Rotary woofer

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 varies in exact relation to the voltage (audio signal) presented to the voice coil, and the fan blade pitch 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 can reproduce audio frequencies from near zero Hz to 20 Hz, much of which is outside the normal hearing range.

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 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 (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 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.

ee also

*Subwoofer

Citations

* [http://www.mst.dk/transportuk/02030000.htm Danish study on Infrasound] dk icon
* [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JASMAN000055000004000814000001&idtype=cvips&gifs=yes Journal of Acoustic Society paper by Yeowart and Evans]
* [http://www.defra.gov.uk/environment/noise/research/lowfrequency/pdf/lowfreqnoise.pdf#search=%22Thresholds%20of%20audibility%20for%20very%20low-frequency%20pure%20tones%22 UK government paper on very low frequency sound]

External links

* [http://www.rotarywoofer.com Rotary Woofer] site
* [http://www.iar-80.com/page142.html International Audio Review Technical Article]
* [http://bassment.wordpress.com Rotary Woofer Installation Blog]
* [http://blog.stereophile.com/cedia2006/091606thigpen/ Stereophile] blog
* [http://www.ultimateavmag.com/features/1005rocky/ Ultimate AV Magazine]
* [http://blog.ultimateavmag.com/michaelfremer/102705something/ Michael Fremer] writeup on Eminent Technology and the Rotary Woofer breakthrough