Yeah, this information is a bunch skewed. It's
not exactly that the two objects are vibrating and causing the drone; technically, yes, all sound is vibration, but
vibration on exhaust (primarily) is a result of suspension and flex.. This is why BMW uses mass dampers on certain OEM systems- it stops the natural harmonic frequencies that are a result of the exhaust gas passing through the entire system. But, to address the finer points on why many (but definitely not all) aftermarket exhaust systems tend to have 'drone':
First, lets nail down what "drone" is- drone is a frequency that is felt as well as heard, and for the average human, it is most significant between 94hz-125hz. Some people may be more or less sensitive, but that's just human dynamics. As discussed before, sound is the subject byproduct we are discussing, and in that frequency range, it creates a pitch that can be felt. Easy parallel - big speakers at a concert. When you're far away in the back and its loud as hell, thats because the mid and upper range speakers are hammering away. Higher frequencies tend to cut through and over deeper sounds, but travel a shorter distance. But, as you get closer to the big woofers pounding away up front, you start to not hear as much, but instead, FEEL the bass.
Drone is the magic range at which you can both hear AND feel the exhaust sound.
Factory systems do loads to prevent you from hearing this. I can't even begin to cover all of that, but most notable examples include mass dampers, valves, and specially chambered devices, which we'll get to.
Sound travels as a wave. Exhaust gas pulses travel like waves through the tubes.
It's about to get tough right here: The sound that creates drone, that pitch 94hz-125hz, these are long waves. You know how waves have wavelengths, and all that jazz? These pitches are long, basically from 9ft to 12ft long. As the pulses accumulate and travel down the exhaust (lets use a straight tube as a reference), the sound created will be a constant drone at that range. Bends and resonators help by breaking up the pathway of that wave, and absorbing the actual volume of the wave... but the low frequency tone is still there.. which is how you can have quiet exhaust that still drones. That sucks. Also, that's sometimes why louder exhaust doesn't seem to drone; you let the high frequency waves cut above it... like back at the concert reference we made before.
The best way to cut down the drone is via destructive interference. What you need to do, is reflect the trouble-causing wave back at itself, 180* out of phase.
This is like two equal people pushing an object in opposite directions- nothing happens!
However, that effectively means you need a chamber that can reflect the wave back in a straight line for 4.5 - 6ft. First up, thats difficult under our cars, to have dead tube space like that. One of the easy ways to do it would be to have a muffler or resonator that had a dead stop in a straight line down the center of the car, but the world will assume it can't flow well enough to work. This is the balance between power/flow and sound/comfort. It's always a balance.
Take APR for example, they use RSC technology. They are licensing this from Corsa, which is awesome, because Corsa has a patent on it. Because it works pretty well. What they do, is create that long length of tubing INSIDE a smaller canister- tough to manufacture as well- but it creates that destructive interference. If you've been in a car with a proper RSC system, you know what this is like- you hear the tone that should cause drone, but it doesnt. You don't feel it.
There are other ways to kick that drone though. Take for example, that listed Stratmosphere exhaust- there's a mini magic trick happening in the center resonator, which works awesome on a B7 A4 because of the long straight length of tube created by the DP/center section. Where the exhaust gas splits to go to duals, its in a perforated "Y" piece.. so the waves can travel through it, hit the back of the resonator, and travel backwards (as a wave, not as exhaust gas pressure) oh, I dunno, from about 4.5-6ft right back towards the source!
So basically, there are ways to cut down drone that you can't see. There are loads of ways to deal with it.
With the mustang, using different size mufflers is just another trick. But it's not always the answer. I know older mustang guys used to claim that having two 26.5" long tubes welded at 90* off a section before the muffler would also eliminate the drone- all sorts of ways.
What further makes things interesting, is how different cylinder configurations create drone at different ranges, but all the same (to an extent).. You can calculate out where 4, 6, 8, 10, 12, 16 cylinder configs will drone. I am trying to remember without digging through documents (As this is all off the top of my head), but 4bangers create the 94-125hz pitches between 3500 and 4500rpm. Add a turbo to the mix, and its more difficult to sit at that RPM range while cruising. So often, you can have a system that is minimal, large diameter, and flows well, without having intrusive drone.
What sucks, is 6 cylinders. They create the 94-125hz tone between roughly 2450 and 3250rpm. Riiiight around the range you'll likely be cruising at on the highway. This also primarily explains why BMW has some of the fanciest tricks for exhaust stuff from the factory, what with a plethora of 6 cylinders along the lineage. It's also why the B8 S4 has light drone BONE STOCK!
8 cylinder motors are easier, as the pitches occur low, like 1850-2450rpm, which in say, the 4.2L, is barely moving along.
No, swapping various Brands mufflers will not likely be the answer. And if it was, it'd be a total fluke. A more interesting scientific test would be to make sure an exhaust system could handle full flow out of only ONE side of a dual exit exhaust.. and then cap off the other side... that'd create your long reflecting chamber..
Anywho- the point is, it is all about waves, and how to bounce them around. You might not see how it's done. And yes, true, not every aftermarket company bothers- but the ones that do, typically are worth the cash.
Open to clarify or draw pictures to explain further.
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Originally Posted by StratJohn
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