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.

Typical subwoofers using moving cones do not transmit energy very well to the air below 20 Hz, and thus their sound pressure level (SPL) falls off significantly below this frequency.

[3][4] To help people to perceive the very-low-frequency content available in recorded material, Bruce Thigpen of Eminent Technology experimented with new methods of producing the required SPL.

[6][7] As an analogy, the hub of the rotary woofer's fan is somewhat like a helicopter's swashplate which allows a stationary source of reciprocating motion—the voice coil of the subwoofer—to change the angle of the spinning set of blades.

Many DIY rotary woofers use swashplates of remote-control helicopters connected to modified conventional subwoofer drivers acting as linear actuators in order to modulate the pitch of the blades.

Without a baffle or enclosure, sound pressure from the rear of the unit being 180 degrees out of phase would almost completely cancel that from the front the end, resulting in very low output.