by "TahoeSTi" (tahoesti)
Published 12/15/2017 at 13:51
by "TahoeSTi" (tahoesti)
Published 12/15/2017 at 13:51
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STARS: 14
So after showing a Jeep Rubicon how to rally over snow banks, I found my factory exhaust snapped and hanging.....the only solution was, of course, a full exhaust upgrade. After which the car sounds great...but on the first road trip the 80dB+ drone at 100hz was killing my wife and I . She puts up with all my loud cars....but this one is her’s and it’s the car we use for longer trips now.
So being a bit of an Audio nerd I knew a Helmholtz resonator could fix it While searching for a way to calculate that I found something even simpler. A 1/4 wave tube, AKA J-pipe, Side branch resonator. The basic idea is the pipe is 1/4 of the wavelength of the frequency you are trying to eliminate. In my case this was 100hz. So my side branch resonator is 34" long. The results are downright amazing. No change in exhaust volume outside, but between 2500 - 3300rpms there’s no ear-rattling drone in the car. In fact, on the freeway, while cruising it sounds stock.
Calculations:
Find the drone frequency use an RTA app or use this:
RPM x engine pulses per RPM x 1min/60sec, a 4 cylinder engine a Subaru boxer has 2 pulses per RPM
In my case it was: 30000 rev/min x 2 pulses/rev x 1/60 = 100Hz
See that bump at 102Hz that’s what we are trying to fix. Note this is not a calibrated mic it’s just the one on my phone I didn’t feel like bring my laptop and full Room analysis setup so I just used a cheap phone app. Basically to confirm my rpm based calculations.
Next: wavelength = Speed of sound/ frequency
Wavelength: 1125.33 ft per sec / 100 Hz = 11.25 ft
Resonator length: 11.25 ft / 4 = 2.81ft of 33.75"
Note the speed of sound changes with temperature and pressure.....but the side branch resonator won’t have any exhaust gasses in it so it won’t get much hotter than outside temp on average. Since everything is relative, the length should work as temps change.
So far the results are amazing, I was able to have a phone conversation with my Bluetooth. I’ll try to post an RTA graph of the after I thought I save a screenshot but I didn’t....the hump at 100hz is completely gone. Next half of this road trip is tomorrow and Sunday I’ll try to get screenshots of results then.
"TheRealBicycleBuck" (therealbicyclebuck)
12/15/2017 at 14:26, STARS: 0
So the resonator is just welded to the exhaust? There are no exhaust gases passing through it?
"Rustholes-Are-Weight-Reduction" (rustholes-are-weight-reduction)
12/15/2017 at 14:26, STARS: 0
Working for an exhaust OEM supplier, that’s amazing work! But I’m on the durability side, so I’m wondering how long that setup lasts on standard hangers
"HammerheadFistpunch" (hammerheadfistpunch)
12/15/2017 at 14:26, STARS: 2
Math! Hells yeah!
"Rock Bottom" (rockbottom81)
12/15/2017 at 14:37, STARS: 3
As an unashamed aerodynamics fanboi, I can say that Helmholtz resonators make my pants tight. We aerodynamicists use them to reduce low frequency wind tunnel pulsation. I never thought of applying one to a droning exhaust system. Brilliance through simplicity. Well done!
"DAWRX - The Herb Strikes Back" (karsonkinja)
12/15/2017 at 14:41, STARS: 2
This is a super interesting write up! Good oppo!
"TahoeSTi" (tahoesti)
12/15/2017 at 14:45, STARS: 0
It’s got a bracket attaching it to the mid pipe, should hold up.
"TahoeSTi" (tahoesti)
12/15/2017 at 14:46, STARS: 1
A hole is drilled in the mid pipe, this is attached there, then the other end is capped off, So exhaust won’t flow because it will be pressurized inside the side chamber.
"gawdzillla" (gawdzillla)
12/15/2017 at 14:50, STARS: 0
wait so you just weld a pipe on there ? you dont need to cut a hole in that “upgraded” exhaust or anything ?
mind blown
edit : where / how do u determine the location on where u putting the J-pipe
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
12/15/2017 at 14:56, STARS: 5
The idea is that by using a blocked pipe 1/4 the wavelength of the problem frequency, the sound travels in and returns exactly halfway out of phase—thereby cancelling out the problem frequency. No passthrough.
"Textured Soy Protein" (texturedsoyprotein)
12/15/2017 at 14:57, STARS: 0
Ok, I’m curious about this.
My 335xi drones because I have an aftermarket downpipe, and I disabled the flapper valve that normally closes the driver’s side muffler when cruising. I did this partly for more fun noises and partly because the valve was going bad so when it was closed it made a high pitch whining noise and I figured it was easier to just disable it by unplugging the connector that controls it than to get a new driver’s side muffler. If I got a new driver’s muffler I could of course just re-connect the valve on highway trips.
The downpipe is a Y-pipe:
The midpipe has a cat on each pipe and a crossover.
The mufflers connect to the midpipe like this (although this midpipe has no cats, unlike mine):
So...any ideas on a way to add add some kind of resonator like yours to a goofy exhaust like mine?
(In an ideal world I would buy these B&B mufflers that have a flapper valve in the driver’s side, and just reconnect the flap control connector on road trips. But that’s $1200+!)
"ateamfan42" (ateamfan42)
12/15/2017 at 15:06, STARS: 1
"thejustache" (thejustache)
12/15/2017 at 15:15, STARS: 0
I want to do the same thing to my XT now for the same reason (pretty much so my wife won’t start to hate the car or not take long road trips with me anymore)! So it just reduces the droning frequency but leaves the rest intact? How many db does it reduce 100hz? I love the deep rumble right now but there is a particular frequency that’s just excessive, and it sounds like its around 100hz for me too.
"TahoeSTi" (tahoesti)
12/15/2017 at 15:52, STARS: 0
It just stops the Resonance at 100hz, It brings it down to match the rest of the tone.... you’ll find you hear the exhaust, not the drone, outside it sounds almost the same. If you like the way it sounds now you’ll love it after you kill the drone.
"TahoeSTi" (tahoesti)
12/15/2017 at 15:54, STARS: 0
you would need 2 one for each side since you have to do it after the Y. To figure out what size you need you need to figure out what frequency the drone is, you can use an app on your phone or calculate it using the RPM it happens at. I found one guy online who only used these resonators on his BMW with the same exhaust setup you have, no other mufflers or resonator.
Both would be the same length and if it’s 100hz you’ll need 2 34" resonators, but since that’s a 6 cylinder it might be higher then 100hz which could mean short resonators.
"awmaster10" (awmaster10)
12/15/2017 at 16:02, STARS: 0
Exhaust stuff mystifies me. I got really lucky this time around and the simple resonator and mid muffler swap made the TL sound incredible but with no drone through the rev range. On my past setups I’ve always had drone problems, the worst of which being on the 4Runner that was thunderously loud when climbing hills or on the highway at 70. The 5vz is a loud ass engine though and the j32 is smoother and infinitely better sounding.
"Textured Soy Protein" (texturedsoyprotein)
12/15/2017 at 16:03, STARS: 0
Do you think that crossover section mess with any of the calculations? TBH I’m not sure where exactly there’s room under the car for any such resonators.
"TahoeSTi" (tahoesti)
12/15/2017 at 16:08, STARS: 1
The resonators only care about the wavelength of the frequency you are trying to deal with. they can curve and bend just have to be smooth bends. so you can wind them up next to the muffler. It also almost doesn’t matter where they go they just have to start at 90 degrees angles off the main tube they connect too. Also, they don’t have to be the full diameter of your exhaust pipes I used a 2" for my 3" exhaust, the closer the match the bigger the decrease in sound I’m told, but my drone is gone at 2".
Also if you have a wider range you’re trying to cancel you can stuff it with steel wool and raise the Q of the resonator, it will do a wider range with slightly less attenuation.
"TahoeSTi" (tahoesti)
12/15/2017 at 16:09, STARS: 2
It’s just sound engineering. :)
"awmaster10" (awmaster10)
12/15/2017 at 16:13, STARS: 1
My buddy who builds big LS motors had to do the same setup on his catfish camaro and I remember running the numbers to figure out the dimensions and it worked perfectly. Hearing a 650whp motor drone with almost straight pipes is not pleasant.
"RacingShark" (racingshark)
12/15/2017 at 16:51, STARS: 0
have you sent this to the manufacturer of the exhaust? This seems like a simple solution. I saw an after market exhaust on a s2000 that had something similar, thanks for explaining what it does!
"TorqueToYield" (torquetoyield)
12/15/2017 at 17:36, STARS: 0
This makes my engineery bits all tingly.
I hate so many after-market exhausts for that stupid highway drone.
I’m assuming you welded it up yourself? I really need more tools...
"TahoeSTi" (tahoesti)
12/15/2017 at 17:41, STARS: 1
No welder yet myself just explained to the muffler shop exactly what I wanted....the guy had even made them before back in the 70s.
I do need a welder this has proven that to me.
"TahoeSTi" (tahoesti)
12/15/2017 at 18:22, STARS: 0
If your car is droning at 3000rpms it’s most likely 100hz.....the math says so.
"TahoeSTi" (tahoesti)
12/15/2017 at 18:25, STARS: 1
you do have to cut a hole, the J pipe is capped at one end and open to the exhaust on the other. It can go anywhere in the exhaust, mid pipe was best for me, so I only needed 1 if I did it after the Y I would need 2.....my new exhaust is 3" so I used a 2" j pipe, it can be smaller than the main exhaust. The key is the length, the 90 degrees port to the j pipe, then smooth transitions in the pipe if it bends. it could be a straight out dog leg at 90 degrees but i didn’t have room for that.
"TorqueToYield" (torquetoyield)
12/15/2017 at 20:49, STARS: 1
Cool. Sometimes a guy/shop with a tool you need is just as good if not better. Especially something like a welder that takes some skill and you might not use too often.
"TahoeSTi" (tahoesti)
12/18/2017 at 11:02, STARS: 0
So just got back from a 3 hour and 5-hour drive, and it’s much much better. you hear the exhaust rumble when you start the lay on the gas pedal, but there’s no Drone. You can hear the exhaust but not the resonance. My wife and my ears were ringing after road trips before now it’s almost as quiet as stock with nice exhaust rumble in the back.
The drone is very much reduced over a wide range and is completely gone from 2900-3100 rpms .
"Wrong Wheel Drive (41%)" (rduncan5678)
01/19/2018 at 11:42, STARS: 0
I enjoy that my Miata doesnt drone on the highway because its pushing 4k lol. The exhaust will most certainly drone excessively around 2.5k but its rarely ever there at cruising speeds. I can only get that if I try cruising at about 40mph in 6th gear and at that point any sort of hill will lug the car.