KEF R107 loudspeaker

The Model R107 represents the flagship of KEF's Reference Series, and is second only to the Professional Series KM-1 in KEF's product line. Anatomically, the 107 resembles a person. Beneath a decorative "hat," there's a special head assembly akin to the head on the old Model R105. This head assembly contains the brains of the 107, namely a T33 ferrofluid-cooled tweeter and an improved version of the classic B110 midrange driver, featuring a better voice-coil and a new polypropylene cone. The nerve center is also here, in the form of two passive dividing networks and load-impedance equalizing network. Level equalization of the drivers is performed actively within the KUBE, the second brain of the 107—about which you'll hear more shortly.

The head assembly swivels at the neck, allowing the mid and treble units to be toed-in to maintain a good stereo image when the main enclosure is placed square against a wall. The rounded shape of the 105 head has been retained in order to minimize treble diffraction effects, which can smear or defocus the stereo image. Otherwise, the head enclosure has been re-engineered to increase its rigidity, and to slightly increase the cavity volume. A mineral-loaded polymer "goop" is injected into the cavity walls of the head enclosure for damping purposes.

The guts of the 107 are housed in the main low-frequency enclosure that forms the body trunk. The bass loading used by KEF has the intimidating name of "twin coupled-cavity bass loading" and was first seen in the Model 104/2. (In retrospect, the 107 may be seen as a hybrid of the 104/2 and 105 technologies, with a few improvements thrown in.) The "twin" in the title refers to the use of two 10" paper-cone woofers, each working in its own sealed enclosure and firing into a common central cavity. Acoustic energy from the cavity is vented via a tuned port that radiates from the top face of the LF enclosure adjacent to, or at the neck of, the head assembly.

This is where my anthropomorphic analogy breaks down. After all, who's ever heard of a bottom end venting at the neck? Positioning the head assembly close to the upward-vented bass energy enables the entire audio bandwidth to be, as KEF puts it, "directed towards the listener from an area little larger than the human head." Output integration is thus optimized, while low-frequency directional effects are minimized. Also, because of the height of the bass vent above the floor, reinforcement effects are minimized. All of which should add up to an exceptionally smooth in-room response. Does it? Stay tuned.

The bass driver system is effectively an extension of the bass reflex principle, but with the port supplying all the output rather than reinforcing the main driver above resonance and cancelling its output below. The air mass in the port, driven by the concealed woofers, acts as a long-throw, low-mass drive-unit with an intrinsic bandpass response tuned to 90Hz. The woofers' magnet structures are rigidly linked by a force-cancelling rod that minimizes vibration transfer to the cabinet from the driver frames or baskets. The cabinet itself is very nicely finished and exudes elegance, as an expensive speaker should. Gold-plated binding posts are provided on the rear that will accept bare wire, banana plugs, or spade lugs. These posts feature large, easy-to-use knurled nuts that can be finger-tightened for a really tight connection. However, the posts are not spaced on ¾ centers so dual banana plugs cannot be used.

KUBE or Not KUBE?
That is the question, and the 107 answers in the affirmative. The KUBE cuts through several vexing problems that have traditionally compromised subwoofer designs. Even my six-year-old, Dahlia, associates bass sensitivity and extension with large enclosures. And, indeed, the laws of physics conspire to make it so, as Richard Small (now working as Head of Research at KEF in the UK) and A.N. Thiele so aptly demonstrated almost 20 years ago. I'll get a bit didactic and elaborate:

In order to achieve flat LF response to, say, 20Hz, you have to start with a woofer whose free-air resonance is no higher than 20Hz, and preferably even a bit lower. Remember that below the driver's resonant frequency, the output will droop as the driver is no longer mass-controlled and cannot maintain a constant acceleration as frequency decreases. Next, it turns out that a large woofer is much more adept at pressurizing a given-size box than a small woofer, and a very large air volume is needed to prevent the air stiffness of the enclosure from significantly raising the system's resonant frequency. Reduce the volume and low bass can still be squeezed out of the design, but only at the expense of severely curtailing sensitivity.

If you want low bass and a usable sensitivity, therefore, what you end up with is a very large box that is not only expensive to construct and ship, but also, most importantly, almost impossible to make rigid enough to minimize colorful cabinet radiations. Have you ever seen the WAMM subwoofer? Do you know how much carpenters get paid these days? Now you know why that system costs over $40k. So it appears that, at least for realistically sized production-line speakers, the hopes of realizing a 20Hz response using a natural alignment are doomed.

The intelligent engineering solution is to use active equalization between the pre- and power amplifiers to "artificially" extend and flatten the LF response. There has been a recent trend to do just that: recall for a moment the Enigma subwoofer, or, more recently, the free-standing Celestion woofer (footnote 1). (Of course, the latter two designs minimize cabinet colorations by doing away with the cabinet altogether, another plus.) KEF's strategy is to align the coupled-cavity for optimum upper-bass sensitivity, then boost the frequencies below the natural turnover to give a flat extended response.

(Don't run out now and buy yourself an equalizer to do just that with your speakers; odds are that you'll simply succeed in destroying the woofers. You should know that woofers are unhappy about being pushed into the deep bass because cone velocity doubles and excursion quadruples for each lower octave. Any design that uses bass boost, therefore, must necessarily include woofers that can cope with the excursion demands.)



Footnote 1: Actually, the only people who can "recall" the Celestion must have been in attendance at the last CES, since the woofer is only now in production.—Larry Archibald
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