I googled and came up with nothing, I found some interesting articles on noise cancellation though.

What causes exhaust drone? I've read random bits from several car forums and have come up with:

1. leaks in the exhaust
2. if any part of the exhaust system is physically touching the bottom of the car

what do you guys think?

 

Muffler drone often is a result of the design of the baffles within the muffler, and the resulting harmonics rather than the size of pipes or whether one has an x or h or y or o pipe or headers. The sound pulses (waves) from each cylinder's exhaust valve burst have a certain frequency that increases as the RPMs increase. At a certain RPM, the muffler enhances these frequencies much like a guitar case, shape, and opening will enhance the sound of the strings' frequencies. This phenomenon is called resonance, and can be annoying when using certain muffler systems. Many popular two and three chambered mufflers have some sort of resonance frequency, although many aren't noticeable because the frequency is either canceled by other baffles within the muffler or is at too low or high of an RPM to be obvious.

 

lets say an exhaust is droning that was designed not to. what problems can be causing it?

 

Neither.
1) makes simple noise.
2) makes chassi vibration

Drone is a high amplitude sinusoidal oscillation of the exhaust gases in the exhaust pipe.

You can think of the exhaust system as working like a single note pipe organ. When it hits about ~125 Hz, it rings out loud and causes what is known as drone.

Drone is like driving around with a HIGH power, single bass note pipe organ hooked up to the *** end of your car.

 

the design of the exhuast is the problem. nothing u can change about that besides redesigning it with resonators or x pipes or something else to change the harmonics.

 

I agree with all the above. If you are having trouble with a drone/resonant frequency, all you can do to quite it down is to eliminate the resonant spot.

Everything in the world has a natural resonant frequency as does your whole car and independant parts. Since the exhaust system is nothing more than a long pipe, it will have it's own natural resonant spot. Thus you add a divice in to cancel or re-range the resonant point. The small muffler in the middle is called just that and it added for that reason. If your exhaust exits under the car, the car will act like an amplifier, thus we move the exit point just far enough away that it doesn't happen.