sentences:  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.The rotary woofer displaces far more air than is possible using moving cones, which makes very-low-frequency reproduction possible.Instead of using a moving electromagnet (voice coil) placed within the field of a stationary permanent magnet to drive a cone, like a conventional subwoofer, on a rotary woofer, the voice coil's motion is used to change the angle of a fixed rotation speed set of fan blades in order to generate sound pressure waves.