So most aftermarket exhausts have a lot of drone, the OEM exhaust, and some aftermarket exhausts like the EVS tuning one avoid it. They correctly use mufflers and resonance chambers to cancel the drone at the 3,000-4,200 rpm range. That means that these exhausts have a droning frequency in the 50-75hz range.

3,000/60 = 50hz

4,200/60 = 70hz

The resonator formula is
fr = .25 C D/Sqrt(Pi V (L+.75 D)).

fr is the resonant frequency in Hz.
C is the speed of sound in meters/sec. All other quantities use mm.
pi = 3.1415962
D = hole Diameter, L= hole length
V= cavity volume

Also note: The resonant frequency is lowered by making the cavity volume larger and raised by making the hole larger.

So the combination of both the muffler resonance pipe and the quarter wave tube both move the drone into a range that both pipes work effectively at canceling out. This is why having just a pipe welded off the side of the exhaust doesn't totally eliminate the drone, you need another resonator to move the drone into the range the long pipe will cancel out the most drone.

If you only have the long pipe welded off the side, you move the resonant frequency from 50 to ~45, so you need another quarter wave tube to raise the resonant frequency back to 50hz. Hence the "raised by making the hole larger" , and "lowered by making the cavity volume larger"

 

The "3000/60" is basically a unit conversion? RPM to RPS?

What is the quarter wave tube you are referring to?

 

Yep hz is frequency per second.

Quarter wave tube is the side chamber at the beginning of the oem exhaust. The hemholtz chamber is on both mufflers.

 

So the drone is associated with the engine's exhaust pulses within that range and is something we cannot (effectively) tune out so the next best solution is to tune out the drone and we do that by trying to match these parameters so that the frequencies cancel each other out. One resonates at 50hz while the other resonates at an -50hz. Does placement of the either chamber matter?

The Tanabe Medallion Touring which is reportedly one of the quietest and drone-free exhausts out seems to just use three straight-through mufflers with no quarter wave tube. Theories on this?

 

Milltek has a good rep
https://www.s2ki.com/forums/uk-irela...mance-1157525/

 

Whoa, I thought production on those had stopped!

But another example of an exhaust without the frequency tuning (or at least not obvious anyways) that has a good reputation of ko droning.

The Fujitsubo Legalis is also another one.

Is it maybe that third muffler that is the trick maybe?

 

I have a milltek dual. It's great, WAY better than my Greddy single, but it still has a lot of drone, even with stock cat. I wear earplugs on drives > 1 hour.