Drone Eliminating and Branch Resonators
#1
Drone Eliminating and Branch Resonators
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
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
Last edited by DM_kota; 01-27-2012 at 12:01 PM.
#2
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
#3
#5
#6
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
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.
Last edited by DM_kota; 01-28-2012 at 07:12 PM.
#7
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#8
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.
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.
#9
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.
#10
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?