DIY - Killing Drone - 1/4 or Branch Resonator

[BRZ.PTY]

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.

[Bergen23]

That's pretty neat. My exhaust doesn't drone too much so I'm not all that worried. Plus the exhaust cost me less than 375 bucks so if it does get annoying I'm not taking a big hit to replace a catback system.

[edeekeos]

Excellent information here. Thanks!

[EAGLE5]

https://en.wikipedia.org/wiki/Helmholtz_resonance

You're doing it wrong-ish.

[BRZ.PTY]

@jsimon7777 do you care to elaborate?

Thanks

[8R6]

so basically, this is a DIY hemholtz resonator. My HKS exhaust (and some other exhaust systems) incorporate this into their designs. pretty cool for those that dont have it though.

[EAGLE5]

You can change the sound of the car in many ways, including at the exhaust and with a tune. Those resonators are more about volume than length, and there are many other factors, like metal thickness and how they're installed, that matter.

More importantly, you need to identify what the sound problem actually is. Is it that the exhaust is rubbing on something? Is it resonating? Is it making sound that is making something else resonate? Adding another exhaust hangar or two could dampen a resonating exhaust by absorbing some of the vibration. Adding a well-located weight to the exhaust could change the frequency of resonance to something that sounds good instead of bothering you, or to something you don't even hear. Sometimes you just need some vibration damping material on a body panel. The resonator you have is one way to deal with the issue, but it's a hard way. You'd need some kind of sliding trombone system to get the tune right, and even then, you might have another frequency that drives you nuts.

It just doesn't seem like you 100% know what the problem is.

[BRZ.PTY]

Originally Posted by jsimon7777

You can change the sound of the car in many ways, including at the exhaust and with a tune. Those resonators are more about volume than length, and there are many other factors, like metal thickness and how they're installed, that matter.

More importantly, you need to identify what the sound problem actually is. Is it that the exhaust is rubbing on something? Is it resonating? Is it making sound that is making something else resonate? Adding another exhaust hangar or two could dampen a resonating exhaust by absorbing some of the vibration. Adding a well-located weight to the exhaust could change the frequency of resonance to something that sounds good instead of bothering you, or to something you don't even hear. Sometimes you just need some vibration damping material on a body panel. The resonator you have is one way to deal with the issue, but it's a hard way. You'd need some kind of sliding trombone system to get the tune right, and even then, you might have another frequency that drives you nuts.

It just doesn't seem like you 100% know what the problem is.

Well I do know what the problem is, Drone!

And yes there are different ways to kill it. Adding a weight is an option but I dont know how to do it properly? Do you know how to calculate how much weight and where? I would love to know so I don't have to open a hole to my exhaust next time and I'm sure its a lot cheaper. Some people heat wrap the entire exhaust with very good results, this add weights and changes the frequency.

I know it's different than a traditional hemholtz resonator but the math and physics behind this method cannot be argued, at least IMHO.

I can't change the thickness of my exhaust without changing the entire setup. This is why choosing your exhaust and researching about it its so important and even after weeks of research you can end up with drone. There are too many variables.

Vibration damping material will not eliminate drone, its like a band aid. This will only reduce what you hear not what it's actually happening. This is the least effective method.

A trombone system would be awesome but in practice its very difficult to achieve and thats why I said that the easiest way to kill the drone is to change your exhaust but for some people this is not an option.

This is a proven method. You will see many success stories on the forums specially guys running supras (4 inch straight exhaust) and mustangs. I did it and now I can live with my car. As I said on the post, this is not for everyone put your stock exhaust back if you don't want any troubles.

We could argue all day about what is the most effective way of doing it. I did it this way and worked pretty good. Take this a grain of salt, I will encourage all forum members to read more about this topic but I believe this is a good start.

[Turdinator]

This is an interesting thread. Thank you for posting it OP. If i were to do this I would look into getting two pipes that were a slide fit, cut four slits in the end of the bigger one and use a clamp to hold them together so you could fine tune it. Hell if you were really keen you could probably hook up an linear actuator add some sort of seal and have a fully variable setup. Not that it matters now but I suspect your exhaust gas is hotter than 80 - 100c.

[mid_life_crisis]

Thoughts I had because of this thread.
1) I learned something new. I didn't know that the reason the speed of sound is different at different altitudes is about the air temperature and not barometric pressure.


2) I was paying more attention to the sound in my car this morning and found it interesting that some trick of acoustics makes it sound like the drone is coming from the front of the car.