A rotary woofer is a

subwoofer A subwoofer (or sub) is a loudspeaker designed to reproduce low-pitched audio frequencies, known as bass and sub-bass, that are lower in frequency than those which can be (optimally) generated by a woofer. The typical frequency range that is ...

-style loudspeaker which reproduces very-low-frequency content by using a conventional speaker

voice coil A voice coil (consisting of a former, collar, and winding) 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. Th ...

's motion to change the pitch (angle) of the blades of an

impeller An impeller, or impellor, is a driven rotor used to increase the pressure and flow of a fluid. It is the opposite of a turbine, which extracts energy from, and reduces the pressure of, a flowing fluid. Strictly speaking, propellers are a sub-clas ...

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 blade position. Since the

audio amplifier An audio power amplifier (or power amp) electronic amplifier, amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup (music technology), pickup, to a level that is high enough for dr ...

only changes the pitch of the blades, it takes much less power for a given sound level to drive a rotary woofer than a conventional subwoofer, which uses a moving electromagnet (voice coil) placed within the field of a stationary

permanent magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, c ...

to move a diaphragm to displace air. 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.

Description

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 level Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...

s are required to perceive these sounds. These frequencies are often not audible but still subliminally detected by humans (see:

Infrasound Infrasound, sometimes referred to as low frequency sound or incorrectly subsonic (subsonic being a descriptor for "less than the speed of sound"), describes sound waves with a Audio frequency, frequency below the lower limit of human audibility ...

). Typical

s 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 is an American audio electronics company based in Florida, established in 1983 by Bruce Thigpen. The company manufactures tonearms and speakers mostly at its own facility in Tallahassee, Florida. History Their first product ...

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 A swashplate, also known as slant disk, is a mechanical engineering device used to translate the motion of a rotating shaft into reciprocating motion, or vice versa. The working principle is similar to crankshaft, Scotch yoke, or wobble, nutat ...

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.

Drawbacks

A rotary woofer is designed to produce only frequencies lower than 20 Hz; distortion increases as the input frequency approaches the frequency of blades passing a given point. Fan speed is limited to reduce the continuous noise generated by the fan blades. Current models typically spin at around 800 RPM (13 Hz). Another issue with rotary woofers is the space requirement. Where a conventional subwoofer needs a baffle or enclosure to prevent back waves from destructively interfering with the output, a rotary woofer also requires a baffle or enclosure. However, because the frequencies are so low, the enclosure or baffle must be very large in comparison. Without a baffle or enclosure, sound pressure from the rear of the unit being 180 degrees out of phase would almost completely cancel that from the front the end, resulting in very low output. An infinite baffle can be created by mounting the woofer and circular cutout into a window. Alternatively an attic, basement, or spare room can act as the enclosure.

Installations

Six rotary woofers were installed as part of an immersive

Niagara Falls Niagara Falls is a group of three waterfalls at the southern end of Niagara Gorge, spanning the Canada–United States border, border between the Provinces and territories of Canada, province of Ontario in Canada and the state of New York (s ...

attraction known as ''Niagara's Fury'', located in the Table Rock House, to provide low-frequency extension down to below 1hertz, to emulate the waves created by the falls. However, the rotary woofers were removed some time before September 2023.

References

Sources

Danish study on Infrasound
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Journal of Acoustic Society paper by Yeowart and Evans

UK government paper on very low frequency sound

External links