Its not to hard to design, I took a course on wave physics. You just figure out the frequency you want to cancel out (which is the one responsible for the drone) and make a resonator to a size that will effectively cancel this wave out. Back in my Motocross days I looked at doing this to my own exhaust on my quad.
All the equations to determine the size and length of the chamber can be determined from the equations on the Wiki page
Helmholtz resonance - Wikipedia, the free encyclopedia