KEF Q900 Speaker System
Cue the Qs
KEF’s Q Series, improved over multiple generations since 1994, has long been the British speaker company’s bread-and-butter line. The new Qs, which began shipping earlier this year, were designed in the U.K. and are manufactured in China.
Much of the technology in the latest Q loudspeakers was first developed for use in KEF’s Concept Blade, a cost-no-object design study that is not (yet) a commercial product. The Qs incorporate the company’s latest thinking on loudspeaker design—or at least as much of it as they can offer at relatively affordable prices.
KEF developed the first Uni-Q drivers in 1988 and has been using them in most of its speakers, with ongoing refinement, ever since. The Uni-Q concept positions the tweeter coaxially, at the apex of the woofer cone (or, in a three-way design, the midrange cone). This solves a number of problems with traditional speaker design. The most significant is the nonuniform dispersion, particularly in the crossover region, that occurs when a woofer and tweeter are physically displaced from each other. This displacement is nearly universal among speakers. It creates challenges to obtaining uniform response throughout the listening area and, many would argue, a convincing three- dimensional image. Careful (and more complex) crossover design can minimize these effects, but it can’t eliminate them.
However, the process of positioning a tweeter at the center of the woofer isn’t straight- forward. It requires skillful design to avoid creating more problems than it solves. Fortunately, KEF has been at this Uni-Q business a long time. The 11th-generation Uni-Q drivers used in the new Q Series incorporate stiffened, optimally shaped aluminum tweeter domes fronted by KEF’s Tangerine Waveguides. The tweeters’ rear chambers also extend through the center of the midrange/woofer voice coils to reduce back pressure on the tweeter domes.
The cones in the new Q Series drivers are also fabricated from aluminum. This simplifies pro- duction and allows the cones to be shaped to perform two duties that aren’t necessarily compatible: acting as a waveguide for the tweeter and also providing opti- mized woofer or midrange response. The distinctive Z-Flex surrounds on the Uni-Q woofers and midrange drivers are also designed to minimize undesirable resonances that could unfavorably color the sound.
To that end, KEF offers another technological twist: Cone Breakup Control. This is designed to overcome metal cones’ natural tendency to ring at high fre- quencies. Most speakers that use metal cones deal with this problem by using trap filters in their crossovers, often combined with steep crossover slopes. This adds yet more complexity (and cost) to the design.
Of the three concave woofer cones that are visible on the front of the Q900 tower speakers that served as the front left and right channels in this review system, only one (the middle driver) is active. The other two are Auxiliary Bass Radiators (ABRs). One of the two woofers in each of the Q Series’ three-way center-channel speakers is also an ABR. ABRs, also known as passive radiators, are cones with flexible surrounds but no magnet or voice coil. They perform the same function as a port, but they have advantages over a port that some designers prefer. They eliminate the internal resonances escaping through the port, as well as noise caused by air turbulence in the port itself (although we’ve never found either to be a problem in a properly designed ported speaker). An extra cone or two is also more visually seductive than a port, although it’s more expensive to implement.
Along with this dedicated 8-inch woofer, the business end of the Q900 sports a Uni-Q array that mates another 8-inch cone with a large, 1.5-inch tweeter. The oversized tweeter better sup- ports the lower crossover point desirable with a large midrange driver, although the cone portion of this particular Uni-Q covers more than just the midrange. It works in conjunction with the single, dedicated 8-inch woofer and the two ABRs to operate down to the speaker’s bass limit.
The Q900’s main crossover to the tweeter is at 1.8 kilohertz, with second-order acoustical slopes. (When combined, the drivers’ natural mechanical rolloff and the electrical rolloff from the crossover network produce, in total, a second-order, or 12- decibel-per-octave, acoustical slope.) KEF doesn’t specify whether it uses an electrical crossover, or even the point above which the dedicated woofer rolls off, leaving the rest of the fre- quency range to the Uni-Q alone. The Q900 is rated for a maximum output level of 114 dB (unfortunately without the details required for the numbers to be meaningful). In practice, this is far more than my ears can handle. Its sensitivity is rated at 91 dB.