Branch resonators - ways of reducing exhaust drone

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Enraged

A HG job took HOW long??
Mar 30, 2005
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looks good!

the temp would change depending on lots of things (engine load, rpm, etc) wouldn't it? I suppose the best idea would be to simply get an average number that works, or even measure the temperature at the rpm/engine load that creates the worst droning sound?

I'm following this thread closely, I think I'll try something similar on my car when I build the new exhaust.
 
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Kristian_Wraae

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I've been working on the resonator a bit more. I wasn't satisfied with the performance and luckily I had recorded a sound clip of the exhaust before adding the resonator.

I imported the sound into Audacity and did a frequency analysis:

Exhaust%20pre%20branch%202000%20rpm.jpg


As you can see there is a peak around 99 hz and one around 200 hz.

The 200 hz peak is what the resonator was tuned for. But as you can see the peak at 100 hz is the largest.

After adding the resonator the spectrum looked like this:

Exhaust%20post%20branch%202000%20rpm.jpg


As you can see the peak at 200 hz is almost gone but the 100 hz one remains. And that is the really annoying one.

So I need to make a bigger resonator to get at that.

I'm going to do a Helmholtz resonator looking like this:

Helmholtz-Resonator-300x230.jpg


The neck is 4 inch and 30 cm and the volume is 10 liters. That should tune it to around 100 hz if we assume a speed of sound of 380 m/s. I'm still not sure about the speed of the sound but I found a pdf dealing with muffler design and they used 380 m/s. This is equivalent to 100 C. The temperature used is the temp of the gas inside the resonator and not the temp of the exhaust gas. If someone has a temp gun I'd appreciate if the could measure the temperature of the exhaust pipe around the test pipe. The Helholtz resonator is going to be significantly cooler. I'd just like to get a ball park idea.

I'll report back when it is done.
 
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spdu4ea

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Apr 15, 2011
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Saw this thread linked from another forum -- have there been any updates? I also took the liberty of overlaying before/after sound spectrum graphs:

 
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Kristian_Wraae

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I haven't done more since I have swapped in a 2JZ and at the moment I'm on the stock exhaust.

You would probably need a branch resonator that is twice as long as the one I used. The drone at 2000 rpm is around 100 hz.

---------- Post added at 12:32 PM ---------- Previous post was at 12:28 PM ----------

So around 90 cm branch. I was thinking of putting is right in front of the tank. I think there is room there. You could try doing it in 3 or 2½ or maybe 2 inch piping. The smaller the diameter the smaller the effect, though.
 
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Kristian_Wraae

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Nosechunks;1700783 said:
I run a 4 inch exhaust from the downpipe back. So im assuming i should use that size pipe?

What RPM was the drone issue you were having?
Click to expand...

You can use any size. The bigger the pipe the bigger the effect - but it'll also be more difficult to make it fit.

I had the same drone rpm. It simply relates to the total length of the exhaust.

But if you want to make sure - make a recording of the sound and use Audacity to make a spectrum analysis. Then you can see the peaks you are having trouble with.

Run the car where the drone is maximun and record the sound and take note of the rpm. Then you have something to compare with after.
 
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TurboStreetCar

TurboStreetCar

Formerly Nosechunks
Feb 25, 2006
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Kristian_Wraae;1700802 said:
You can use any size. The bigger the pipe the bigger the effect - but it'll also be more difficult to make it fit.

I had the same drone rpm. It simply relates to the total length of the exhaust.

But if you want to make sure - make a recording of the sound and use Audacity to make a spectrum analysis. Then you can see the peaks you are having trouble with.

Run the car where the drone is maximun and record the sound and take note of the rpm. Then you have something to compare with after.
Click to expand...

You said i should use one branch with twice the length. But you said you have the drone at the same RPM. Shouldnt our branches be the same?

How much of an effect did this have when you were all said and done? On a scale of 1-10, 1 bieng no effect and 10 being the same volume threw out the rpm, how would you rate the effectiveness of this?

I already put a second $60 2 foot long glasspack under the car, i dont want to go threw the trouble to have not so much gain. Adding the second muffler was very disappointing, Made little to no difference. If i do this i want it to be silent inside the car haha.

When you analize the sound file in Audacity, How do you know exactly what your looking at?
 
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Kristian_Wraae

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I made mine half the length it was supposed to be (I did mine before I analysed the sound - so I thought my peak was at 200 hz).

As you can see from the sound spectrum it got rid of the 200 hz peak, but it should have gotten rid of the 100 hz peak instead.

So you need to make one that is twice the length or you could try to do a Helmholz resonator tuned for 100 hz (or what ever you drone frequency is).

My guess would be a branch of appr 90 cm. If you make it so it could slide a little you could tune it to max performance. Just make a sound recording (I just used my camera) so you have something to compare with.

---------- Post added at 07:42 AM ---------- Previous post was at 07:36 AM ----------

When analysing the sound you look for peaks. The peak at 100 hz is the drone. The 200 hz is the octave of the drone.

I only got rid of the octave but even that was very noticeable. The sound was way more relaxed.

But since I never got around to do the 100 hz resonator I can't guarantee you that you'll be "drone free", but I have a strong feeling that it'll make a difference unlike the extra mufflers. At least it works for the Mustang guys - so why shouldn't work for us?
 
TurboStreetCar

TurboStreetCar

Formerly Nosechunks
Feb 25, 2006
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I see. Im going to try to record a sound file inside the car at the drone RPM.

When you anaylise it in audacity, how exactly do you do it? Do you select a certain part of the sound file? If so how much?

Also what did you use to make the sound file? When i open a video from my camera it sounds like electronic noise, not the sound from the video.

Thanks for all the info, Im going to look into this in the next week or so.

-Dan
 
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