Dutch said:
You hear drone in a car when the sound coming out both pipes resonates harmonically and creates an abnormally loud sound. If you think back to high school physics you may remember the experiments where you move back and forth between two identical sound sources and the sound will get louder and softer as you move. In a car with dual exhaust, you will hear a similar warble as you accellerate. It will get louder and softer as it goes up through the RPM range. If you happen to be cruising with the engine in the right RPM range, you will be stuck listening to the loud droning noise.
Solutions include changing speed, changing engine speed by shifting gears, having single exhaust, or the magical approach used by car manufacturers: the mufflers are slightly different left-to-right so the sound sources are not identical and will not resonate.
Not necessarily the case ...
Drone or resonance is a function of harmonics created directly by the firing pulse and how it is expelled and how these sound pressure waves are either altered by mass and lack of dampning or how they are reflected through or on other adjascent surfaces. Finding the resonant frequency of a particular motor or vehicle is a matter of considering motor dynamics ... compression / firing order / volume of exhaust gas expelled / manifold design. Then Combine that with the dynamics of the silencer system ... exhaust system mass / hanger locations / routing / dampening method / tubing diameter. Together you will have introduced some 30-40 variables. NOT an easy computation.
In the case of the MDS 5.7L Hemi, Damlier-Chrysler went through some 200 computer generated model iterations before they came actaully building a sample prototype. The LX exhaust system was engineered using just about ALL the technologies available to minimize noise. Careful examination of the OEM system reveals the use of a crossover/balance tube, packed/perforation core based silencer, chambered silencer, helmholtz resonator and usual pipe/tube size alterations to control heat and gas velocity. The MDS component proves to be a large challenge as it introduces a 4 cylinder (V4 pattern) leading to very long pulse widths and dramatic changes in the pressure of the exhaust system between pulses. You have heard the distinct exhasut note of a boxer type motor (porshce/subaru.) Fortunately, you are only trying to control this exhaust pulse under low throttle position. All in all one can see where the OEM exhaust systems gets its nearly $900 price tag.
But back to resonance ...
Changing speed or shift gears is nto really a solution as it doesnt change the problem but only avoids a symptom.
Having a single or dual exhaust will NOT make a system any less resonant by design alone.
If simply changing a system's left and right banks would reduce drone/resonance why is the stock system entirely symetrical? Symetry alone cannot control resonance ... remember that exhaust gas will travel the path of least resistance (resistance in terms of pressure/heat)
One of the single largest overlooked part of controlling resonance is understanding how mass and system suspension effect harmonics. Look at your exhaust system as a large tuning fork ... this opens a whole new discussion. Just as a brief example: In designing our GTO system we PURPOSELY ommited OEM hangers in 3 locations utiliznig only 3 of 6 hangers due to the change in mass from the OEM to our system. Allowing the system the same degree of movement after having lightened the system kept the sytem from being overly rigid which woudl in turn INCREASE its ability to transmit harmonics through adjascent componets ...
Just something to think about ... BTW this is what happens @ 4am when you can't sleep due the FLU!!!
uch:
