Helmholtz Resonator Calculators – Exhaust J-Pipe Length
This page contains two calculators. The Helmholtz resonator calculator below will tell you the hypothetical ideal length of a quarter wave resonator/J-Pipe. The calculator needs two numbers, the temperature of the exhaust gas entering the resonator and the frequency of the sound you want to reduce. (Spectroid app linked below can help you find the frequency) – At the bottom of the page is a calculator for a Helmholtz resonator chamber. For an explanation of the difference between a quarter wave resonator/J-Pipe and a Helmnoltz resonator, see this excellent post.
Disclaimer: As always use these calculators at your own risk. There are links below to excel spreadsheets for 1/4 wave and the Helmholtz chamber which can be used for double checking.
Sources and Links
The videos and articles that made this Helmholtz calculator possible are listed below
Includes a link to a spreadsheet to cross reference/check the numbers you get from this calculator if you wish.
It also shows how important it is to get the temperature of the gas correct by showing the difference in length according to different temps.
Spectroid Android App
Download Spectroid APKs for Android – APKMirror
In real time shows you the loudest frequencies. Displays volume with a line graph and heat graph. Excellent app.
Again, choosing the right temperature is key. It might be cooler than you think
Start with a slightly long resonator because it is easier to shorten that to lengthen.
Choose the frequency that is the loudest rather than trying to target a frequency based on RPM
Excellent results on Porsche 928 race car
Race Muffler / Track Noise Compliance Tech – Page 3 (corner-carvers.com)
Trial and error finding the best length
Helmholtz Resonator Calculator (1/4 Wave) – Final Thoughts
With this calculator and the info for the two videos linked to above I think you’ll be in a good spot to get your own Helmholtz made.
Both videos show why temperature is so important in the calculation.
For this reason, I think I would have an adjustable J-Pipe. Either by having a larger diameter end section I can slide over the end, or make up some end sections of different lengths which can be clamped onto the main part of the resonator. This lets you experiment with different resonator lengths quickly and easily.
Comments
If you have a Helmholtz resonator on your car, feel free to leave the specs below to help other people find the right size for their car.
Include the diameter of the piping, the number or silencers the exhaust has, the length of your Helmholtz and what it did for you.
Helmholtz Resonator Chamber Calculator
Final Thoughts
I’m not pretending to be any sort of expert on this subject. Having said that personally I am not convinced about the Helmholtz chamber being used as a sound suppressor in the exhaust. My biggest reservation is that the numbers in the calculators above do not address the phase of the sound. I am not sure if this is relevant or not, but I am inclined to believe it is, especially considering how the quarter wave resonator works. Sure we can calculate the resonant frequency of the Helmholtz resonator but is this going to be in phase with the drone frequency ie make it louder or is it going to be exactly out of phase and cancel out the drone. Feel free to leave a comment below to address my reservations. I am sure they will work because many companies use the Helmholtz chamber in their exhaust systems, I’m just not sure if they use the same calculations as the ones on this page.
Links & Sources
The inspiration for this calculator were taken from these sources
Drone Reduction – Wilhelm Raceworks, LLC
https://wilhelmraceworks.com/calculators/helmholtz_resonator.xlsx
https://wilhelmraceworks.com/calculators/quarter_wave_resonator.xlsx
Helmholtz resonance – Wikipedia
MX5 NC RoadsterSport HELMHOLTZ RACE Exhaust (racingbeateurope.com)
