> > I'm sure the output from your amp to the speaker is a DC signal, and therefore
> > has direction.
> Sound is produced by having a surface vibrate back and forth, pushing and
> pulling air. The vibrating surface is moved by means of an electromagnet, driven
> by the voltage from the wire. The "relaxed" position of the surface is
> equivalent to zero voltage, if you want sound you have to make the surface move
> "above" and "below" the zero position, which means positive and negative
> voltages. Et voila', AC signal.