Deozaan, they are the muffler on a car. but they are not stock mufflers. Glass packs are usually a little louder that stock. If done correctly they will have that sweet rumble sound and cackle when you back off the throttle.
What SeraphimLabs is talking about is your 4 and 6 cylinders sound like a buzz or sometime a sewing machine.
And then you have those, usually pickups that are just LOUD and sound horrible.
Sorry for the hijack