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Freebyrd pretty much on it. Pipe and pipe design has more to do with it than motor. Motor difference not much except DOHC will rev higher than SOHC. Pipes change noise and sound depending on how big or long pipe is, and whether cylinders joined in pipe. Joining two or more cylinders actually has a quieting effect because the exhaust pulses then must fight each other, the interference kills the wave action. That's why 4 into1 with no megaphone on it can be quieter than 4 separate pipes. Also why putting a crossover pipe between both sides of dual exhaust of car can quiet it down some. Want proof? Put your hand at back of straight or header pipe, the straight will shoot out puffs that will actually move your hand. Header will blow against hand but more like lots of flow, not individual pulses. I had friend in the '60s with Honda 305 with straight high scrambler pipes, sometimes when he would turn inside me while we were catting about, I could feel those exhaust pulses hit me in face as far back as several feet behind him. Same principle behind rifle barrel, long straight length concentrates power of pulse going down it to make more powerful. Now after that, sound wave tuning can increase noise, say megaphone used on 4 into 1. Once you start wave tuning, noise will flat increase. That's evidence of it working in right direction. Motor may contribute a bit then as DOHC in a slightly higher state of tune than SOHC and responds to wave tuning better. One only has to listen to motor note change that occurs once you hit around 5500 rpm plus with a good working pipe to know that, engine gets noticeably louder then as it enters bottom of main power range. Header sound wave tuning beginning to work well with rest of engine...................