Drone in an exhaust system has the same operational mechanism as a pipe organ tube.
The tubes of the exhaust system can resonate just like a pipe organ. The problem is there is only one note (or set of notes) that the exhaust can play. And this is why drone BOOMs at some RPM's (frequencies).
When the engine exhaust pulse frequency matches the exhausts natural pipe organ frequency the engine and exhaust sing together making one loud & penetrating note. DRONE.
The exhaust system must be designed to notch out that drone frequency. And that is why we use the side branch resonators on the axleback section of the Shockwave))).
The side branch resonator is tuned to 1.2X the natural frequency of the pipe organ. 1.2 insures the side branch will always be out of phase with the natural frequency and thus they cancel each other out.
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