OT in another thread made me think about making a thread on this subject. Probably 99% of us know how drivers and amps work, but for those that are not clear on the subject I want to write a simple answer:
DC, modulated or not, will only move the cone into one direction. This doesn't make any sense, except for testing the polarity of drivers. Hook up a small battery to a driver and it will move either in out out. Lets say the way you connected it, it moves out. if you reverse the poles of the battery, it will move in. If you do this 50 times a second, you have a 50Hz "hum". To this 10000 times per second and you have a high beep tone. You don't want DC near your headphones! Measure DC at the amps outputs and it should be only +-20mV or less, up to +-120mV on bigger speaker amps.
Music, as such a tone, which is an oscillation, moves the microphone diaphram. This is converted into current, + everythime the mics cone moves in from its resting point and - every time it goes outwards from its resting point. So, a 50 Hz vibration produces 50 Hz alternating current. Music is just many 10000s of various AC moving your headphones cones.
The amplifier makes the AC signal "stronger" thus moving the driver "louder".
In essence this is why "directional" speaker wires are not actually directional. (EDIT: For clarification, the "system" positive and negative leads are "directional" in a sense. If you have to loudspeakers and wire one red to black and black to red by mistake then the sound waves that are the same will cancel each other out because one speaker is "out of phase". This is how noise cancelling works, it takes the microphone signal on the outside of the cup, inverts it 180° and plays it on the driver, effectively canceling out the sound from the outside. It is still AC on the red wire that is moving the driver, meaning the "power" moves backwards and forwards through the positive lead)
Below is a animated graphic on that page in the link that shows how a driver works.
Edited by ev13wt - 2/1/13 at 3:03am