sentences:  The pitch of the blades change according to the signal the amplifier supplies, producing a modulated sound wave due to the air moved by the spinning blades.If there is no signal applied, the blades simply rotate "flat" at zero pitch, producing no sound.Since the audio amplifier only changes the pitch of the blades, it takes much less power to drive a rotary woofer, although a secondary power source is required to drive the fan motor.