Rotary Woofer
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The model TRW-17 Rotary Woofer, invented by Bruce Thigpen of Eminent Technology, is designed to reproduce, for the first time, audio frequencies from DC (zero) to 20 Hz and is aimed at the home theater and professional audio markets. Typical subwoofers using moving cones don't couple well with air below 20Hz, and thus their Sound pressure level (SPL) falls off significantly below 20Hz. This is not to say that traditional cone subwoofers cannot create sound pressure between 0 and 20Hz, because they certainly can. One can drive a subwoofer with a 1 Hz tone, and see the cone driver move back and forth, creating sound pressure waves. However, the human ear is quite insensitive to acoustics below 20Hz and as sound frequency goes below 20Hz down to 0Hz, the sound pressure level needs to increase in order to remain audible to the human ear. The Rotary Woofer technology has the capability of reaching these audible sound pressure levels at these low frequencies.
Conventional knowledge is that the human being cannot hear below 20Hz, however, as early as 1974, researchers were aware that the human could hear below 20Hz, but that the ear was much less sensitive to sounds below 20Hz. Thus, in order to 'hear' below 20Hz, one had to be subjected to increasing sound pressure levels. Here is the abstract from Yeowarts and Evan's paper:
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. ©1974 Acoustical Society of America
This suggests that for sound to be perceived at 7-8Hz, that the 7-8Hz SPL would have to be 12.3dB higher than the 92.0dB SPL required to hear 15.5Hz. This means that 7-8Hz SPL would require at least 104dB of SPL to be perceived. For 3Hz to be perceived, the SPL would need to add another 12.3dB, or reach 116dB of SPL to be perceived.
Traditional cone-based subwoofer transducers fall off rapidly below 20Hz, and a new concept was needed to produce the required SPL levels, if the human being was to perceive the very low frequency content available in recorded material, if any. Thus the rotary woofer was conceived as a way to displace far more air than was possible using oscillation cones.
[edit] Citations
- Danish study on Infrasound
- Journal of Acoustic Society paper by Yeowart and Evans
- UK government paper on very low frequency sound
[edit] External links
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