What gives motorcycle motors their sounds?
 
The short answer to this is the internal combustion engine firing.
 
Fuel jetted into the engine explodes creating noise. The spent fuel and some of the sound is then pumped out through the exhaust.
 
Some of the sound is lost to the engine. That metal casing and the piston absorb some of the noise. We’ll discuss that more in a moment.
 
http://auto.howstuffworks.com/question325.htm
 
How Stuff Works details how the Harley Davidson gets that characteristic sound in this article. That sound is created by an engine running on a crankshaft with one pin. Instead of the motor firing every time the shaft spins 360 degrees, the Harley motor fires first at 315 degrees and then rotates 405 degrees to fire. Then 315 degrees to 415 and so on.
 
http://www.straightdope.com/columns/read/2352/did-harley-davidson-patent-the-sound-of-its-motorcycles
 
Harley and Honda famously came to legal blows over the issue of motorcycle motor sounds. Harley finally threw in the towel. The sound of the controlled explosion inside the engine, as Honda argued, is pretty much the same.
 
But there’s a lot that can be done with that sound. Pipes can greatly affect it.
 
To start with, there are baffles. You can read plenty of Internet forums on baffles and get plenty of opinions on how baffles affect motorcycle performance. To be sure, they create more back pressure in the engine than running without baffles. They certainly greatly affect motor noise. Baffles function exactly like a muffler on a cage.
 
http://www.ehow.com/how-does_5044307_exhaust-baffles-work.html
 
eHow has a good article on acoustics and baffles in bikes. Richard Rowe writes that baffles create competing sound waves that serve to dampen the noise. True, but baffles also physically absorb sounds.
 
Noise is created by vibrating air molecules. When the air molecules bounce into something solid, this changes their vibration. Part of the vibration is imparted into the physical item (baffles) which gradually dampens the vibration. The air molecules also lose vibration naturally. Physics calls this entropy.
 
Keep that in mind.
 
Pull the baffles and the bouncing air molecules have less interference. That means more sound escapes through the pipes.
 
Twin pipes are going to deliver a different sound than a single exhaust. It’s that principal of vibration and competing sound waves.
 
A dual system with a pipe running down each side of the ride is going to be a tad different than one with both come tubes on one side. This one might be harder to notice than some.
 
Now think about musical instruments. Brass instruments have their pipes bent and curved in a certain way to produce certain sounds. Exhaust pipes can be bent to provide different sounds on the same bike. Custom pipes produce different sounds than stock exhaust.
 
Look at a trumpet. This brass instrument has an adjustable pipe. Make it longer or shorter to get different noises. Same thing with bike pipes.
 
Let’s go back to the vibration and solid material again. The longer a sound wave has to travel, the more vibration the air molecules are going to lose. This is why shorter pipes are louder than long pipes.