Sound Deadening & Exhaust Droning

[CWI]

Originally Posted by SSE 4 2SS

This exhaust drone can be fixed pretty simply. As was mentioned before, check out the Helmholtz fix.

This "exhaust drone" is caused by sound waves traveling through the exhaust pipes and the larger or close proximity sheet metal panels picking up the vibration and amplifying it through the car body.

The fix is a Helmholtz chamber. This is basically a section of pipe welded on pretty much anywhere in the exhaust system, though for space considerations they are usually located right around the muffler.

The length of the chamber is critical.

Here is a nut shell is how they work.

You have sound pulses traveling through the exhaust. If you install a properly sized chamber, the sound waves are split with some of it traveling out of the exhaust as is normal, and some of it traveling up the helmholtz chamber. When the sound waves hit the capped end of the chamber, they bounce back, at the same frequency as those traveling through the exhaust pipe. When the waves from the chamber reenter the exhaust stream, the sounds waves colliding at the same frequency cancel each other out.... Thus, no drone... or substantially less....

The easiest way to determine the proper length is to get in your car, and find a fairly long stretch of highway... Accelerate up into the top gear.... or manually shift it up...

Hold it in that gear and slow to approximately 50 mph... You should be very low in the RPM range.... Accelerate in steps, maybe three to five mph per step... Then cruise in that step... Note the drone level.... again accelerate 3-5 mph... then cruise at that rpm level... do this until you have reached a level where the drone is the worst and continue on until little or no drone is noticed... Make a note of where the loudest drone was... then use the attached chart to determine the proper length for your chambers... My drone was worst at 72-78 mph and about 1800-1950 RPM....

The pic below is of a typical Helmholtz chamber.Again, the location of it does not matter, and it needs to be fairly close to the same diameter, at least not any smaller. The next item is the chart for determining the length needed based on the RPM of the worst levels of exhaust drone.

As for the Stereo related sound stuff... it's not in my area... Others can help you with that...

Hope some of this helps.

Is that chart reliable? I thought we had to know the actual frequency being emitted to determine the length for the j pipe.

[SSE 4 2SS]

Originally Posted by CWIweldace

Is that chart reliable? I thought we had to know the actual frequency being emitted to determine the length for the j pipe.

It would be best if you could use a meter to determine the frequency, but I know of a few members here that have used the chart and pretty much hit it....The nice thing is it is so easy to shorten or extend them a little if needed. Especially with your skills as a welder...

[LBV]

The stuff I used btw was Damplifier Pro.

[CWI]

Originally Posted by SSE 4 2SS

It would be best if you could use a meter to determine the frequency, but I know of a few members here that have used the chart and pretty much hit it....The nice thing is it is so easy to shorten or extend them a little if needed. Especially with your skills as a welder...

I have been told it is not that simple but i'm sure it may get some in the ballpark. The drone in mine is in more than one gear, speed or RPM. I plan on going to a sound shop and find the frequency and order accordingly. I only want to do one install rather than fishing for the right length.

[CWI]

Here is the response I got when contacting Flowmaster about drone. Thought I would share:

Hello Ron,

Thank you for contacting Flowmaster. The best way to reduce interior resonance is by swapping out the current exhaust setup. A pair of Super 44's on a late model V8 Camaro is not a quiet setup. The smoothest sounding kit that produces the least amount of interior resonance is our Force II system (part# 817506). Designed to increase exterior sound as you get on the throttle while maintaining a mellow interior resonance (drone) level. Hearing the interior drone level amplify at certain RPM is common especially when the vehicle is under load. Please keep in mind that any aftermarket performance exhaust system will always produce more interior drone but this kit will be dramatically less than what your currently experiencing. Another great feature about this system is that it's designed to hook up to your stock tubing without having to purchase any additional components or hardware. 100% American Made and fully constructed out of 409S stainless steel which is backed by our lifet ime warranty. Below I've attached a link that will direct you to further details regarding this kit. In case I've missed anything or if you have any further questions, please don't hesitate to contact me.

Thank you for contacting Flowmaster.
Alex Ortega
Inside Sales Representative
Flowmaster, Inc.
1 (800) 544-4761, ext. 444 Kinda sounds like a sales pitch but it does help me decide to go with J pipes.

[VR Baron]

I used 2 layers of durakool matting in the trunk and around town lessoned the drone with the Flowmaster outlaws alittle bit, freeway none that I could tell, but outlaws like a muffler delete that is tuned. Magnaflow resonated x pipe killed drone from 2000-2400 but not below that. My J pipes killed the rest from 1500-2000 rpm. In my case used length of 29-29.5" fot the J's. this Is for the LS3 only.

[Smkymts]

The members that suggested looking into the " helmholtz resonance" and the "J pipes" seem to understand what I am concerned with.

In my research, I have come to understand that Stereophiles create their own noise issues with the thrumming bass they have added bouncing off the flexable metal panels in the car and benifit from adding eavy material. The exhaust droning issue from adding power and eliminating exhaust baffling to increase exhaust flow creates a totally different noise issue.

The Helmholtz principal is key to addressing the exhaust droning issue only. It is the same principa used in noise cancelling headphones. While it does cancel out noise, it does allow human voices to be heard while wearing the headphones.

This is because the headphones are creating a resonance frequency that cancels out the frequency of the engine, squeak of machine noise but doesn't block the frequency the human voice creates. The "J pipes" will cancel out the exhaust drone by duplicating the frequency but will not cut out other noise frequencies. Since I am only concerned with exhaust drone for now, I believe this is my best solution and I will be looking into installing the appropriate J pipes for my car.

Your scale is one way to move forward, finding a way to exactly indentify the exhaust drone pitch and tone using a tone oscillator is another. Anyone have tips on where to find one or a company that can perform this examination?

If others have pictures of the J-pipes they have had installed, please add them and notes. Also add how you determined your J-pipe size.

Thanks so much for your input.

Others please continue to use this thread to add your input in to exhaust droning as well as adding sound deadening material.

[CWI]

Originally Posted by Smkymts

[

Your scale is one way to move forward, finding a way to exactly indentify the exhaust drone pitch and tone using a tone oscillator is another. Anyone have tips on where to find one or a company that can perform this examination?
If others have pictures of the J-pipes they have had installed, please add them and notes. Also add how you determined your J-pipe size.
.

I asked the same question and have got no answer about where to go to find the frequency of the drone. Lots of people can measure the dB part of it but it's not about dBs. I can measure dB on my phone but not frequency. No ideas where to get this measurement?

[myold88]

Originally Posted by CWIweldace

Solo j pipes are in my near future, as soon as I find out my frequency target. Most likely will order from Steve at Solo. I like that they are adjustable to fine tune them. I would make my own if I had a shop, pretty simlpe design and not hard to fab.

I bought my SS for turnpike/interstate travel. I didn't believe the amount of road noise.
Imo It souned like I was in a tin can at highway speeds.
I installed full raammat & dynamat material to the unibody even before I changed my exhaust on my L99.
I then installed the Solo Axle Back J-Pipes. No adjustments necessary after my install.
No AMF sound, no drone and now quiet as can be inside and deep rumble outside.
Easy as 1, 2, 3.