Floor Loudspeaker Reviews

Of all the speakers I have most enjoyed in recent years, two were from British manufacturer KEF: the LS50 Anniversary Model ($1500/pair), which I reviewed in December 2012; and the Blade Two ($25,000/pair), which I reviewed in June 2015. Though these two speakers lie at opposite ends of the price scale, they have in common KEF's unique Uni-Q drive-unit, in which the tweeter is mounted on the front of the midrange unit's pole piece, so that the lower-frequency cone acts as a waveguide for the higher-frequency output. Not all Uni-Q drivers are identical, of course—the one in the LS50 must cover frequencies down into the upper bass, whereas the Blade Two's Uni-Q is relieved of having to undergo high excursions by being crossed over to four woofers. But having one drive-unit with two concentric diaphragms that together cover almost all of the audioband gives you, in effect, a single sound source with no interference between the drivers, and with all the benefits that confers on dispersion and soundstaging.

The Blade Two may sound superb, but its idiosyncratic styling—it resembles an old-fashioned straight-edge razor stood on end—was not for everyone. KEF therefore introduced the Reference 5 ($19,000/pair), which uses almost identical drive-units—a similar Uni-Q driver with a 1" tweeter and a 5" midrange, plus four 6.5" woofers—in a conventional cabinet of the same internal volume.

Design
Whereas the Blade Two's woofers are mounted in pairs on its side panels, the Reference 5's 6.5" woofers are all mounted on the front baffle. These each have a shallow aluminum diaphragm, and as with the Blade, the voice-coil drives the diaphragm via radial ribs to allow the free flow of air, and also to control the diaphragm's breakup modes. Each pair of woofers is loaded by a port on the rear, these offset from the cabinet's center line and flared on both ends to minimize turbulence. KEF provides a choice between interchangeable long and short ports to optimize the speaker's low-frequency performance for specific rooms. While the woofer frames are internally braced, they're connected to the aluminum baffle via lossy damping.

Jack Oclee-Brown, KEF's Head of Acoustics, visited to help set up the Reference 5s in my room. Among other topics we discussed in a video'd interview, I asked him what, specifically, KEF had done to get so much of the Blade's sound quality from a conventionally styled cabinet with the woofers on the front baffle, as they claim they have.

"The focus of the Uni-Q is to deliver sound as if . . . the sound has to originate from one point in space, but it also has to be matched [in terms of] directivity, so at the crossover you need the tweeter directivity to match up with the midrange. . . . With Blade, the whole idea was then to say, 'Why are we restricting ourselves just to the mid and treble? What would happen if we tried to do the bass as well, and how would that look?' My colleague Mark [Dodd] was really the guy who originated the idea of saying, 'Let's put just the Uni-Q on the front, and you could put the bass drivers much closer to the Uni-Q than you would be able to otherwise. You've then only got the single Uni-Q on the front, so the front of the cabinet can be shaped whatever you like, and so we could make it a very smooth surface to minimize diffraction.'"

The sound from the midrange unit therefore doesn't meet any boundaries as it propagates?

"Exactly. So the whole thrust of [the Blade] is really to take this idea of let's try and create a single, pure sonic source as far as we possibly can. [But] with the Reference we are very aware that these speakers are things that people have to put in their houses, and they have to live with and see every day—and the Blade looks, in my mind, spectacular, but in other people's minds, 'dreadful' . . . "

Idiosyncratic is a better word, I suggested.

"Yes, it splits opinion. So when we came to do the Reference series, we decided it would be sensible to do the opposite, in terms of styling, and that handed the engineering team a big challenge. How do we get as much as possible of the Blade's performance, but now do it in a conventional loudspeaker and put the drivers on the front?"

Oclee-Brown explained that, with a conventional enclosure, the ways to create a single sound source are limited, but one thing they can do is use a symmetrical driver array. "It is exactly the same thing [as a D'Appolito configuration], except that now you don't have a tweeter, you have a Uni-Q instead, and that means that [the] crossover frequency for the D'Appolito array can be much lower. . . . [I]f you go low enough, you get lobe-free vertical dispersion."

The low diffraction is obviously a big challenge, Oclee-Brown continued, but he pointed to the "shadowing"—the profiled ring on the front baffle around the Uni-Q. "Diffraction depends on how much you illuminate any hard edges," he explained. "The waveguide of the Uni-Q naturally narrows the directivity as you go higher in frequency, and if we guide it a little bit more with this flare, we can minimize how much sound arrives at [the baffle edges]. So for something that looks like a hard-edged box, you get much less diffraction than you would see if the tweeter were just on the baffle."

KEF's Uni-Q concept, introduced in 1989, is now in its eleventh generation. I asked Oclee-Brown if the shape of the Uni-Q tweeter's dome is different from that of a conventional tweeter.

"It is a spherical dome," he explained, "[but] you need to get the spherical shape of the dome exactly right compared to the walls of the Uni-Q . . . and that's just to get the sound from the tweeter started traveling down the waveguide. Then you come to the next issue, which is you arrive at the edge of the cone, where you would normally have a big half-roll surround, and it will scatter off that. . . . [T]o fix that problem, we use very low-profile surrounds. . . . [T]he surround is a continuation of the cone."

In current Uni-Q drivers the tweeter dome is mounted behind a complex grille called the Tangerine. But this is more than a grille, as the ribs are wider next to the dome than they are at the front. I asked what that does.

"The Tangerine—technically, it is a type of compression driver. . . . [Y]ou cover up the diaphragm, and then you can choose exactly which areas of the dome you want to radiate sound. You normally, in pro audio, do that to make it more efficient, but in this case we are doing it to solve a particular problem. . . . [I]n acoustics you really like pulsating sources—sources which are moving radially, so the surface gets bigger and smaller, as if you're blowing up and letting air out of a balloon. A dome is the right shape, but it's moving axially, because you are driving it with a voice-coil. . . . [B]y covering up the dome, then choosing exactly where you allow the sound to radiate, you can get it to behave more like a pulsating source."