Helmholtz resonator project I will start as soon as I can

[RAAMaudio]

My DIY system has a bit of drone, not real loud but annoying on a longer drive. I have a big resonator up front but small race muffler in the back, when I put on the straight pipe for the track there is not a lot more drone when accelerating or cruising but a huge jump to a very high level under deceleration, odd it is mostly there.

I am going to work on a Helmholtz resonator for the pipe right before the Vband for the straight pipe/muffler quick change so the resonator is always on. Much less weight than a bigger muffler and I have a lot of pipe sections to work with. I will find something that I can slip a pipe over a pipe and can tune it that way and then cut off what is not needed and weld it up nicely.

Before Vband was welded in, after the last bend before going into the muffler.
There is room on top of the muffler for the Helmholtz resonator.

It would not be exact for any other system but might be close on some other 3" ones that are true 3" from turbo back with straight through resonator and muffler and single tip so I will share the info when I get mine dialed in.

It will have to be done when I get the EFR kit here and tuned first.

Stay tuned, add comments, etc......

Rick

 

[Chuckable]

Nothing constructive to say - since the engineering of a Helmholtz resonator is over my head - except for looking forward to seeing your progress!

 

[RAAMaudio]

Since I have a lift and all the tools and retired I am just going to build one, see what happens, adjust it, make it work, no engineering at all

That is basically how most aftermarket companies make parts, very few really engineer anything, sad but true.

How many insanely loud Subaru's have you heard?

I had a Cobb DP and exhaust on my highly modded Forester XT, big hatch/wagon, absolutely no drone and it was proven to make great power, it was engineered properly, not just slammed together and sold as most exhausts are.

I have not read up on their ST exhaust, just noticed some mention of it, likely pretty quiet and not a drone prone part I am sure but this little engine is a bit of a challenge in that department so it might have a bit.

 

[iso100]

I suggest a long pipe with a ton of overlap with another so you can slip joint them. Then you can adjust the length to tune exactly the frequency you want to cancel out. Once nailed down you can cut, tack and then seam weld.

You might be able to eyeball the CP-E system for a ballpark measurement to get started.

 

[RAAMaudio]

That is exactly my plan, you just said it better

I will take a look, I forgot who make the exhaust with one already on it, since I made my own I do not look at others much.

Thanks
Rick

 

[no-pistons]

I plan on doing the same thing to my MBRP if I don't just end up selling the stupid thing. Yeah, there is a lot of room where the original muffler used to be. According to the formulas I found online and using a free frequency detector on my iPhone, mine needs to be about 30 inches long.

 

[iso100]

I plan on doing the same thing to my MBRP if I don't just end up selling the stupid thing. Yeah, there is a lot of room where the original muffler used to be. According to the formulas I found online and using a free frequency detector on my iPhone, mine needs to be about 30 inches long.

That's longer than I expected. The CPE one seems to be only about 16 inches long. I wonder if you made the chamber slightly larger than 3 inches if you could get away with a shorter chamber.

 

[RAAMaudio]

Is part of the equation volume vice length or both included?

I have a pretty good idea of how wavelengths work, I should after all the years in electronics and then sound deadening, etc....but sometimes volume is as critical as length or the length, and diameter, is used to get the volume....

I can come off the pipe with a 2.5" section and then make the sliding tuning section 3" as I have a section of flared pipe so it would be easy to use a clamp and move it back and forth to change the length and volume at the same time but that also creates a different kind of resonate cavity with a pipe within a pipe......

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I used to have an RTA from my car audio comp days but now I just have a freq counter I could use, maybe I should test and do the math but I have to wait to get the car running first. I could do it the guestimate, sliding test setup now and itching to start a project....

I spent 20 years in the USN and many of those on insanely complex electronics systems and became rather burned out on it so prefer to not think so much now but I know I can figure most anything out if I wanted to, I like to built stuff more than think about it



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[RAAMaudio]

I have a y pipe I could use and get more volume if needed as 3" is the max diameter I could fit above the muffler. I have some 4" tubing which would add less weight but just to big in diameter.

I could also cut a section of 2.5 or 3" and add material to make it an oval.

I will consider all this as want the least weight possible even though I am taking all the weight off the front of the car I can so more rear weight helps balance it out....

 

[iso100]

I say make a recording of the conditions you want to reduce. Then on the computer determine the actual Hz you want to cancel out.

Then plug it into the second calculator here:
http://www.vk2zay.net/calculators/helmholtz.php

Hopefully that will give you a port length you can deal with. If it's too long, then try varying port area to see how it affects the length.

 

[iso100]

Hopefully that will give you a port length you can deal with. If it's too long, then try varying port area to see how it affects the length.

Crap, it doesn't. It just gives volume.

 

[iso100]

Useful thread from elsewhere:
http://www.ls1gto.com/forums/showthread.php?t=449416

Make note of post #30.

 

[RAAMaudio]

I read that whole thread this morning

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I guess I have to use my brain instead of pickling it while I work on just pounding something out....thanks

Though I never built ported sub enclosures, I was more into SQ than SPL, I have an understanding and they tune using volume, length and diameter combined, as far as I know the end result is area not actual length.

That make sense to a degree when you know the length some frequencies require to fully propagate, which I no longer remember but in lower freqs, rather long.

Wavelength calculator: http://www.mcsquared.com/wavelength.htm

Example: 32 hz is over 35'

Vehicles are probably more in the 60-120 range for exhaust, just guessing really, 60 hz is still 18'+ but the end being blocked off must at least double the effect of the resonator.....still ends up being 9', there has to more to this to end up with shorter lengths we will need.

 

[KKaWing]

Can't you weld in a 90 degree bend to the exhaust and have a v-band on the end so you can just attach different length "resonators"? Like a quick detach resonator to experiment with?

 

[RAAMaudio]

Good idea!

But I had to think it over, more work in the end doing it that way as have to make more parts, more time to swap between tests....

This thread is turning out quite interesting, thanks for all the inputs!

Rick