Rotary woofer

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The Rotary Woofer is a subwoofer-style loudspeaker which reproduces very low frequency content by using the voice coil's motion to change the angle of a constantly rotating set of fan blades which generate sound waves. Since the audio amplifier only changes the pitch of the blades, it takes much less power 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. Rotary woofers can reproduce audio frequencies from near zero Hz to 20 Hz, much of which is outside the normal hearing range.

1 Description
2 See also
3 Citations
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[edit] 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: Infrasound). 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 constantly rotating set of fan blades in order to generate sound 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 they require a secondary power source to spin the fan.

As an analogy, the hub of the rotary woofer's fan is somewhat like a helicopter's swashplate (or more accurately, a fenestron) 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; the input signal cannot exceed the fan's speed in number of revolutions per second, or else distortion will be introduced. Current models use an AC induction motor spinning at 800 RPM. 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 sound pressure level would be much less.

This paper presents new binaural hearing threshold data obtained (a) by an earphone method over the frequency range 5–100 Hz and (b) by a whole body chamber method over the range 2–20 Hz. The results obtained are in excellent agreement with recent reported data. The binaural to monaural listening advantage appears to remain at 3 dB throughout the frequency range. A good approximation to the binaural threshold of hearing may be formed by lines from the point 92.0 dB SPL at 15.5 Hz with slopes of 12.3 dB/octave for frequencies below 15.5 Hz and 22.2 dB/octave above.

—Yeowart and Evans (1974) - Acoustical Society of America

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.