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  1. #1
    Senior Member Four Rings ericpaulyoung's Avatar
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    Car at shop, helmholtz resonators being installed.

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    Ok, so last weekend I installed a new stasis exhaust and loved the sound volume but not the drone. So I just dropped my car off at Goofy's Custom here in Eugene, and having them cut in a 34" long stainless resonator into the stasis. I am having just one installed right before the exhaust splits into the 2 x 2.5" sections. I have before pictures, and FFT resonance spectrums from recording my exhaust under several different loads earlier today. After I pick up the car in a few hours, I will record new sound clips and do another FFT analysis to have an exact measured difference in drone. I am keeping my fingers crossed because this is already close to $400 for all the materials and fabrication work.
    Eric

  2. #2
    Active Member One Ring
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    2006 Audi A4 2.0TQ 6MT
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    Upstate NY

    What did you use to record the sound? What was the main frequency? Can you post the FFT?

  3. #3
    Senior Member Four Rings ericpaulyoung's Avatar
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    I used SignalScope from Faberacoustical. It is an iphone app that is $25, and works great. The only drawback is that the built in iphone mic has a low pass filter (which the software maker indicates as a reason to use an external mic if you have one. I didn't have an external mic so the readings that I get for <150 Hz will be artificially low. However, the resonance is still very visible at ~70-90 Hz and another one at between 150-200 hz. Depending on the rpms that I am at, the two peaks will float within those two peaks.

    OK, update. The thing ROCKS!!! Basically the "STASIS exhaust with 034 HFC" is close to the "stock exhaust with the 034 HFC". Seriously I don't think I can overstate how much better it sounds. The bad part is, the total system cost went from about a grand to around $1400 with the extra fab work, so it might be easier to just get the APR GenII with Sound Cancelling (if they offer it for the B7). Now if you didn't pay someone $100/hr, and you did it yourself, or if you already have the system or a similar one and you just want to fix the thing, than I say GO FOR IT ASAP. I used a 2" dead end stainless steel 304 pipe that is adjustable from 30" to about 40", and I can tune the thing to have pretty much zero drone, to having a little drone (if you wanted to,,,not me).

    I don't have any online image storage, so if someone wants to host the spectrums and pictures for before/after, just PM me and I will email you the photos. I am still moving the pipe longer and shorter trying to tune it exactly, as the sound varies depending on how long the chamber is, so I am not finished with my tailoring yet; but I am in love with the car again.

    - Eric

  4. #4
    Veteran Member Four Rings DRAKLORE's Avatar
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    You can email them to me. Shanedrakeairforce@gmail.com
    I'll post them as soon as I get them
    -Shane Drake
    2006 A4 Avant Dolphin Grey
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  5. #5
    Senior Member Four Rings ericpaulyoung's Avatar
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    Sent the fab images to you.

    I am at the bar watching the Duck game, so too busy to add the spectrums with the needed descriptions right now. Maybe at halftime or when I get home (if standing straight enough to want to surf the forums :-)
    Eric

  6. #6
    Veteran Member Four Rings DRAKLORE's Avatar
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    I'll get the rest up in a bit, I have to admit the whole setup looks pretty good :-)
    -Shane Drake
    2006 A4 Avant Dolphin Grey
    2007 GT3071R A4 Moro Blue-RIP
    >>DRAKLOREs Build<<
    DRAKES-PERFORMANCE
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  7. #7
    Veteran Member Four Rings DRAKLORE's Avatar
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    BEFORE:



    AFTER:



    The loss seems fairly clear. Where do you place the phone for the sound test?
    -Shane Drake
    2006 A4 Avant Dolphin Grey
    2007 GT3071R A4 Moro Blue-RIP
    >>DRAKLOREs Build<<
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  8. #8
    Veteran Member Four Rings drumnjuny's Avatar
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    wow, that looks great!!
    Arjun's Ibis B7 2.0TQ-S GTX2867R

    404awhp/355awtq (91 oct)

    Ask me why RAI should not have a business license.

  9. #9
    Senior Member Four Rings ericpaulyoung's Avatar
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    Go ducks !!

  10. #10
    Veteran Member Four Rings DRAKLORE's Avatar
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    Paul young, like on desperate housewives... Right? Lol
    -Shane Drake
    2006 A4 Avant Dolphin Grey
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  11. #11
    Veteran Member Four Rings D0lphinGrey's Avatar
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    Look great! Good to know it solved your problem. I went with a different method, got my shop to weld in a custom flange with a removable block off plate on one side of the exhaust.

    Your setup is similar to what they do with on a 350Z,
    B7 A4 GT28

  12. #12
    Veteran Member Four Rings DRAKLORE's Avatar
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    My friend has that sane exhaust on that same color z lol
    -Shane Drake
    2006 A4 Avant Dolphin Grey
    2007 GT3071R A4 Moro Blue-RIP
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  13. #13
    Senior Member Four Rings ericpaulyoung's Avatar
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    Thanks guys. I am frankly amazed it is working so well. The 350Z setups are actually different, but work on the same principle. This is the whole argument between AWE and APR that showed up in another thread, where the there was a pretty fare amount of flame going back and forth about sound cancellation theory. Basically this is not a Helmholtz chamber, though it could be working as one in addition to its real intended purpose. The difference is in how the cancelation wave is generated. I will skip all the vague arguments about which is better or who has a bigger shop and just jump to the end.

    1. In a Helmholtz chamber the air in the throat of the chamber is oscillating back and forth because of the spring action within the chamber (the 350Z idea above, and what is used in AWE/Magnaflow/etc exhausts). If this is at 1/4 or 1/2 (or some 1/4 or 1/2 multiple), the two pressure waves eat each other's lunch and we are happy. Here, the sound in the exhaust is not needed as a driving function, the velocity of the gasses travelling across the connection produce the oscillation, and the volume of the chamber and the relative geometry of the connecting throat are the critical items to get it to produce the correct cancelation pressure wave.
    http://en.wikipedia.org/wiki/Helmholtz_resonance

    2. In a reactive silencer, the standing wave produced by the driving function in the exhaust tone is sitting pretty within a length of side passage. The distance travelled from the entrance to the end cap length of the chamber is 1/4 that of the driving wave, and again they eat lunch and we are happy. What I designed and built was a quarter wave length reactive silencer similar to what APR has licensed from CORSA, and is part of the famed NASA reference that shows up in forum posts regularly.
    http://www.universalaet.com/en/docs/...-silencers.pdf

    Everyone (including myself when I started this thread) keeps describing reactive silencers and Helmholtz chambers the same thing. They are not. But here is the catch, they both require to be tuned to specific frequencies (neither are broad band regardless of what any vendor tries to sell you), both both do the same thing, both suffer from changes in the temperature of the chamber, and both can do their job just as well.

    Now here is the part that fuels all the arguments, the Helmholtz chamber is potentially more sensitive to pressure and temperature fluctuations than the reactive silencer. So even though a correctly designed Helmholtz chamber will work 100% as well as a reactive silencer, it:
    1. Suffers from changes in the environment easier and won't work as well over as many conditions. Basically it is a touchy biatch and if it ain't happy, it don't go shopping. You could fix this by ensuring that the chamber temperature is always spot on in all times of the year, and tune it right, but it is just damn touchy.
    2. THIS IS IN MY OPINION THE BIG ONE - It doesn't need a driving function. In other words, it will produce the blowing across the beer bottle sound even if there is no sound to cancel, in which case it is ADDING TO THE EXHAUST NOISE in the absence of drone. Basically it becomes the drone when its evil twin is not around for it to have lunch with. Don't know if this is the cause of the reactive silencer being better or not, but it is my speculation that it is part of it.

    Happy holidays, and I am excited to see if the vendors chime in her and correct me or start biatch slapping each other again. :-) Come on, it is Sunday, I don't attend church and we need entertainment.

    - Eric

  14. #14
    Established Member Two Rings
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    do you think doing what you did on an awe exhaust would yield noticeable results? I have awe quad tip with 034 HFC in my avant with a ridiculous amount of drone. I love how it sounds, but I'm thinking of getting rid of it because of the drone.
    06 B7 A4 2.0T Q6MT avant - BB

    http://foreignautomotive.ca/

    Mods:Uni stg2 HPFP 91oct, KMD HPFP, IE Billet VC, Moroso CC vented to DP, ER SMIC's w/034 hoses, 034 turbo inlet, 034 HFC, Custom quad-tip, a4 sport suspension, RS4 rear sway, RNS-E, RS4 carbon trim pedals & shifter, S4 Recaros, AWE vent mount boost gauge, Black RS4 look grill, Votex front splitters & side skirts, Ti window trim, S4 mirrors, LLtek LED headlights, DDM HID Fogs, Black 19" O.Z. Ultraleggera

  15. #15
    Senior Member Four Rings ericpaulyoung's Avatar
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    Yup, sound cancelation works everywhere. BUTTT, it has to be tuned to your specific exhaust. So just record the drone, FFT the frequency, calculate the "rough" length, then have one installed that allows you to adjust +- about 5 inches each way, and you should be able to knock it out. Now here's the catch, you probably have at least 2 different low end peaks that are pissing your ears off, and the pipe length for one is going to be way the hell off of what is needed for the other. In my case, I have no idea how I am hitting both peaks ??? Maybe the change in exhaust dimensions made it no longer resonate at one peak, so that one didn't have to be treated so much, and then at the same time I was able to hit the one that was still there. At least this is my guess. A rough shot at it (and many hours looking at what people have done for everything from 4 bangers to V8 Porches), if you cut in a pipe that is adjustable from 28 - 38 inches, you will likely be able to get at least one of the little guys that are hurting your hard earned Audi! ;-)

    Hint #2, make the dead end portion uphill so you don't collect moisture in the pipe. I had mine shaped downward so that I can get a really long pipe end (total of 60") just for testing to see if I can reach down and hit the 80 Hz range for science. And now it is collecting water vapor in it :( So when I through the K04 on next month, I will take the downpipe reactive silencer combo to the shop and have them mandral bend the daddy up ! ;-)

    - Eric

  16. #16
    Veteran Member Four Rings DRAKLORE's Avatar
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    Your welcome btw...
    Lol
    -Shane Drake
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  17. #17
    Senior Member Four Rings ericpaulyoung's Avatar
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    Oh snap. Thank you for posting the pictures. Forgive the tardy appreciation; I can be an asshat about remembering to say thanks sometimes!

    - Eric

  18. #18
    Veteran Member Four Rings DRAKLORE's Avatar
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    NP- anything that benefeits the community is worth the effort :-)
    -Shane Drake
    2006 A4 Avant Dolphin Grey
    2007 GT3071R A4 Moro Blue-RIP
    >>DRAKLOREs Build<<
    DRAKES-PERFORMANCE
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  19. #19
    Senior Member Four Rings ericpaulyoung's Avatar
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    One last update then we can let the thread die. This thing is much more sensitive to adjustments than I expected; I was moving the pipe back and forth a couple of inches at a time, but it is actually sensitive to 1/2" changes in length! So if you do it, expect to fine tune the little guy by doing plenty of crawling under the car to make small changes, then going for a drive, then crawling under the car, then going for a test drive, etc. I like doing this, so no biggy.

    Sky Capitan Out
    - Eric

  20. #20
    Senior Member Four Rings ericpaulyoung's Avatar
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    Oh ya, and my final adjusted length to kill the 2000 rpm drone is approximately 34".

    - Eric

  21. #21
    Veteran Member Four Rings NorcalPB's Avatar
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    Don't mean to necrobump this thread, but where did you get the adjustable dead end 304 pipe, and why did it cost you close to $400? I can do dual 18" resonators after my y-pipe for $200 installed, but if this is a more potent way to eliminate drone then I'll go ahead and do this.

  22. #22
    Veteran Member Four Rings Stewy's Avatar
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    Quote Originally Posted by ericpaulyoung View Post
    Go ducks !!
    That was a crappy game...we didn't even show up!
    2007 Audi A4 S-Line 2.0T FSI - APR Stage III GT2871R, APR HPFP, Carbonio intake, AWE FMIC, BSH CC / PCV delete, OEM "D" DV, APR downpipe, Neuspeed exhaust, H&R coils, ECS Stage 4 BBK, JHM 4:1 center diff, Neuspeed short shift, RS4 rear sway bar, Neuspeed snub mount, Defi MeterBF gauges with Defi-Link, Vag'd, de-badged, black rear valence, Cupra R lip, Huper Optik tint, RS4 grille, EU amber tails, 19" Hartmann RS4 replicas with H&R Trak+ spacers

  23. #23
    Senior Member Four Rings ericpaulyoung's Avatar
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    Quote Originally Posted by NorcalPB View Post
    Don't mean to necrobump this thread, but where did you get the adjustable dead end 304 pipe, and why did it cost you close to $400? I can do dual 18" resonators after my y-pipe for $200 installed, but if this is a more potent way to eliminate drone then I'll go ahead and do this.
    The muffler guy welded a short section with a dead end to it, that was swaged to fit over the open end of the branch resonator. That way you can adjust the length to get it perfect. It is surprising that small variations will make a noticeable difference. And given that the exact location of the branch tee, and setup you are running is going to be slight different, this adjustability is a must. I guess the other option would be to give the calculation your best shot and live with it, but I noticed a substantial difference in even small variations in overall length.

    Just make sure it runs up hill towards the end so it doesnt fill with condensate and screw your tuning up. Or you can do like I did and drill a 1/16th inch hole at the lowest point as a drain.
    Eric

  24. #24
    Senior Member Four Rings ericpaulyoung's Avatar
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    Quote Originally Posted by Stewy View Post
    That was a crappy game...we didn't even show up!
    GO DUCKS!!



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