KEF R107 loudspeaker John Atkinson 1991
Bass. A small word. But a profound one: bass is the base of all Western music, the underpinning that organizes the overlying structure. Basically speaking, music without bass is music without roots or soul. Yet how many loudspeakers do justice to that concept? "You don't understand bass," said a St. Louis reader in a recent letter, who concluded that I (once a professional bass player) was "either deaf below 60Hz, didn't have room for decent-sized speakers, or never listened to bass-heavy rock." (Not one of the three, as it happens, is true.) But did he understand? Did he have a true feel for the concept of "bass," citing as he did in his letter a veritable litany of best-selling thunderboxes—loudspeakers that are only loud; underdamped speakers that, in general, I have found to confuse the musically aware listener with a vague, monochromatic sea of low-frequency nonsense that obscures rather than clarifies harmonic structure.
My abhorrence of such proletarian boomboxes explains why over the years I have resisted the blowzy, bosomy beckonings of "full-range" speakers, why I have tended to stick with musically accurate, tonally precise minimonitors like the Celestion SL600 and SL700. While sacrificing the true pedal weight of the bass register, these thoroughbreds present sufficient of its outline and pitch to preserve musical sense.
Either of these superb Celestions represents the triumph of quality over quantity...yet, at regular intervals, I forget my asceticism. My monkey bone hankers for music's full measure. If only it wasn't so rare to find a full-range speaker that hung onto the delicacy of the leading edges of bass notes with the pedal pressed to the metal.
Occasionally—very occasionally—I do find such a paradigm: "There was suddenly a moment in the Ashkenazy Sibelius 5—the double-bass discord in the last movement before the final flare-up—when I realized that I was hearing low-frequency reproduction quite an order of magnitude more transparent than I had previously experienced from an apparently conventional loudspeaker," read the start of a loudspeaker review I wrote almost five years ago (footnote 1). That discord, underlying the symphony's sense of rhythmic and tonal uncertainty, concerns two low notes a major second apart, E-flat and F, which are almost identical in frequency but far enough apart in pitch to be distinguished. Unfortunately, distinguishing those notes is a hurdle at which just about every dynamic loudspeaker to feature true 20Hz extension falls. But the speaker that was the subject of that review, the original KEF R107, was hardly ordinary; it handled its task with harmonically accurate aplomb.
It was therefore with considerable anticipation that I awaited the arrival of the R107/2 in my listening room...
Means of Ascent
In a 1979 AES paper entitled "A bandpass loudspeaker enclosure" (footnote 2), KEF's Laurie Fincham offered a theoretical analysis of a different kind of woofer loading. In a normal reflex enclosure, a port added to a sealed box allows a cavity resonance to extend the woofer's response downward by a useful amount. But what, asked Fincham, if the woofer were mounted so that it had no direct connection with the outside world, instead firing into an internal cavity which communicates with the listening room via a port? In this manner, the air in the port would act as the bass driver proper, having very low mass and capable of very high excursions. In an ideal world, this will result in lower distortion and a higher sensitivity than an equivalent sealed-box alignment using the same drive-unit (equivalent, that is, in that it features the same lower cut-off frequency and that the total internal enclosure volume is the same). The unit's intrinsic response will resemble a band-pass electrical filter centered on the port-tuning frequency, with smooth out-of-band behavior.
In the real world, you will not be surprised to learn that things are not so simple, the bandpass enclosure suffering a propensity for coloration-inducing high-Q resonances above its passband "at frequencies where the wavelength of the sound is less than eight times the smallest dimension of the second cavity," found Mr. F.
But hey, if you can arrange for those resonances to be pushed into a corner, you've got one hell of a woofer! Which is why the '80s saw a proliferation of such devices, both from KEF in the form of otherwise conventional loudspeakers and from the manufacturers of those cute little three-way systems that promise everything but deliver...well.
The original R107 and the new R107/2 implement the bandpass woofer with two 10" pulp-cone units mounted vertically in separate sealed internal enclosures and firing in push-pull into a cavity. The driver frames are linked by a nonmagnetic tie-bar, and as the reaction motions in the frames are identical and out of phase, they cancel, leading, it is to be hoped, to minimal excitation of the enclosure walls. The bandpass cavity communicates with the outside world via a port sited on the top of the bass enclosure to position the effective bass driver very close to the midrange unit. (The vent opening is covered with black wire mesh to prevent animals and small children from venturing inside.)
The R107/2 reveals its evolution from KEF's R105 of 1977 with its swiveling treble/mid "head" unit, which connects to the bass bin via a gold-plated XLR plug. The head is constructed from a honeycomb plastic material, its cavity filled with a mineral-loaded polymer damping compound. However, whereas the 105 used a B110 driver, the R107 head uses a polypropylene-cone midrange driver, this mounted on rubber O-rings to give a degree of compliance between the driver chassis and the enclosure to eliminate specific resonances. The original R107 used KEF's T33 tweeter, a large soft fabric-dome unit which I and other reviewers felt not to be equal in terms of clarity to the rest of the drivers used. Perhaps not uncoincidentally, the revised 107 features a metal-dome tweeter, this featuring a short horn flare around the dome and constructed on a cast front-plate with "KEF" prominently displayed. Surprisingly, the tweeter magnet is still marked "T33." More surprisingly, some of the bolts holding the tweeters in place were a little loose, presumably from shipping vibration. (The midrange bolts, which I assume have to be torqued correctly to give the right behavior with the O-ring gaskets, were all screwed tight.)
Footnote 1: HFN/RR, July 1986, p.65.—John Atkinson
Footnote 2: Presented at the 63rd Audio Engineering Society Convention, May 1979. Preprints are available from the Audio Engineering Society, 60 East 42nd Street, New York, NY 10165. Tel: (212) 661-8528. Fax: (212) 682-0477.—John Atkinson