I like many Corvette owners have experienced the dreaded exhaust "drone" and have been struggling to eliminate it. I have read through the hundreds of posts about the subject, talked with exhaust manufacturers, and a few close friends about the subject but have found mostly myth and superstition about why this phenomenon occurs. So, I would like to offer an explanation and perhaps a definition of what, exactly "drone" is and what can be done about it.

Let me start by giving you my background... Years ago I spent a great deal of time designing and constructing audio systems for IASCA show cars. I have an engineering and musical background and currently work for a precision measurement company.

Many people have described drone in different ways. Some associate the term simply with volume and others with increased volume at a certain opearting (RPM) range. Manufacturers often refer to it as "resonance". For my purposes, I define drone as the disproportionate increase/level of volume at a specific engine operating load... here's some explanation...

When I used to build sound competition vehicles, it was a commonly known fact that each speaker enclosure, especially the ported or bandpass variety, had a range of frequencies that it would be most efficient. A properly tuned speaker enclosure dramatically increased (amplified) the amount of volume for a specific range of frequencies. In addition, the shape and size of the car in which the speaker/enclusure operate (the listening environment) will also have a range or collection of ranges in which it is most efficient. The net effect of the speaker/enclosure/listening environment is similar to that which can be observed in a lake; when waves meet, they either cancel eachother out (opposites do this) or reinforce each other (similar waves do this). If reinforcement occurs, the result is a much bigger (more powerful) wave. Sound Pressure Level (SPL) vehicles, for example, compete to make the most powerful wave and rely heavily on wave reinforcement to make this happen. As another example, take a soda bottle and blow over the top. You have to blow on it just right to make reinforcement occur and the bottle "drone"

So back to our Corvettes. If you think of our engine as the speaker, the exhaust as the tuned enclosure, and the interior of the car as the listening environment, you can start to figure out how to address this issue. The interior of our cars provide a reinforcment potential for a specific range of low frequencies. This is inherent in the Corvette model design and does not vary from vehicle to vehicle. Because the interior of a Coupe is different from that of a vert or a FRC, each model will exhibit different reinforcing capabilities. Of the three, you will hear the coupe owners complaining about drone because the coupe offers the most potent reinforcing environment (with the removable top in).

So how to combat drone? Five choices:

1. Tune the exhaust to minimize or not hit that specific range of undesirable frequencies (e.g. Corsa)

2. Make the exhaust system quiet as a whole so it doesn't provide a powerful enough note at the specific range of frequencies (e.g. Stock exhaust)

3. Sheild/dampen the vehicle from incoming exhaust noise (insulation, dynamat, etc.)

4. Modify the listening environment (roll the windows down, take the top off, etc.)

5. Ignore it or shift to get out of the engine load (rpm) at which the drone occurs

The above choices have all been performed by forum members and have been proven effective in combating drone. Which route you choose is highly a matter of personal preference. some poeple like drone and don't eliminate it at all.

I've typed enough... now your turn!

 

Thank you for explaining in terms we, the non-engineer types, can understand. Without turning this into a "my exhaust is better than your's" thread, I have experienced the dreaded drone with a previous C4 with muffler eliminators. I solved the problem with the "no drone" cat-back system on that C4 and our current C5. Now, if they made a system for our `56 Chevy station wagon - talk about a "listening environment"!!

 

Great post, seems to be right on the money. The reason this topic keeps coming up is because owners:
1. Don't want to pay the $$$ for a well designed exhaust like Corsa (myself included).
2. Don't want the restriction and loss of hp associated with a high backpressure (quiet) exhaust.
3. Don't want to insulate/mat the interior to solve a problem GM "should have anticipated".
4. Don't want the extra noise from having windows open or top off.
5. Don't want to lose fuel economy by shifting to a different gear.

I agree with all of your proposed solutions...just offering a counterpoint from what I have heard from various threads.

As a side note I have quite a drone with my Borla Stingers, LG street LT's (no cats) and some rear compartment insulation. I'm looking into changing out my mufflers in an attempt to reduce resonation...

 

I think my exhaust system is fits the desciption for zero drone

 

I'm sure it does, if you're deaf that is...

 

I gotta admit it is rediculously loud especially with this monster cam, but it sure does sound intimidating.

 

With 3.90's or 4.10's out back, I don't experience drone either, just a lot of noise..music to my ears though.

As for a cheap and simple help, install lock washers between the rear muffler hangers and the frame.

 

One way I've heard to combat drone is to clamp or weld slugs of metal to the exhaust to cause the exhaust to resonate at a different frequency.

I've never tried it so I can't say if it works or not.

 

I think part of the problem is the path that the exhaust gases must take. My 68's exhaust followed a straighter path and never had any drone issues, despite having LT headers and Dynomax Super-Turbo mufflers, along with 2.5-inch pipes.

 

See I am deaf 80 %..a lot of deaf may still have some kind of hearing, believe it or not, of course I can still pick very low frequencies.Now there's a curse. What did help was dynamat and using one of Jim Vose partitions (Southerncarparts).. all of a sudden I could turn my hearing aids up and found out I had a stereo in my car I was telling Jim that at the beginning it was 109 db behind my head. This weekend I'm going to setup the meters just to see how much it has dropped. Sometimes I felt like a mac truck driver having to make sure I hit the right RPM on the tach so I could stay out of that drone range...call me old fashion but I just like to shift by feel not drone...

Knowing how some people can take comments on forums This in no way should be taken as disrespectful to anything concerning deafness please, just an example . The drone is what can be disrespectful

Hoping not OOffend

Thanks
Jarad

 

Great post! I hated the drone of various catbacks that I tried. As stated, the worst was when I had to have the top up and windows closed. My solution was to insulate and go back to stock mufflers with cutouts. I leave them open all the time the top is down(not as loud as you'd think with 4 cats.), and close them when the top is up. Works perfect for me.

 

GHL exhaust..
Period.
No drone.
No resonance.
Period.
GHL.
Get it.
Smile.
Period.

 

I like it. Music to my ears, that's why I got B&B Bullets. Once I do cam and LTs it's going to be obnoxious, which is exactly what I want.

 

the word "period" and "smile" are rarely used in the same paragraph. Married guys know what I am talking about.

 

corsa cat backs w/dyntech supermaxx system....no drone...but my wallet is much thinner....but my ears are happy!!

 

I've tried several setups to finally settle on a combo that is actually a "tunable" system - Headers, hi-flow cats, x-pipe, electric cutouts and Z06 Ti's. Took 3 tries to get there. First I swapped the stock catback with MagnaFlows with the MF x-pipe. No resonance/drone. Very sweet but a bit too mellow. Then, I installed FLP headers/x-pipe. Sounded great. No resonance/drone. Did heads/cam. Still a bit mellow. Swapped the MF's for a set of dual-tip Borla Stingers. Excellent race car note -like a banshee wail at full moon. All my friends were jealous. But the drone was horrendous . . . I couldn't drive the car for more than 45 minutes without getting nauseous.

Funny - my wife was not affected by them at all and, in fact, was upset when I removed them from the car. People are affected differently by the same frequency/power combination.

Anyway, I bought a set of Z06 Ti's, installed them and sold the Stingers (Tearfully, I tell you. They did sound awesome.)

Then I decided to try a set of cutouts. Installed them and have been happy ever since. Cutouts closed, the car is somewhat agressive, yet controlled with no drone/resonance, and I can drive for hours with the stereo easily heard and without "sound fatigue". (I did insulate the entire interior during the Stinger episode.) Low volume conversation is even easily accomplished with the windows and top in place. With the cutouts open, very aggressive, very mean-sounding. As good, if not better, than with the Stingers. Dogs run and children cry when they hear the open cutouts. But very little to no resonance, depending on the valve-opening setting. The "tuned enclosure" is significantly different than with the Stingers but the "speaker" and "listening environment" have not been altered since.

For me, a nice compromise.

 

Corsas are where it's at...

long tubes, no cats, x-pipe...loud at WOT, no drone, and stock quiet at highway cruise speeds.

Hey Duckvett...Nice car!!

 

Interesting. I had all kinds of drone with my GHLs. But wow, I love the sound of that exhaust.

 

I discovered the "roll down your window" solution early on too. Sucks in the winter time, but it works. I discovered it by accident. While driving with my wife, I noticed that she complained less about the drone when my window was rolled down. I tested my theory a few times to confirm - we'd be talking fine until I rolled up the window, at which point she'd start making comments about how loud the car was.

 

Had quite a drone with the Borla stingers and stock exhaust manifolds/cats. Installation of supercharger, American Racing 1 7/8 headers, and high flo cats seems to have eliminated it.