A Mighty Mini

Mankind has steadily sought to produce more and deeper bass from smaller and smaller enclosures ever since the first Neolithic audiophile blew through a conch shell and thought, “Damn, I wish this thing went lower, louder!”

- By Daniel Kumin

Good luck with that. Hoffman’s Iron Law (that’s Josef Hoffman, the “H” in pioneering American hi-fi firm KLH) tells us, essentially, that amongst the three desirables of deep bass, lifelike loudness, and compact enclosure size, you can have any two but never all three from a single design.

It’s not really Joe’s law, of course, but the universe’s. High acoustical levels of truly low frequencies need a large vibrating surface moving a good distance in and out: it’s a function of wavelength, which increases rapidly as frequency decreases. And a big woofer needs a large box to baffle it properly and supply an adequate volume of air to achieve sufficiently low resonance—and, of course, to fit that big driver.

Beating these restrictions requires considerable electroacoustic guile, which is precisely what KEF—one of Britain’s oldest loudspeaker makers—has brought to bear on a new design it terms Uni-Core. Briefly, this is a clever, “force-cancelling” (back-to-back) double-woofer that backs two drivers onto a common magnet structure. One smaller-diameter voice coil moves concentrically inside the other, sharing a common pole-piece, with an intervening aluminum spacer/ structure to hold the whole apparatus in alignment. This topology requires a balletic balancing of magnetic and electrical parameters like flux, resistance, reluctance, and inductance, such that two loudspeaker “motors” with necessarily very different voice-coil dimensions and air gaps maintain perfect electromagnetic balance over their full travels. While Uni-Core incorporates a deal of mechanical cleverness too, it is this high-wire act, no doubt achieved with considerable computer-simulation firepower, where the real Uni-Core magic happens.