Now That's a Subwoofer!
Eminent Technology’s Bruce Thigpen has always taken an interesting slant on how to design audio products—his air-bearing tonearm was one of the best-sounding back in the day and his push-pull planar magnetic speakers are thought by some to be unbeatable. But at THE Show, held next door to the official CEDIA venue, the Convention Center, in the Denver Athletic Club, Bruce was showing off his infrasonic subwoofer. Yes, that’s a fan, which rotates at a constant 800rpm. The wrinkle is that the audio signal is used to vary the pitch of the fan blades. Feathered with no signal, when driven with audio the twisting blades produce a massive acoustic wave with very little power input. The bandwidth is limited by the fan speed to below 30Hz or so—you have to rotate it faster to reproduce higher frequencies but then its self noise increases rapidly— but it will reproduce frequencies as low as 1Hz with a very high spl.
Its effective radiating area increases as the frequency goes down, which is just what you need with a subwoofer. Downside is that it needs to be installed in a separate room, with the wall between acting as an infinite baffle. But then, it really did produce awesome levels of low-distortion infrasonics, literally shaking the walls and the room‘s ceiling!