Fixing The Drone

[SilverSH0]

A couple weeks ago I installed some new "mufflers" (actually resonators) that were Aero AR25's. I heard them on several cars and they sounded the best I've heard of any exhaust on these cars. I always think V6's are a tough engine to make sound good so I don't really like a lot of the muffler options. Once I picked up the car with new mufflers from the exhaust shop it sounded great at idle and when revving. But when driving, the drone in the cab was bad enough that I was going to fix the drone or return to stock. It droned pretty bad from about 2200-2800 RPMs. That drone resulted in the graph below (taken at 2500 rpm) where you can see the bad drone spike and smaller spikes at various octaves of that drone.
http://i184.photobucket.com/albums/x201/PwrRngr/30_Jul_2016_09_15_49_zpsinlvtmmi.jpeg

So I went back to some of my old acoustics knowledge (acoustics Engineer) and decided to build a Helmholtz resonator. I debated between a design with a neck and chamber (more compact design) or simply a longer tube that's capped at the end. After getting out the tape measure I opted for the simple tube since that would mean I only needed to buy one size tubing (2") and not two different sizes.

I looked at pre-bent exhaust tubing but it didn't have the bend radius I wanted so I ended up making my own.
So I broke out the cutter, MIG welder, and started going to town. Now remember, I'm an Engineer and not a welder. I finished restoring an old Mustang 10 years ago and that's the last time I touched a welder. So no, these welds are not pretty and I would not call them good. They are adequate and ugly but functional. Since no one sees under the car, I don't really care. Here I have created the two bends that will be used right out of the stock exhaust pipe.
http://i184.photobucket.com/albums/x201/PwrRngr/0803161920_zpsjer1t0ho.jpg

Then the exhaust came out of the car, I cut some holes, did some more welding, and it all ended up like this.
http://i184.photobucket.com/albums/x201/PwrRngr/2592_zpszwzfaetg.jpeg

Then the exhaust went back into the car, I let it warm up, and took it for a test drive. End result is below. This is a complete night and day difference. It went from an exhaust that I was going to take off and return to stock and changed it to the exhaust tone and volume that I wanted. As expected, it drastically reduced the 129 Hz drone and it also created a small peak at 68.5 Hz. When cruising it gives the car a nice subtle growl (IMO, how it should sound stock) and once you get into the upper RPM's you definitely can hear it. If anyone has drone in their exhaust they want to remove I highly suggest having and exhaust shop do this or doing it yourself.
http://i184.photobucket.com/albums/x201/PwrRngr/Capture_2016-08-06-14-38-18_zpsclka6mkq.png

 

[Emulsifide]

This is amazing!!! I read through your original thread and wondered how you would end up solving the drone issue. Can we get an updated video that includes the standard outside of the car revving and then some cruising from the interior?

 

[SHOdded]

Being a Star Trek fan, I am all in favor of your chosen solution, SilverSHO! Nice work, and thank you for the documentation

 

[Pintony]

Unusual Remedy!

 

[Emulsifide]

Another set of questions. Does the radius of the bend affect the tone, or were you just interested in a tight radius for the sake of packaging?

Do you mind sharing the physical specs of the resonator?

Lastly, what kind of microphone are you using? I see your spectrum analyzer choice is Advanced Spectrum Analyzer PRO for Android.

 

[SilverSH0]

I'm uploading a couple videos and they should be ready in a few hours. You'll have to ignore the video and focus inn the audio as i simply held the camera and didn't pay attention to it as i drove.

Another set of questions. Does the radius of the bend affect the tone, or were you just interested in a tight radius for the sake of packaging?

Do you mind sharing the physical specs of the resonator?

Lastly, what kind of microphone are you using? I see your spectrum analyzer choice is Advanced Spectrum Analyzer PRO for Android.

I just used my phone (G5). I originally brought out my laptop, microphone, and ran real programs. Then i compared it to my phone and they were close enough For what i was doing. So to save time and effort i simply used my phone.

Bend radius only effects tone in terms of it's impact on overall length. The tighter the radius, the shorter the distract of the bend. Pipe diameter has minimal impact on tuning. The smaller the diameter the narrower the notch. I wanted it tight so it would fit and not hit the bottom of the trunk.

The math is pretty simply. The overall length is C/(4*f) where C is the speed of sound and f is frequency. Since i didn't have a thermocouple to stick in the exhaust, i assumed the exhaust temperature was 200 degrees at the back. All that resulted in the tube being 30" long (tuned too 125Hz trying to push the peak it would create down a little).

 

[SilverSH0]

For some reason Photobucket isn't uploading all my videos. So I have one that's a quick drive and another of a warm start and a few revs.
http://vid184.photobucket.com/albums/x201/PwrRngr/Mobile Uploads/20160807_085306_zpsiduk9vz5.mp4

 

[Emulsifide]

Amazing work!!!! Thank you for sharing all of the details!

 

[CZD3MY]

To the OP: I have basically the same setup as you but made by MRT, straight thru resonators. I have an obnoxiously loud drone at almost the same rpm. I was looking at ways of getting rid of it and I saw your Helmholtz solution. I had looked into those but didn't know how to figure out what frequency to aim for, never thought about a phone app. So I have one now, can you walk me through the process? I've tried using your numbers with the formula you posted but I don't get the length you have...I'm doing something wrong here. I'm no engineer, let alone knowing much of anything about acoustics. Any help would be appreciated! Thanks!

 

[SilverSH0]

To the OP: I have basically the same setup as you but made by MRT, straight thru resonators. I have an obnoxiously loud drone at almost the same rpm. I was looking at ways of getting rid of it and I saw your Helmholtz solution. I had looked into those but didn't know how to figure out what frequency to aim for, never thought about a phone app. So I have one now, can you walk me through the process? I've tried using your numbers with the formula you posted but I don't get the length you have...I'm doing something wrong here. I'm no engineer, let alone knowing much of anything about acoustics. Any help would be appreciated! Thanks!

The speed of sound in air at 200 degrees F is 1258 ft/s and frequency is 125 Hz. Plug that into the equation and you get 1258/(4*125) and you get 2.5 feet. Convert that 2.5 feet to inches and you get 30 inches. That's all there is to it in determining length.

 

[CZD3MY]

There's my problem, I had speed of sound at a much lower number. Thanks a bunch. I'll let you know how it goes.

 

[Harp96sho]

I was able to have this done to my 15 at a local metal fab shop. The drone was horrible with the MRT axle-back, but it is all but GONE now and the car is long trip driveable again. Many thanks to SilverSHO for the info, this is the best way, in my opinion, to get the car sounding right. It is a little tight on the passenger side but there's no rattle/contact with the spare tire well. Plenty of room on the driver's side. If anybody is local the place was Regal Metal Works in New Cumberland PA. Also if anybody is looking for a set of OE mufflers/tips hit me up

 

[SilverSH0]

I was able to have this done to my 15 at a local metal fab shop. The drone was horrible with the MRT axle-back, but it is all but GONE now and the car is long trip driveable again. Many thanks to SilverSHO for the info, this is the best way, in my opinion, to get the car sounding right. It is a little tight on the passenger side but there's no rattle/contact with the spare tire well. Plenty of room on the driver's side. If anybody is local the place was Regal Metal Works in New Cumberland PA. Also if anybody is looking for a set of OE mufflers/tips hit me up

View attachment 14295 View attachment 14296

Glad it worked out for you. I'm with you that it made the car driveable by removing the drone.

 

[Chuck Johnson]

Check out this guy on Youtube named Markell Mims. He recently put J pipes on his and said it helped the drone alot but when he floors it the car whistles.

 

[Harp96sho]

Dropped the exhaust to do my RDU fluid change today so I figured I'd share.

 

[thenewguy]

Nice man, so the shop knew what measurements to do and how long of a pipe they need to make or just made one and kinda stuck it there?

 

[Harp96sho]

He knew exactly what needed to be done. Even though I had figured it out before I went there he asked the frequency and rpm of the drone. He was familiar from other projects he had done.

 

[thenewguy]

Cool! Im not familiar with what needs to be done so how did you determine that?

 

[Harp96sho]

Cool! Im not familiar with what needs to be done so how did you determine that?

Go to the beginning of this thread, silversho explains it.

 

[thenewguy]

Go to the beginning of this thread, silversho explains it.

I saw that but his pics are not working. Hes mentioning graphs, what are they? How were they obtained? What was the process?