rotary woofer

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.

• Illustration — A homemade rotary subwoofer with 3D-printed fan blades attached to a remote-control helicopter swashplate The rotary subwoofer design was first patented on December 1, 1942, by R.C. Sanders, Jr., under U.S. Patent Number 2,304,022. In the early 1970s, researchers noted that while humans could detect frequencies below 20 Hz, the ear was much less sensitive to these frequencies. As a result, increased sound pressure levels are required to perceive these sounds. These frequencies are often not audible but still subliminally detected by humans (see: Infrasound). 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. 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. The pitch of the blades changes 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. 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.