Acoustic Waves Generated by a Loudspeaker
A loudspeaker converts electrical energy to accoustic (sound) energy generated by an electromagnet in a magnetic field. The electrical current flowing through the coil oscillates between 20 and 20000 times per second (Hz). This causes the diaphragm—a paper cone attached to a metal frame called the basket—to vibrate, thus generating the acoustic waves.
 Wikipedia. "Loudspeaker." (Mar 28, 2012) en.wikipedia.org/wiki/Loudspeaker.