[in progress] identifying and canceling drone

[Clint Beastwood]

So I got exhaust done, posted about it in this thread http://www.fiestastforum.com/forum/threads/17016-Any-thoughts-on-interesting-sounding-mufflers/

There was some drone I wasn't satisfied with, so rather than just throw parts at it (i.e. add a muffler, etc.) I figured I would determine the frequencies I need to cancel out and work out a helmholtz or 1/4 wave resonator to clean it up a bit. The first step is to record audio of the process and do a spectral analysis. A screenshot of the spectral analysis is below.



-- edit/update --

(Apologies if I ramble, on pain meds!)

mission accomplished! First is an analysis of my car just idling before installing my quarter wave resonator

Then is my car idling in the exact same place, but after the installation of my "math-ed" resonator

(yes the pictures are 2 different programs, both programs showed the same thing, but one was done on my computer one was done live while sitting in the car).

The head splitting drone between 3000-3400 rpm is gone while driving, and resonance while idling is gone. I like very much.


In the end, I targeted 3200 rpm, it wasn't the *worst* resonance, but it was right in the middle of the resonant range. I drove around with my Zoom sound recorder on a suction mount down by my tailpipe (@ 35% sensitivity) to get sound recordings and used spectrum analysis to find my drone frequency, and completely separately used resonant RPM to calculate the frequency, which wound up being identical numbers (validation!) So, I did the following to determine my overall length required (this is a rehash of some of the stuff above, though I adjusted numbers a bit as I learned).

Resonant RPM: 3200
Cycles: 2 (4 cylinder=2, 6 cylinder=3, 8 cylinder=4 (unless you have true duals with no crossover, then its 2 per side)

(RPM*Cycles)/60 = frequency
(3200*2)/60 = 106.67

Speed of sound(in FPS, changes w/ temp, I used 150 degrees F) / frequency = wavelength
1210/106.67 = 11.34 (feet)

wavelength/4 = quarter wave length (feet)
11.34/4 = 2.84
quarter wave length*12 = resonator length in inches
2.84*12=34.08 inches.

I have a 34" length of pipe looped where the muffler would normally go, and it worked absolutely fabulously. Make sure you branch in at a 90 degree angle, otherwise you could introduce turbulence into the exhaust flow.

Learnings: I didn't *really* save any money vs buying an mbrp or something, I could've just bought an exhaust system, but I had a *ton* of Amazon gift cards saved up over the years (and forgotten about!) and I couldn't spend Amazon cards @ whoosh, plus I kinda wanted to design my own system for fun because I am a nerd. There were MBRP and other systems on Amazon, but I didn't know the vendor and would rather spend money at Whoosh because Ron rocks. I guess if it came down to it, I could've gone with just a center resonator and my 1/4 wave pipe and called it a day, but I like the smoothness of the vibrant ultra quiet, so that was an extra hundred bucks (amazon, again, gift cards!). Another learning, doing the symposer delete got rid of some sound "oscillation" that made me want to barf once I got the exhaust installed, but it didn't cure the resonance, just an oscillation.

While researching exhausts it looks like many complain about the MBRP's drone - I'd bet if you bought an MBRP (or other exhaust) and used the above to calculate out the length of a side branch resonator, you could have a nearly drone-free MBRP exhaust that still sounded ridiculously visceral. If anyone was wondering, no, the side branch resonator will not affect performance, as it's not a path of travel for gases, just an acoustic wave, once the branch is pressurized it no longer affects gases in the pipe.


Update 08-21-2018 - I had the tailpipe extended 2” so it’s past the bumper to reduce bumper spot and the residual exhaust booming is gone now. So, if you can’t fully defeat your drone make sure your exhaust exits past the bumper. I *knew* this but got caught up in the resonator design and skipped a very basic step.

 

[Clint Beastwood]

[relevant info bumped to my OP]

 

[kivnul]

Looking at the availability of monitors per Cobb, "Exhaust Gas Temperature (Exh. Gas Temp) -> Exhaust gas temperature (EGT) based on the EGT sensor located in the uppipe." is an option if you have a access port. If this is what you mean by EGT, I will try to remember to plug mine in on the way home tonight.

EDIT: I was looking at a subaru list. I am not seeing it on a Ford list. There is "Exh Flange Temp – Inferred Exhaust Flange Temperature." dunno if that helps

 

[Clint Beastwood]

Looking at the availability of monitors per Cobb, "Exhaust Gas Temperature (Exh. Gas Temp) -> Exhaust gas temperature (EGT) based on the EGT sensor located in the uppipe." is an option if you have a access port. If this is what you mean by EGT, I will try to remember to plug mine in on the way home tonight.

No access port, I don't think I'll reflash until after my warranty is up

If you could grab the EGT that would be awesome, the speed of sound goes up as the temp does

 

[XanRules]

This is with JUST the Vibrant Resonator, right, no muffler?

 

[Clint Beastwood]

This is with JUST the Vibrant Resonator, right, no muffler?

2 vibrants, an ultra quiet and a bottle res. I had a pile of amazon gift cards so I ordered up the resonators with the intention of trying a vibrant muffler if the two resonators didn’t work out. Right now, if I can cancel the ~125hz drone it’s great. If I can’t cancel it out I’ll probably put a long round case muffler where the stock res lives. At the moment it looks like I need a quarter wave res about 27” long, I’m still gathering data with a high end sound recorder.

From downloading a bunch of other people’s exhaust vids it looks like everyone with a 3” drones in the 125-140hz range except cpe, so they definitely did their research with their Helmholtz chamber.

And I know it would be cheaper to just buy exhaust, but I’m having fun

note: edit on 04/25/2018 the 125-140hz drone I recorded and extracted from people's youtube videos was secondary resonance from the interior of the car working like the body of an acoustic guitar, it wasn't the actual genesis of the audible resonance. Targeting the 125-140 range would've done nothing for me.

 

[dmb]

exhaust temp can be as high as 1100 degrees on a pull, but just driving anywhere from 300* to 900* depending on boost. aluminum melts at 1220* that's why people use coatings on pistons on their race engines. to try to cool things down.

 

[Clint Beastwood]

exhaust temp can be as high as 1100 degrees on a pull, but just driving anywhere from 300* to 900* depending on boost. aluminum melts at 1220* that's why people use coatings on pistons on their race engines. to try to cool things down.

I’m going to harbor freight today to pick up an IR thermometer - since I’ll branch the res towards the end of the system, tailpipe temp should be close enough.

I have the appropriate length for my quarter wave resonator, almost 34 inches. I’m having a harder time figuring out dimensions for an actual helmholtz chamber - I wasn’t a math major, I studied network engineering and security

If I can get the proper helmholtz chamber size it should be a lot smaller, I think 17” or so.

 

[Clint Beastwood]

I’m going to pick up some pvc pipe and attach it to one of my exhaust tips and adjust the length until I get a proper wave cancellation. That should allow me to verify before cutting a hole in my exhaust

NOT driving with it and not for a long period of time, just a few minutes at a time.

 

[dmb]

ARK exhaust has some pic's, you can check out his chambers. it looks like his chambers are the same size

 

[dmb]

ARK exhaust has some pic's, you can check out his chambers. it looks like he uses the same chamber on all his stuff.

 

[Clint Beastwood]

Going in to have a 33.7” quarter wave resonator tomorrow.

 

[Clint Beastwood]

moved the results and extra nerd info to my first post in this thread

 

[Clint Beastwood]

Is it possible to rename the thread title? I want to update it to "solved - identifying and canceling drone"

 

[kivnul]

Next project, canceling out the X47 whistle!

(I loathe the whistle)

 

[Clint Beastwood]

Next project, canceling out the X47 whistle!

(I loathe the whistle)

Do a spectrum analysis (there are a bunch of phone apps) or record a clip w/ good sound and tell me what time in the clip the sound appears and I might be able to tell you what length you'd need to cancel it

though high frequency whistling you could probably cancel with a vibrant ultra quiet straight-through resonator.

 

[Clint Beastwood]

Video clips with a/b comparisons before and after the branch resonator. I think I could probably lose 1" off the branch resonator to target the remaining drone, but as-is it completely cured the pain inducing drone that was present before the branch resonator.

[video]https://youtu.be/lyoZm-DrErw[/video]

 

[TyphoonFiST]

Video clips with a/b comparisons before and after the branch resonator. I think I could probably lose 1" off the branch resonator to target the remaining drone, but as-is it completely cured the pain inducing drone that was present before the branch resonator.

[video]https://youtu.be/lyoZm-DrErw[/video]

That does sound very nice... I will admit that its way way easier on the ears and bearable than others out there! You should compose a list of parts so others could follow!

 

[Clint Beastwood]

That does sound very nice... I will admit that its way way easier on the ears and bearable than others out there! You should compose a list of parts so others could follow!

Thanks, it was fun to design and do the nerd work.

I'll put up a build sheet at some point, after I can get the car up on some ramps and get some pics. I still need to go back and have one joint cut and v-banded, I missed the lack of a vband joint where the pipe goes over the rear axle when I was inspecting the welds

After I get my right arm out of a cast I'm going to take some welding classes at a community college, I want to learn stainless steel and aluminum. Last time I tried welding I did some stick welding and just kept blowing holes in things.

 

[TyphoonFiST]

Thanks, it was fun to design and do the nerd work.

I'll put up a build sheet at some point, after I can get the car up on some ramps and get some pics. I still need to go back and have one joint cut and v-banded, I missed the lack of a vband joint where the pipe goes over the rear axle when I was inspecting the welds

After I get my right arm out of a cast I'm going to take some welding classes at a community college, I want to learn stainless steel and aluminum. Last time I tried welding I did some stick welding and just kept blowing holes in things.

Just curious...What monetary wise have you got into this setup?