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Old 2012-09-26, 12:24 AM   #1
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Default Killing exhaust drone with 1/4-Wave Resonator Tubes

I've been following this really long thread on the corral and others like it for a long time. They detail the science between adding capped resonator pipes branched off the main exhaust that are exactly 1/4 of the length of the soundwaves of the "drone zone". The idea being the sound wave enters the capped pipe and bounces back so that when it gets back out, it's exactly 1/2 a wave out of phase, so it meets the next pulse in the system head-on, totally cancelling it out. I can't do this on my mustang because the aftermarket suspension components leave me no room to add the extra pipes. I've added Dynomax VT mufflers to it, which work, but I just bought a Pontiac G8 GT with a Magnaflow exhaust that droned like crazy at 1900 RPM, 125 Hz, but otherwise sounded great. I didn't want to change it, I just wanted to kill the drone.


The G8 folks are hip to the phenomenon, and a company called SOLO makes a catback that incorporates adjustable-length capped J-shaped pipes that mount back by the mufflers. These aren't them, but the principal, the shape, and the location are the same.



It's important to note that the phenomenon doesn't care where the pipes are mounted, or how they're shaped, just how long they are. It just so happens that there's a perfect spot for them on the G8's platform back by the mufflers near the bumper. These ones are adjustable kind of like a trombone, but they come pre-assembled at the perfect length to kill a 125 HZ resonance. Lower frequencies require longer pipes, and higher frequencies require shorter pipes.

The math works like this:

1900 rev per minute * 4 pulses per rev / 60 seconds per minute = 125 pulses per second


The speed of sound in air depends upon the temperature. I assumed 1075 feet per second, which according to my research is appropiate for 113 degrees temperature. Seems fair for a pipe that's connected to the exhaust system, but which doesn't actually see any flow, because it's capped.


To find the wavelength of each sound pulse:


1075 feet per second / 125 pulses per second = 8.5 feet or 102 inches. You want the quarter-wave tube to be 1/4 of the wavelength, or 25.5 inches, in my case.


I had them installed yesterday, and the difference is incredible. And measurable. As seen by this screen shot from my handy-dandy sound meter app on my iphone, the big hump in the 125hz range is the drone, before I added the 1/4-wave tubes:










I don't have any sound files, but the meter tells the story: Same RPM, same stretch of road, same time of day. Look how much smaller that hump is, and look how many fewer decibels it's reading. That 10-decibel difference is huge. The way the decibel system works, every 10 decibels higher means twice as loud. That means the old setup was twice as loud as the new one, or put another way, the new setup is half as loud as before:






The drone has been virtually eliminated, and I'm a happy camper. This shit works, and it works on any car with enough clearance to add pipes of the appropriate length.



.
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Old 2012-09-26, 01:00 AM   #2
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I'm surprised no one has tried to copy Motordyne's "ART" downpipes made for Nissan Z and Infiniti G. Same concept, but they're upstream in the downpipe and pack a huge wallop of low rpm torque (~25-30wtq) with the Helmholtz resonators while also canceling out some of the nasty Hz. The owner of Motordyne was formerly an aerospace engineer. His parts make the power as advertized unlike many companies out there.

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Old 2012-09-26, 06:08 AM   #3
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very cool. I had installed a couple of those exhausts with the J pipe before. They have one that has adjustable sections to "tune" your exhaust.
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Old 2012-09-26, 09:00 AM   #4
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Been wanting to try this out when I finally build a different exhaust for my turd.
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Old 2012-09-26, 09:51 AM   #5
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The OEM's use this a lot on intakes to dampen the undesirable intake noise so the ride is quiet all the time..
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Old 2012-09-27, 12:51 PM   #6
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A couple of the exhausts for 350z's have resonators tubes like that by the mufflers too. I hadn't seen the resonated downpipes, those are pretty cool.

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Old 2012-09-27, 05:27 PM   #7
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Corsa makes high quality drone free exhausts that sound killer with acceleration. Each exhaust is tuned specific to the vehicle application so they don't need to use the above method. They can be rather expensive though.
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Old 2012-09-27, 06:35 PM   #8
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My god, the logic and science behind that is awesome... Rarely do I think this gets said but, nice thread sir.
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Old 2012-09-27, 07:36 PM   #9
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I had a corsa db black on my old 94 f150. I fucking loved it. Quite, but noticeable at idle, but screamed when you stood on it. It sounded odd since it was half a fox catback.

http://s111.photobucket.com/albums/n...t=MVI_1576.mp4

Meh. I liked it. $300 muffler for free on a $1900 truck.
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Old 2012-09-27, 11:13 PM   #10
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Drone is the one thing that I dislike about my exhaust. Had a resonator installed and it helped a bit, but it can still be annoying at times. Swear I've lost some hearing from it too.
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Old 2012-09-28, 12:38 AM   #11
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Drone is the one thing that I dislike about my exhaust. Had a resonator installed and it helped a bit, but it can still be annoying at times. Swear I've lost some hearing from it too.
What?
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Old 2012-09-28, 02:42 PM   #12
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Very nice! I'm curious if you have a plot of exhaust noise +- say 100 rpm, was the drone really that peaky?
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Old 2012-09-28, 03:06 PM   #13
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Which SOLO did you pick up? My dad wants one for his G8 but he's worried about drone. I've been telling him for a while to pick up a SOLO but he's never heard testimony from a credible source.
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Old 2012-09-28, 03:35 PM   #14
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I only picked up their J-pipes, I didn't get the whole system. I simply added them to the Magnaflow cat-back.
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Old 2012-09-28, 03:40 PM   #15
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Quote:
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Very nice! I'm curious if you have a plot of exhaust noise +- say 100 rpm, was the drone really that peaky?
I don't have it graphed, but it didnt' drone below about 1700 and it was gone by 2000.
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Old 2012-09-28, 03:53 PM   #16
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Originally Posted by joesstripclub View Post
A couple of the exhausts for 350z's have resonators tubes like that by the mufflers too. I hadn't seen the resonated downpipes, those are pretty cool.

Looks like an ARK kit. Nice, but pricey.
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Old 2012-09-28, 05:10 PM   #17
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Quote:
Originally Posted by Dave B View Post
I'm surprised no one has tried to copy Motordyne's "ART" downpipes made for Nissan Z and Infiniti G. Same concept, but they're upstream in the downpipe and pack a huge wallop of low rpm torque (~25-30wtq) with the Helmholtz resonators while also canceling out some of the nasty Hz. The owner of Motordyne was formerly an aerospace engineer. His parts make the power as advertized unlike many companies out there.
This part made me eye roll and lol hard. If you live in Wichita you know at least 30 retarded ass aeronautical engineers. Literally still laughing.
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Old 2012-09-28, 05:23 PM   #18
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Looks like an ARK kit. Nice, but pricey.
could be an ark or any of the other 1k knockoffs there are of that style hks exhaust
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Old 2012-09-28, 06:22 PM   #19
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Magnaflow is terrible for drone in the G8. I think the J-pipe is the best answer besides switching your entire exhaust system.

edit - also REEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
http://kcsr.org/showpost.php?p=1767744&postcount=13

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Old 2012-09-29, 10:33 AM   #20
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so i assume the math is slightly different for figuring out for a 4cyl ?

i'll fab up one today for my DD if i can get the length figured
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Old 2012-09-29, 11:24 AM   #21
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Yes, a 4-cyl generates 2 pulses per revolution instead of 4. So the math is
tube length in feet = 1075/(((drone revs)x2)/60)

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Old 2012-09-29, 12:00 PM   #22
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Does diameter have to be same?
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Old 2012-09-29, 12:38 PM   #23
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I don't think so
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Old 2012-09-29, 12:43 PM   #24
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im coming up with 27 and 5/8ths for mine. my drone is ~3400-3800rpm so i went with 3500rpm since that seems to be the absolute loudest.
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Old 2012-10-01, 05:21 PM   #25
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Seems about right.
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