In this week’s video I want to talk about eliminating exhaust drone. I have done an article for StrikeEngine.com about Helmholtz resonators and quarter wave resonators. The video shows you how to use the calculators
DISCLAIMER: These calculators may or may not be correct for your application. Compare results with other sources.
First a Bit of Background
Why do I want to reduce exhaust drone?
The Octavia RS project car has a straight through exhaust system with just a catalytic convertor and a small jap-style real silencer.
For sure the exhaust is quite loud in general, but the biggest problem is a big droning noise at what seems at anything above around 3500rpm.
Initial Thoughts & Solution.
I’ve found active exhaust silencers on AliExpress. They look good quality, the size of them is not completely tragic ie huge. The issue I have is that when the valve is closed they look like they will be very restrictive. The holes going into the silencer section are small. And I’ve read reviews where people have said they noticeably reduce power above 4000rpm (not sure on the engine or the power of the reviewer)
I haven’t completely ruled out this solution but it’s not my top choice.
Another possibility is to fit a valve in the exhaust with a less restrictive silencer. This would entail quite a bit of cutting to the standard exhaust and probably a new exhaust tip. Honestly, I think I’d rather just get a silencer that isn’t too restrictive and forget the valve.
I thought those were the only options but then I stumbled upon quarter wave resonators and Helmholtz resonators. People use these resonators to eliminate specific frequency drones from their exhaust sound.
Eliminating Exhaust Drone – Exactly what I want
Theoretically I just have to find the frequency that is causing the problem and then uses the calculators to find out what size of resonator I need to cancel out this noise.
The article on StrikeEngine.com contains a couple of calculators which gives us the dimensions for a quarter wave resonator/J-Pipe and Helmholtz resonator volume.
Check the video above to see how the calculators work.
I’ve read some forum posts which have raved about the J-Pipe solution. Not so many people raving about the Helmholtz resonator.
I’m not an expert but I’ll give my thoughts
I can understand how the quarter wave resonator can work. It sends the same frequency back into the exhaust exactly out of phase to eliminate the desired frequency. Makes sense and I can see how it happens.
Helmholtz Resonator 
Quarter Wave
The Helmholtz resonator on the other hand I don’t really get. Sure, I know a Helmholtz resonator is like blowing across the top of a bottle. Depending on the volume of the bottle the sound it makes changes. However, I don’t see how we can control the phase of that tone.
I can see the exhaust gases passing over the opening to the Helmholtz resonator on the exhaust and creating the frequency of the drone. What I don’t see is how we will know if that resonance will be in phase or out of phase with the drone frequency in the exhaust. If you see what I mean?
Unless I find some more concrete/convincing information on the use of a Helmholtz resonator I’ll pass on that and concentrate on the J-Pipe/Quater wave resonator.