FOR EXAMPLE:
Base reference velocity is 1128 ft/sec @ 20 C or 68 F.
The Math:
Frequency (f) = Velocity of Sound (V) / WaveLength (Lw)
assume a working resonator temp of ~50 degrees C (122 F):
V is 1187 Ft/sec.
125Hz = 1187 / Lw, or, Lw = 1187 / 129 = 9.2 feet.
The resonator is a
quarter-wave resonator, so that its length must be 9.2
/4 = 2.3 feet or 27.6 inches.
Here is a few before/after idle and 2300RPM wave profiles. Notice the low frequency peak? This is drone. The branch resonator will reduce significantly a narrow frequency range and ONLY that range. Cruise in 6th in peace. Last picture is a Porsche muffler delete(LOUD AS FUCK!) but without drone because of the use of branch resonators. It also looks like there is some x-pipe principle being used as well. So regardless of what you use, H-pipe, J-pipe, branch, whatever, this is nothing new. Hot rod muscle cars have been using this to avoid drone.
X-OST Maranello(an Italian Ferrari aftermarket) states on their website:
"X-OST Maranello hand assemble the finest aftermarket exhaust systems available today for your Ferrari. Fitting all current Ferrari models, X-OST Maranello also has applications for virtually all Ferraris' going back to the early 12 cylinder classics of the sixties and everything in between. Special attention is given to the elimination of drone which many other aftermarket mufflers suffer from." They certainly didn't tell their customers,
"there is no such thing as drone" and
"go buy a Prius", or
"get some earplugs",
"turn up your stereo" etc....
DINAN(
http://www.dinancars.com/webresource...92Exhaust3.gif) and AA(
http://cdn.nexternal.com/aaw/images/Large5.jpg) have developed exhaust systems for the E9x M3 and both implement some sort of closed end resonance absorber to avoid drone while increasing performance and sound!