Rotary Woofer
Encyclopedia
The Rotary Woofer is a subwoofer
-style loudspeaker which reproduces very low frequency content by using a conventional speaker voice coil
's motion to change the pitch of a set of fan blades rotating at a constant speed. The pitch of the fan blades is controlled by the audio signal presented to the voice coil, and is able to swing both positive and negative, with respect to a zero pitch spinning blade position. Since the audio amplifier
only changes the pitch of the blades, it takes much less power, per dB of generated acoustic sound level, to drive a rotary woofer than to power a conventional subwoofer, which uses a moving electromagnet (voice coil) placed within the field of a stationary permanent magnet to drive a cone which then displaces air. Rotary woofers excel at producing sounds below 20 Hz, below the normal hearing range; when installed in the wall of a sealed room, they can produce audio frequencies down to zero Hz, a static pressure differential, by basically just compressing the air in the sealed room.
). 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 change 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.
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, 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 (13Hz). The woofer is installed and carefully braced so that the blades lie in a circular opening. This is so that air can be moved between an external chamber, such as the attic of a house, and the main listening space; if the rotary woofer were not installed in such a "baffle" and placed directly in the main space instead, the generated sound pressure level into the listening area would be much less.
This suggests that for sound to be perceived at 7–8 Hz, that the 7–8 Hz SPL would have to be 12.3 dB higher than the 92.0 dB SPL required at 15.5 Hz. This means that 7–8 Hz SPL would require at least 104 dB of SPL to be perceived. For 3 Hz to be perceived, the SPL would need to add another 12.3 dB, or reach 116 dB of SPL to be perceived.
, to provide low frequency extension down to less than a single Hertz, to emulate those waves created by the falls.
Subwoofer
A subwoofer is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below...
-style loudspeaker which reproduces very low frequency content by using a conventional speaker voice coil
Voice coil
A voice coil is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it...
's motion to change the pitch of a set of fan blades rotating at a constant speed. The pitch of the fan blades is controlled by the audio signal presented to the voice coil, and is able to swing both positive and negative, with respect to a zero pitch spinning blade position. Since the audio amplifier
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
only changes the pitch of the blades, it takes much less power, per dB of generated acoustic sound level, to drive a rotary woofer than to power a conventional subwoofer, which uses a moving electromagnet (voice coil) placed within the field of a stationary permanent magnet to drive a cone which then displaces air. Rotary woofers excel at producing sounds below 20 Hz, below the normal hearing range; when installed in the wall of a sealed room, they can produce audio frequencies down to zero Hz, a static pressure differential, by basically just compressing the air in the sealed room.
Description
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: InfrasoundInfrasound
Infrasound is sound that is lower in frequency than 20 Hz or cycles per second, the "normal" limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high...
). 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
Eminent Technology
thumb|200px|Eminent Technology is a company established in 1983 by Bruce Thigpen to create innovative products for the audio field. Their first product was an air bearing straight line tracking tonearm for phonograph playback, this was the first implementation of a captured air bearing for tonearm...
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
Subwoofer
A subwoofer is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below...
, 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 change 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
Swashplate (helicopter)
A swashplate is a device that translates input via the helicopter flight controls into motion of the main rotor blades. Because the main rotor blades are spinning, the swashplate is used to transmit three of the pilot's commands from the non-rotating fuselage to the rotating rotor hub and...
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, 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 (13Hz). The woofer is installed and carefully braced so that the blades lie in a circular opening. This is so that air can be moved between an external chamber, such as the attic of a house, and the main listening space; if the rotary woofer were not installed in such a "baffle" and placed directly in the main space instead, the generated sound pressure level into the listening area would be much less.
This suggests that for sound to be perceived at 7–8 Hz, that the 7–8 Hz SPL would have to be 12.3 dB higher than the 92.0 dB SPL required at 15.5 Hz. This means that 7–8 Hz SPL would require at least 104 dB of SPL to be perceived. For 3 Hz to be perceived, the SPL would need to add another 12.3 dB, or reach 116 dB of SPL to be perceived.
Installations
Six rotary woofers are installed as part of an immersive Niagara Falls attraction known as Niagara's Fury, located in the Table Rock HouseTable Rock Center
The Table Rock Welcome Centre is a retail and observation complex located in Niagara Falls, Ontario, Canada at the brink of the Canadian Horseshoe Falls, several hundred feet south of the former rock formation which bears its name...
, to provide low frequency extension down to less than a single Hertz, to emulate those waves created by the falls.