In this guide I will explain how I killed the drone I had in my car with a branch resonator.
A branch resonator is basically a capped pipe welded to your exhaust at a 90 degree angle. What it does is cancel the sound wave that is causing the drone.
This is not for everyone, the easiest way to kill drone is to buy a new exhaust but I love the sound and performance of my 3 inch exhaust.
Benefits:
- You can maintain your current setup
- It does not affect performance
- Kills drone very effectively
- It has been tested for many years and it WORKS!
Cons:
- It adds weight
- You have to open a hole to your exhaust
- Involves some guessing
Well I'm doing this because as many of you I had a drone in my car that was killing me. Since I installed my Ptuning header back exhaust I had a drone from 2600 to 3000 rpm. It was so annoying that I stopped driving the car (DD) and switched to a Honda CRV for a couple of weeks (I think this says it all)
There are two ways to do this:
1) The simple way:
Step 1: Drive around with your car and look at what RPM you are experiencing drone the most. In my case was 2600 rpm
Step 2: We need to find the drone frequency so we use the formula
RPM x engine pulses per RPM x 1min/60sec
A 4 cylinder engine like our boxer has 2 pulses per RPM
In my case it was: 2600 rev/min x 2 pulses/rev x 1/60 = 86.7 Hz
Step 3: To find the drone wavelength we have to divide the speed of sound in the exhaust by the frequency in step 2
Since the speed of sound varies depending on the temperature we have to guess the average temperature of the air in the exhaust were the pipe will be welded.
I personally assumed a temperature between 80C and 100C which yields a sound speed of around 380meters/second
wavelength = Speed of sound/drone frequency
= 380/86.7
= 4.38 meters
Step 4: the idea here is to have sound wave cancel your drone frequency. To do this, you need a resonator which is 1/4 the length of the drone wavelength
Resonator size: drone wavelength/4
Resonator size: 4.38/4
Resonator size: 1.1 meter
Step 5: Since all the calculation is done you need to find a 1.1 meter tube and welded at
90 degrees at your exhaust, this is very important since you don't want any gases going through the resonator. Also, you resonator has to be as straight as possible after the 90 degree connection because sound waves don't travel in the same way as gas.
Ideally you will need a tube the same diameter as your exhaust but since space could be a problem you can use a smaller diameter tube. In my case I had to use a 2 inch pipe.
Where you should weld the resonator is still up for debate, but the determining factor will be available space. In my case I chose the front pipe since I want the drone waves to be cancelled as far up as possible. Some people do it to the mufflers in the back I would say it depends on your personal choice.
Attached is a picture of my setup.
Step 6: Put your exhaust back together and test the new setup.
2) The not so easy way.
Step 1: Record with your phone or any audio recording device the rpm range where you are getting the most drone.
Step 2: Use a software to analyze your recordings and determine the highest DB level and the drone frequency.
Step 3: Same as before.
I did the easy way and I can say the results are amazing, there is still a lil bit of drone under really heavy load (going uphill and shifting from 3 to 6) but now I can live with the car. I would say there is a 90% improvement.
What I'm going to do next is record the drone and determine the pipe length again to see if I can kill it once for all.
Sorry for the choppiness and grammar of the post, English is not my first language and I do not post much.
Feel free to ask me any question you have.
Disclaimer: I'm not responsible for what you do to your car, please use this only as reference. Results may vary depending on the quality of the calculations and materials used. Do this at your own risk.