i believe the word your looking for here is super impose .. if memory serves me correctly
super imposition is caused by two sound waves, when they meet they either cancel eachother out or add to eachother to make a greater sound. I'd say its got more to do with resonance than super imposinig.
Its all about harmonics, the resonant frequency depends on the length of the pipe. i have the formulars around here somewhere, if the exhaust gas is shooting through the exhaust pipe at a certain frequency or wave length if you like, and the pipe is the right length you would set up a standing (sound) wave (refered to as a harmonic) this causes resonance, and resonance is what you dont want, resonance is waht amplifies the sound wave through the pipe, hence drone. resonance would occur at different rpm's deppending on exhaust, but a jap exhaust would probably be at tuned lengths to try and eliminate this i would think
if your budget allows try to get a good quality one, the cheap ones are thin walled and tend to sound a bit tinny. Im in the market for an exhaust and i dont think i can go past the trust pe2, ive heard it first hand and it sounds so nice especialy when you give it some