Kef Muon (£70,000)

Hi-Fi News presents the first technical review of KEF's statement Muon - a shining obelisk to the art of loudspeaker design

For readers whose knowledge of particle physics is as lamentable as mine, the muon is the name given to an important elementary particle and one that has a relatively extended lifetime of 2.2µsec. Muons are difficult to create – something which unquestionably applies to these Ross Lovegrove designed KEFs too, whose superformed aluminium cabinet takes about 160 man-hours to manufacture the shining megaliths you see here.
   Numbers for the Muon are impressive even before you get to its £70,000 price tag (per pair). It stands 2m tall, weighs 115kg, has eight drivers – nine, if you separate the Uni-Q drive unit into its cone midrange and dome tweeter sections – and incorporates a massive four-way crossover including impedance correction elements, distributed across two massive circuit boards. To describe it as a statement product is superfluous: it shouts its intent to be the best KEF loudspeaker ever from the moment you set eyes on it.

The superforming process used to create the Muon’s curvaceous cabinet is principally employed in the aerospace and automotive industries. It exploits the fact that certain aluminium alloys, when heated to around 500° C to soften them, exhibit behaviour called ‘superplasticity’, which allows them to be stretched without fracture, sometimes over elongations of ten times or more. Only a few aluminium alloys display this behaviour. In the aerospace industry high-strength 2000-series and 7000-series alloys are superformed for structural components; more generally the Al-Mg-Mn alloy 5083 is used, which has good weldability and corrosion resistance.
   In something akin to the vacuum-forming process used with plastics, the two halves of the Muon cabinet are created by draping heated sheets of 6mm-thick 5083 over a male mould and evacuating the air from between them. The fronts and backs of the cabinet are then CNC machined to create the drive unit holes, welded together after fitment of internal chamber dividers and braces, and lastly hand-finished to a mirror polish.
   KEF has taken the opportunities afforded by superforming to create a cabinet which is not only aesthetically arresting but also structurally and acoustically superior to a conventional box cabinet. The gently curved back panels (in horizontal cross-section the cabinet is for most part roughly a triangle with rounded-off apexes) add stiffness and help suppress internal air resonances. The pronounced waisting of the cabinet around the upper midrange and Uni-Q drivers serves to control diffraction effects.

Aluminium is a good loudspeaker cabinet material in terms of stiffness but not in respect of self-damping, so the Muon cabinet would ‘ring’ if not damped on its inner surfaces. Over much of the interior KEF kills two birds with one stone by bonding to the cabinet walls the porous bags of activated carbon that exploit its ACE (acoustical compliance enhancement) technology.
   This is an idea KEF originally worked on in the 1980s. As the air in the cabinet is compressed by the bass driver diaphragms moving inwards, the activated carbon adsorbs air molecules within its complex surface pores. When the diaphragm moves in the opposite direction, lowering internal air pressure, the adsorbed air molecules are released. As a result, the drive unit ‘sees’ what appears to be a larger cabinet volume.
   Bass extension and sensitivity are intimately related to cabinet size, allowing this apparent increase in volume to be exploited in various ways. Both can be enhanced while keeping the cabinet size unchanged, or the cabinet could be made smaller without sacrifices. Apparent volume increases of 1.5 to 3x are achievable, with about 2x used in the Muon. Without ACE its cabinet would have to have twice the internal volume to achieve its combination of impressive bass extension and high sensitivity. Bonding the ACE bags to the interior walls also provides cabinet vibration damping, and additional damping is applied around the lower-mid and Uni-Q driver cut-outs.

Another familiar KEF technology, compliant driver mounting (introduced as long ago as the original Reference 105), is used to suppress cabinet output still further. While the six 250mm bass drivers are bolted firmly to the cabinet, the 250mm lower midrange driver and Uni-Q upper-midrange/tweeter driver are isolated from it via elastomeric mountings. In concert with the mass of the driver these form a mechanical low-pass filter which attenuates the amount of the magnet reaction force communicated to the cabinet.
   KEF’s use of a four-way configuration for the Muon is interesting, as the 250mm lower-midrange driver covers only 1.3 octaves from 120–300Hz. (Graphs in KEF’s Muon brochure show the crossover to be at 400Hz, but that was in an earlier, pre-production iteration.) It is tempting to suppose that a good deal of complication could have been avoided by having the bass drivers and Uni-Q meet at, say, 200Hz, but as Andrew Watson, KEF’s head of project acoustics says, ‘KEF has never been frightened of complication where it is of genuine benefit acoustically. It’s a sophisticated system and you need to engineer it properly to get it to work. When you do, the sonic benefits are clear’.
   A key part of engineering it properly involves the use of another long-standing KEF technology: conjugate load matching. CLM involves the addition of extra components to the crossover whose job it is to convert the highly frequency-dependent impedance of the drivers into something close to a constant resistance. When originally introduced, CLM was used to achieve a 4ohm nominal impedance, and thereby a gain of 6dB in sensitivity, without making the speaker any harder to drive than a conventional 8ohm design. In the Muon, this is only one of the purposes that CLM serves. Applied to the bass and lower-mid drivers as well as the upper-mid section of the Uni-Q unit, it allows the crossover filters at 120 and 300Hz to be properly terminated. Without this it would be impossible to achieve well controlled crossover slopes. Only the upper midrange/tweeter crossover point is clearly visible as a peak in the Muon’s impedance modulus graph.

As Andrew Watson explains [see Origin of a Species box-out, page 25], the Muon was conceived as a way to exploit the improvements made to the 165mm Uni-Q driver. As in previous Uni-Qs, this comprises a dome tweeter, now with a titanium diaphragm, mounted coaxially in the centre of a midrange cone driver, where the dust cap or phase plug would normally be, to form a coincident driver pair.
   Directivity of the two drivers is the same at crossover, thereby avoiding the step-change in off-axis output that often occurs in speakers with separate midrange and treble drivers. Their coincidence also prevents the off-axis lobing that can occur when some types of crossover are applied to non-coincident drive units.
   Downsides are that tweeter directivity is increased, which can rob some ‘air’ from the sound, and that the midrange cone acts as a waveguide – ie, a horn – for the tweeter, which raises the possibility of mouth reflections and resonances. In this latest iteration, the cone has been made shallower to widen the directivity and KEF has closely researched the optimum cone flare which, in addition to providing the required acoustic loading for the tweeter, has also to be formed so as to suppress cone breakup resonances.
   A further enhancement of the revised Uni-Q driver (KEF now prefers the term ‘Uni-Q array’) is venting for the back of the tweeter. This reduces pressure variations behind the moving dome, claimed to give ‘a smoother sound with enhanced dynamics’.

A problem with all loudspeakers having prodigious bass capability is how to prevent room modes clouding and colouring the low frequency output. KEF has addressed this in the Muon by placing two of the six bass drivers on the rear of the cabinet, which can be used to create an optional cardioid LF directivity pattern.
   Because bass output to the back, and to some degree to the sides, of the speaker is suppressed as a result, some room modes are excited less energetically than with conventional omnidirectional bass output. Whether this option is activated or not is controlled by an extra, removable, terminal above those of the split crossover. When this is in place the rear drivers are activated and the bass directivity is cardioid; when it is removed the rear drivers are disconnected and the bass output is omnidirectional.
   We experimented with both configurations during our listening session, performing most of our assessment with the Muon’s bass in omni mode. Preference will probably depend on room and system circumstances as well as individual taste. Cardioid mode removed some upper-bass thickening in KEF’s listening room and made the bass more tuneful overall, but it also lessened bass weight and warmth.
   All told, then, the Muon is everything you would expect and hope a statement KEF loudspeaker to be. Its visual impact is unquestionable, yet that striking aluminium cabinet’s beauty is more than skin deep. Our listening impressions bear out that this hi-tech recipe has been cooked with consummate skill and care.

Had I made my mind up about the Muon on the basis of the first 45 minutes of listening, I’d be telling you now that it is fine at low SPLs but runs into the dynamic buffers at higher levels, sounding amorphous and constricted. But within a period of minutes it changed character, apparently as a result of its aluminium cabinet coming closer to thermal equilibrium with the air in the listening room. Suddenly, here was a speaker that handled dynamics effortlessly, controlled different strands within the music with ease and snapped, crackled and popped with vitality better than any puffed-up breakfast cereal.
   The first track to benefit from the transformation was James Taylor’s ‘Sweet Baby James’, followed in short order by Lou Rawls’ ‘Fine Brown Frame’. I knew of both, of course, but it was the Muon unpicking their unexpectedly fine recording qualities that had me ordering them on Amazon next day. Likewise The Legendary Eva Taylor, instantly recognisable as Scandiwegian jazz on the Opus 3 label. These recordings aren’t easy to reproduce well but the Muon sailed through, crisp but never harsh, warm but never thickened.
   Linn’s Mendelssohn Hebrides Overture led me to suspect that the Muon didn’t do image depth as well as it should but then a burst of vintage Decca (Peter and the Wolf with Sir Malcolm Sargent conducting) put paid to that notion. In short: marvellous.

OK, I resisted the urge to bring along any pun-tastic cow-related test discs but perhaps I should have thrown in Pink Floyd’s Animals – as these silver obelisks do like a bit of rock. Highlight for me was ‘No Inbetween’ from Supertramp’s Brother Where You Bound CD. Despite their size the Muons are amazingly agile, the airy vocals, top-end detail and soaring sax solo towards the end of the track finding great projection into the room. The keyboard arrangement is played with some force and the Muons have a hard enough edge to convey convincingly the percussive nature of the sound. There is micro-detailing aplenty which really brings out the leading edge attack and decay of individual notes. The balance and bent for finer detail work equally well with strings and the soaring arrangement of The Hebrides Overture test disc got a double thumbs-up for sheer realism alone.
   At the bottom end of the scale I found the Muons less sure-footed, at least within the confines of the large, open-sided showroom at KEF’s Maidstone offices. The LF extension of the six-bass driver arrangement is unquestionable. They plumb depths that precious few single cabinet, full-range loudspeakers have gone before, and make the pronounced bass of the Supertramp track qualitatively excellent – particularly in ‘omni bass’ mode. I just had a hankering for more of it. As I tweaked the volume high enough for my colleagues to complain (they’re worse than neighbours) the top-end moves to the front of the mix, leaving the bass a little lighter than ideal. A good room is essential.

Three extended listening sessions, three venues, two countries: so complex and ornery is the KEF Muon that it wasn’t until the last of the three – at KEF in Maidstone – that I discovered why the speaker sounded different each time. It was all down to temperature.
   Think about it: the Muon is a 6ft tall chunk of metal. It should come with built-in heating, water piped through a tubular network. My first hearing was at a penthouse in New York – all hard walls, a warm October day. The second was at CES, a hotel room in Las Vegas. But it was only at the factory, after a long warm-up and judicious repositioning courtesy of Keith and Richard (as well as flitting from cardioid to omni) that the speaker showed its – ahem! – mettle.
   My remarks deal only with what we agreed was the fully warmed-up playback. All of the high-end/big speaker signature details were there, what you’d expect of a behemoth costing the same as four Mini-Coopers: a broad and deep soundstage, but less generous with its hot seat than, say Quad ESLs. The Muon is, surprisingly, focused in the way you’d anticipate of an audiophile’s selfish set-up. Which meant musical chairs for the four of us.
   Vocals, especially Keb’ Mo’s even before the tweaking, were rich and realistic, but sibilance was never far away. Bass? Solid, rich, extended. But I know the speakers were doing their job with authority, because Leonard Cohen was as miserable as ever. Bottom line? A detailed speaker, but less cuddly than I prefer. On the other hand, one hundred lucky music lovers will own a functional sculpture.

As befits a substantial tube of metal, these speakers have a very obvious run-up period, sounding rather too tight with a restricted, compressed sense of dynamic contrast over the first hour or so. Once warmed-up, the music relaxes, flowing with an unforced ease and underpinned by a tremendously deep bass. Indeed, the Muons even exposed the pumping bass rumble – rarely heard with other speakers – from the ’58 Decca recording of Peter and the Wolf. There’s plenty of controlled bite to the treble too, as the sharply-etched percussion and crack of ivory from Supertramp’s ‘No Inbetween’ served to demonstrate.
   The characteristically hard intonation of Eva Taylor was revealed by the Muon to wonderful effect, the edged articulation of her voice betraying the vintage of this live but ‘clubby’ acoustic recording. Neither this nor the metallic resonance of the sax could dampen our enthusiasm for the passion of the players themselves.
   Still in omni bass mode, the flowing introduction to Mendelssohn’s Hebrides Overture found us nodding in sympathy with the sweeping gestures of the massed strings and visceral impact of the timpani. Driven via bi-amped Musical Fidelity kW750s, these speakers betrayed a real flair for the dramatic, the ebb and flow of this orchestral theme flooded and receded from the room with a grace that belied the massive proportions of the recording, the speakers themselves and the scale of the ‘soundscape’ they developed. How something so visually arresting can sound so acoustically transparent remains a mystery!

Throughout its 47-year history, KEF has produced a series of Reference flagship speakers, but these were mere sailboats to the streamlined super-yacht that is the Muon. As much elegant architecture as state-of-the-art loudspeaker, the Muon is destined for residence in capacious, high-ceilinged rooms populated with similarly high-end sources and amplification. Stunning to behold and no less captivating to hear, the Muon sets new standards in the neutral and yet unmistakably tuneful and emotive reproduction of music, regardless of colour, creed or genre.

Originally published in the May 2008 issue