Rotary woofers are a type of loudspeaker that uses the coil’s movement to modify the pitch of a set of fan blades. Rotary woofers are capable of creating sounds below the normal level of human hearing, which is 20Hz. By compressing the air in a closed room, these woofers are able to produce frequencies below 0Hz. The rotary woofer displaces air by using moving cones, which causes the reproduction of extremely low frequencies.
Rotary woofers are capable of producing frequencies at levels that can be perceived by the human ear. They achieve this by using a traditional cone and voice coil to generate these frequencies at sufficiently high levels. It’s worth mentioning that human beings can’t perceive sounds below 20Hz, but new technologies have it all figured out.
On rotary woofers, the voice coil’s motion changes the angle of a set rotation speed of fan blades. When this happens, sound pressure waves are created. The pitch of the blades changes according to the signal of the amplifier while producing a modulated sound wave due to the air moved by the spinning blades. When there is no apparent signal, the blades don’t produce any sound. Since the audio amp only changes the pitch of the blades, it needs much less power to drive a rotary woofer, but a secondary power source is required to drive the fan motor.
As mentioned before rotary woofers produce frequencies lower than 20Hz. When the input frequency exceeds the fan’s rotation rate, the distortion tends to increase. 20Hz would be equivalent to 1200rpm for the fan. Newer models use an AC induction motor spinning at 800rpm, which is equivalent to 13Hz. Within this setting, the rotary woofer is carefully installed so that the blades are assembled in a circular opening and the air can flow freely.