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All drivers here are aluminium: active and passive, bass and treble. This extends to the voice coils of woofer and midrange. The rationale reads similar to Dynaudio propaganda. Though aluminium is the slightly inferior conductor to copper, it’s significantly lighter to benefit the lower-moving-mass/more-efficient-power transfer equation. The material lends itself to relatively wide and long coils without getting unduly heavy. This also entails larger surface area which improves thermal efficiency. And that results in a/ higher dynamic range, and b/ reduced dynamic compression at high output. That's because increased heat equals increased electrical resistance which equals lower voltage sensitivity. The better a driver’s self cooling properties, the lower its compression under stress.

Midrange diaphragm section, tweeter assembly with wave guide, exploded coaxial driver

To stay with aluminium as diaphragmatic choice, many listeners will reflexively suspect ringing. This refers to breakup or self-resonance modes of metallic membranes which are typically shifted outside the critical pass band but still can impact the actual coverage range, be it in the mid-to-treble transition or at the uppermost limit of the tweeter. Here the Brits at Kef roll out their so-called ‘Cone Breakup Control Device’ for the midrange. Whilst sounding massive on paper, it’s actually just a coupler between the membrane and its voice coil. A combination of material properties and mathematically calculated shape are said to insure pistonic behavior plus a gradual damping at higher frequencies where mechanical resonance would otherwise kick in. Kef compares this scheme to a properly adjusted car suspension.

How about the unusually large 35mm tweeter? Kef’s R&D suggests an innate shape dilemma. Mechanically an elliptical cone that's similar to the flatter end of an egg would be ideal since it resists resonating into the highest frequencies. But the radiation pattern of a semi-spherical profile is superior to the partial egg. Kef’s solution sounds surprisingly simple – ellipse below, semi-sphere above to piggyback one atop the other and cross off both requirements.

There’s more tech talk on Kef’s tweeter such as the ‘Tangerine Waveguide’ to improve off-axis response and acoustic coupling; or the damped tube on the back through which the driver ‘breathes’ to prevent air compression (which echoes B&W). The most profound item of course is the coaxial geometry of a quasi point source. Hence Kef’s nomenclature of coincident driver to stress that the acoustical centers of midrange and tweeter are in the same place or coincident.

The British designers don’t merely bang the drum on reduced vertical interference issues, time coherence and advantages with soundstaging but stress the radiation pattern continuity of their Uni-Q concept. That’s because the shape doesn’t merely influence the directivity of the tweeter but also that of the mid/bass unit to maintain tonal balance even outside the sweet spot. That’s additionally important for the diffusive sound field, i.e. the sum total of reflections from walls, floor and ceiling. Introduced in 1988, the Uni-Q in the Q900 is in its 11th generation by now.