Actually you can't. the whole point of an exhaust is that it is a tuned pipe that creates a negative pressure just outside the exhaust valve of the engine to suck the burnt fuel vapours out. as such it is effectively a musical instrument. Also as the volume of gas is quite high it needs to get rid of it as quick as possible, any 'restriction' such as noise reduction slows down the gases and robs the engine of power as the piston now has to push the gases out. So loud 'usually'[1] equals power. Listen to an F1 car and you will get an idea
[1] Usually means a properly tuned pipe the full length of the exhaust, not just a dustbin shoved on the back of a 2" exhaust normally found on Novas, Imprezas, etc.