beat me to it...

 

Not the exhaust, but the sound waves from the exhaust. By the time they travel to the end of the pipe and back they are out of phase with the rest of the sound traveling through the exhaust, causing a partial cancellation of the sound waves.

 

Yup Thunder has it dead on.

After i read through that whole thread over on that mustang forum, I started google searching branch resonators and learned that they are used in other applications to cancel a specific unwanted frequency.

I just wanted to throw this up on PT for anyone who is interested in learning something new.

The only problem we came across trying to do all the math is we couldnt figure out how hot the branch resonator would get because the temperature the exhaust is changes the speed that sound travels. I think we are a little bit off because it works extremely well when the car first starts up (cold) and when it gets up to operating temp it loses some of its canceling ability.

So I think my fix for that is Im going to buy some 2 inch inner diameter piping and cap the end and get a 2 inch exhaust clamp to slip over and clamp to the 2 inch outer diameter pipe. That way it will be adjustable and we can fine tune it since I cant figure out exactly the temp of the exhaust in the resonator.

Sorry for the long post, but Im at home recouping from my wisdom teeth being pulled and have nothing else to do..

 

Do you need to use the same size pipe as the exhuast? I have an 06 2500HD 6.0L with a 4" magnaflow cat back exhuast, and the drone is horible.

 

This is interesting stuff, I have some highway drone I would love to do away with. How do I determine the size and length of pipe I need to use?

 

there is a link on the first post that goes over calculating length

 

Would it affect power at all? I have a dual inlet single outlet Magnaflow on my truck and it is terrible at 80 of the interstate. The truck is turning 2400rpms and drones aweful

 

from what i understand, exhaust gas doesnt go through it, it is just a resonator connected to the exhaust

 

This is the exact same sound reflecting technology that goes into corsa's mufflers, just done externally and probably ~500 bucks cheaper. look up corsa's website and look at the cutaway of their muffler, its the same principle

pretty cool if you ask me because you can tune your system to your own sound preference by using your own muffler and for specific drone rpm's

 

Ya I saw the cut away picture you are talking about. Its just miniature pipes like I built but inside the muffler.

No you do not. We didnt use the same pipe. The diameter doesnt make a huge difference. Im thinking that the diameter would determine the amplitude of the wavelength meaning it would determine the "loudness" of the note being produced by the closed pipe. But its not crucial; what you are looking for is the out of phase cancelling effect which is dictated by the length of pipe.

The stock exhaust size on the jeep was 2.5" and we used 2" pipe to make the resonator. Home Depot had this really nice 90 degree bend 2 inch pipe so thats what we used.

Yes you could do it on your Magnaflow. The pipe goes after the muffler so you would only have to run one. Even if you had dual exhaust you could run two resonators, one on each exhaust side. More work, yes, but its the same thing.

The exhaust doesnt actually go into the pipe since there is no room for it to go since it cant escape. So the exhause is going to continue out like it normally would. Since its T'ed into the exhaust, there isnt and ducting that is forcing the exhaust gas to flow into the closed pipe. The only thing that is going on in the pipe is that it is producing sound waves because of the air that is being blown over the opening.