Rotary woofer

A rotary woofer is a subwoofer-style loudspeaker which reproduces very-low-frequency content by using a conventional speaker voice coil's motion to change the pitch (angle) of the blades of an impeller rotating at a constant speed. The pitch of the fan blades is controlled by the audio signal presented to the voice coil, and is able to swing both positive and negative, with respect to a zero-pitch blade position. Since the audio amplifier only changes the pitch of the blades, it takes much less power for a given sound level to drive a rotary woofer than a conventional subwoofer, which uses a moving electromagnet (voice coil) placed within the field of a stationary permanent magnet to move a diaphragm to displace air. Rotary woofers excel at producing sounds below 20 Hz, below the normal hearing range; when installed in a wall of a sealed room, they can produce arbitrarily low frequencies, down to a static pressure differential, by simply compressing or decompressing the air in the sealed room.