Generally, human beings can’t detect sounds below 20 Hz. This is why the stated frequency response of many products is from 20 to 20,000 Hz. Part of this impression comes from the challenges in using traditional cone and voice coil subwoofers to produce these frequencies at sufficiently high levels. New technologies such as the remarkable rotary subwoofer now make this possible. Therefore, human beings can, in fact, feel and be affected by these infrasonic frequencies.
What is a Rotary Woofer?
A rotary woofer is a woofer-style loudspeaker that uses a coil’s motion to change the pitch of a set of fan blades. It is not using the cone shape. Since the audio amplifier changes the pitch of the blades, they require much less power than a conventional subwoofer. They are also far superior at creating sounds well below 20 Hz, below the normal level of human hearing. They are also able to produce frequencies down to 0 Hz by compressing the air in a sealed room. 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.
Voice Coil Movement
Instead of using a moving electromagnet (voice coil) in a field of a stationary permanent magnet to drive a cone, like a conventional subwoofer, on a rotary woofer, the voice coil’s motion changes 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. It produces a modulated sound wave due to the air moved by the spinning blades.
Without a signal, 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. However, 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. It allows a stationary source of reciprocating motion—the voice coil of the subwoofer—to change the angle of the spinning set of blades.
Lower than 20Hz
A rotary woofer design enables it to produce only frequencies lower than 20 Hz. The distortion increases as the input frequency exceeds the fan’s rotation rate. 20 Hz would be about 1200 rpm for the fan. The need to avoid making higher-frequency noise (due to the sound of the spinning blades) limits this. Current models use an AC induction motor spinning at 800 RPM (13 Hz). The woofer is installed and carefully braced so that the blades lie in a circular opening.
The air can be moved between an external chamber, such as the attic of a house. If the rotary woofer’s location was in the main space instead, the generated sound pressure from the rear of the unit, being 180 degrees out of phase would almost completely cancel that from the front end. The results would not be fit for its intended application.