4 cylinders.
2. 3,300 / 60 = 55.0 RPS
3. 4 / 2 = 2
4. 55.0 * 2 = 110 Hz
5. 1,260 / 110 = 11.45
6. 11.45 / 4 = 2.86
7. 2.86 * 12 = 34.36 inches

from the result shown, my car needs a resonator of approximately 34.4 inches to add a damper that is tuned for my drone.

the piping should ideally be the same diameter as that of your exhaust pipe. it needs to branch out from the exhaust itself and lead to a dead end so no air can escape (otherwise you'd have quite a nasty exhaust leak). the dead end can be a piece of metal welded onto the tip, or you can even run it back to the outside of the exhaust. this pipe will not hinder performance, as no air will be able to enter it.

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