sentences:  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.A rotary woofer is designed to produce only frequencies lower than 20 Hz; distortion increases as the input frequency exceeds the fan's rotation rate (20 Hz would be about 1200 rpm for the fan), which is itself limited by the need to avoid making higher-frequency noise (due to the sound of the spinning blades).Current models use an AC induction motor spinning at 800 RPM (13 Hz).