Some of you may recall a few months ago I upgraded my Exhaust system with a New Y-pipe and a Magnaflow muffler for some free breathing for the Dakota. I really liked the low rumble I was getting from the Mangaflow. It did have a side effect there was a terrible drone everytime I hit 1700-2000 RPM regardless of what gear I was in.

I spoke with some places, some suggested I put a glasspack in front of the muffler, others said nothing would solve it and just to crank the radio (which I think was a cop-out solution)

So in the name of sience I started to investigate into the drone and found out about the science of muffler frequencies and branch resonators and Helmholtz resonator theory

Basically the drone is a soundwave that is being produced which is not being canceled out and why it is harsh on the ears. So to eliminate the drone, you need to cancel the sound wave. To make a wave to cancel the "drone" wave, you need to create the same sound wavelength at a 180º the current sine-wave and insert it back in. So that means you need to take the traveling sound wave, cut it in half/and send it back into the same system to interfere.

So for exhaust, this is a pipe attached to the exhaust of a certain length, then capped to reflect the sounds waves.Plumbers sometimes do somthing similar to prevent "water hammers"

to determine the length of pipe to create a 180º wave I needed

(v-sound/Hz)
____________ = Length of pipe to create 1/4 wave length offset(cuz 2-1/4's= 1/2 wave)
4

So I had to identify:
-Speed of sound through my exhaust
-Hz Level of the drone

Sound does travel at different speeds when the tempurature changes so this calculation is a mid-range temp of 200ºF
http://www.sengpielaudio.com/calculator-speedsound.htm

So the speed of sound in the exhaust is 1260 Ft/s when 200º
For the Frequency I downloaded this app for my Android and monitored the Hz over a few days to see where my peak frequency was landing at in my truck. The More specific the better.


Once I determined I was a hair over 100 Hz I could figure the length of exhaust I needed to cancel the "drone wave"

Hence 1260ft/s is how fast sound travels divided by 115hz=10.9 then divide that by 4 and u get 2.7ft required to cancel a 115hz sound wave.

According to what I have found, I should place this after the muffler, but I am not certain if it would hurt to put it before the muffler. I haven't found anyone who has done it and posted good results but If I am right, it shouldn't matter. The point is the sound waves getting reflected into the system, not where it gets reflected in; right

It would be more convienently placed before, but I think I can manage a spot after too.

I will post my progress, just wanted you guys to check my math

 

MBRP is the only company I have come across that has implemented this "tuning tube" within there Challenger Kit, like you are trying to achieve. http://www.youtube.com/watch?v=JuQ6Y3f2opw

 

nice job on the maths

my guess is your next step is to mount the reverb pipe
maybe a dual exit muffler and use 1 of the exits for your capped pipe

does it matter if your quietpipe sits parallel or perpendicular? a ypipe perhaps? maybe a t?

 

Interesting man! Nice challenger with the MBRP..I feel like theyre one of the top exhaust companies..up there with Borla and Corsa

 

I would think you would need to put the reverb pipe after the muffler. Because if you put it before wouldn't it need to be a different length because doesn't the muffler change the freq of the sound

 

From what I recall it can be either as long as there are no sharp turns in the pipe. otherwise the sound waves will bounce off the angle and not travel down the full length of the pipe. So it would cancel a higher frequency out, not the right one. Since the longer the pipe-the lower the freq it cancels and the inverse for the higher.

I am going to hopefully add a sweeping 90 or 45 to keep the soundwave moving as smooth as possible.

EDIT: I just waw your dual out muffler idea.... that would have been a perfect Idea, I already have my muffler though, so a cutting i will be

I agree, but it would be fun to try it. It just kills me I have so much extra room in front of the muffler, and I have to cram a 28" tube behind it, which gives me less area to play with.

The good news is since it is sound waves I am reflecting, I can go forward or backward with the pipe I am pretty sure. so maybe I will head forward past the muffler.

 

To test if before the muffler will work ... Remove it and if that freq. is still there then you know it will work .. if (as we suspect) it is a Tone created when adding that Spec. muffler you are best to add it after, or the muffler will just keep making that drone

 

Why not try moving the muffler forward to make room behind it for the reverb pipe. Just a thought I don't know how much cutting you want to do on it.

 

Hey DM Please keep us posted on your results and take plenty of pictures. I have an 08 Dak which I put the cat-back 3-in stainless single Magnaflow on recently. While I like the sound (on the outside) the drone is driving me crazy. I have taken the interior out and put RoadBlocker down and a instillation layer with little effect.

 

Where does your exhaust dump?