The KEF LS50 have been a great commercial success for British manufacturer KEF. But the law of the market is intractable. That’s why KEF has gone back to the drawing board to delve deeper into its familiar technology. An acoustic labyrinth is the weapon of mass destruction of undesirable high-frequency resonances that this new version, the KEF LS50 Meta, is equipped with. And you’ll see that it’s a device that makes all the difference…
For many years, KEF has built its reputation around its coaxial transducers. A configuration called ‘Uni-Q’ by the brand, where the tweeter is placed directly in the centre of the woofer in place of its core cover. This architecture offers the advantage of perfect phase coherence between the sound emitted by the tweeter and that emitted by the woofer. This is a major advantage when it comes to giving the sound image relief and precision in the spatial location of each of its sound elements.
Twelfth generation Uni-Q architecture
What’s more, KEF has continually developed its Uni-Q architecture to optimise its acoustic behaviour. While the overall structure has changed very little, it’s in the details and subtleties that the optimisations are to be found. The KEF LS50 Meta represents the twelfth generation of this system. Here, the quest for perfection concerns the reproduction of high frequencies and, more specifically, the acoustic load associated with the tweeter. Not many brands are concerned with this point. While B&W has taken a very close interest in this problem by associating a Nautilus-type load with its tweeter, as well as with other loudspeakers, to dampen the rear wave, the British manufacturer is the exception. KEF has now joined them in offering a slightly different solution.
An acoustic ‘black body’
It is based on a small labyrinth, the layout of which is reminiscent of a game of skill in which you have to move a small marble towards the centre of the structure.In our case, each ‘corridor’ of this labyrinth has a length and a layout that enables it to eliminate a group of frequencies by diffraction or phase composition. The multiplicity of patterns in this structure makes it possible to cancel the rear wave of the tweeter dome over the entire frequency range it is designed to reproduce. In short, this mini labyrinth acts like an acoustic black body over the frequency range handled by the tweeter. KEF claims that this technology, known as MAT (Material Absorption Technology) and developed in conjunction with the Acoustic Materials Group, is capable of absorbing, and therefore eliminating, 99% of unwanted sound. What’s more, this type of architecture is easy to integrate into the back of a tweeter, as it consists of a relatively thin disc. This element therefore fits easily into the back of the magnetic block of the Uni-Q structure.
Cleverly designed curves
While the use of this MAT damper is the major innovation of this Uni-Q system, the rest of its architecture, like that of the KEF LS50 Meta, remains fairly conventional. The 130mm-diameter bass driver features a cone made from an aluminium/magnesium alloy that combines high rigidity with very low mass.In addition, to further reduce the risk of surface ripples on the diaphragm, particularly during transients, the cone is stiffened by ribs.
In addition, as with any coaxial structure, the cone of the woofer also acts as a horn to give the tweeter a better spatial distribution of high frequencies. To avoid any break in surface continuity between the tweeter dome and the front baffle, KEF uses gentle curves. A first metal piece ensures the coupling between the tweeter and the centre of the woofer membrane, while the front of the LS50 Meta also adopts a sphere-shaped profile for optimum diffusion of sound waves.
Finally, to avoid any break in the curvature between the boomer diaphragm and the cabinet, KEF has banned half-roll perimeter suspensions. Instead, the brand has developed a suspension called Z-Flex, based on ribbed rubber, which gives the woofer plenty of travel without any prominent bulge. This type of suspension, which is fairly rigid, means that the LS50s need to be broken in for quite a long time before they can express themselves fully.
A tweeter that ‘climbs’ very high
The coaxial tweeter occupies the centre of the woofer diaphragm. Its large 25mm dome is made from aluminium. Very light, it gives the LS50 Meta a high frequency response extending beyond 40 kHz, which is rare enough to be worth mentioning and is fully justified when using hi-res audio files. Finally, a front waveguide plays a dual role. Firstly, it acts as a protective grille for the dome. Its second function is to optimise the dispersion of high frequencies by opening up the angle of propagation of sound waves and distributing them evenly over the entire area covered by the speakers.
Finally, the cabinetry of these latest speakers is directly derived from the previous-generation LS50s. The internal bracing for high rigidity and, of course, the curved front panel give these speakers a look that distinguishes them at first glance. The entire internal volume acts as an acoustic load for the woofer. This is decompressed by a vent on the rear panel. As is tradition with KEF, a set of foams is supplied to partially or totally obstruct the vent. This special feature allows you to modify the behaviour of the KEF LS50 Meta depending on its position in the listening room.
The set up
Commissioning the small KEF LS50 Meta does not pose any particular problems. With the vent open, they are still quite sensitive to their position in relation to walls and, especially, room corners. For them to express themselves fully, the ideal position is at least one metre from the corners of the room and at least 30 centimetres from the wall. It’s also a good idea to provide them with a quality stand that’s properly weighted and ensures that the vibrations from their cabinetry drain off properly. On the other hand, despite the name ‘library’, we strongly advise against placing them in a shelving unit and surrounding them with books, which will prevent the free circulation of air around the vent. Finally, near the corners of rooms, we preferred to use foam to conceal the vents in order to preserve the correct balance of the sound reproduction and avoid impasto in the low frequencies.
The sound
As with any coaxial system, the strong point of the small KEF LS50 Meta is their precision in terms of the spatial localisation of the various elements of the soundstage. This was underlined by our traditional listening to William Russo’s Three Pieces for Blues and Symphony Orchestra performed by the San Francisco Symphony Orchestra conducted by Seiji Ozawa. Despite the fairly pronounced reverberation in the San Francisco Opera House, each musician is perfectly located and stands out with excellent precision. The stereophonic space is thus given a breadth that gives the sound image a particularly seductive relief. The intervention of the triangle, heard during one of the work’s rare quieter and more subdued moments, is a perfect illustration of this behaviour. Not only is it perfectly positioned, slightly to the right of the centre of the sound space, but its crystalline note seems to come from the very depths of the hall, which it does.
The excellent midrange performance of this family of loudspeakers and the tonal balance they offer are also evident. Vocals, in particular, are reproduced with great warmth and presence. Listening to various tracks by Bernard Lavilliers enabled us to verify this point. The LS50 Meta always respected his vocal timbre perfectly, without the singer’s deep voice being overwhelmed by the rest of the orchestration, particularly the bass guitar. It’s worth pointing out that the small speakers hold the bass register very firmly. Despite the modest surface area of their woofers, they have a very satisfying impact and provide a good foundation for listening. They even descend into the bass range with surprising ease for such small cabinets. As far as the treble is concerned, the contribution of the MAT damping system is tangible. It provides detailed, even detailed listening, without any shiny effects. The treble remains subdued and remarkably natural, even at sustained listening levels.
Last but not least, the KEF LS50 Meta has a fine dynamic range. Despite their relatively modest output (87dB), the power they deliver is more than enough to cope with the flamboyant flights of a symphony orchestra or the drum solos of a jazz band. This was perfectly illustrated by listening to Grieg’s ‘In The Hall Of Mountain King’, among others. What’s more, we noticed that the timpani had incredible depth in the opening bars.
Our conclusion
Clearly, the arrival of the twelfth generation of the Uni-Q system is not just an announcement by KEF. It’s part of the brand’s ongoing search for optimisation, and its impact on listening is very real. It’s true that the overall behaviour of all the elements in the LS50 family is the same, but if you listen carefully, it’s clear that the reproduction gains in detail, finesse and subtlety while preserving its naturalness. A seductive behaviour that offers a new listening experience whatever the type of music.
Author: Estève Fabry
Technical sheet: KEF LS50 Meta
- Origin: United Kingdom
- Price: €1,199 (per pair)
- Dimensions: 302 x 200 x 280.5 mm
- Weight: 7.8 kg
- Recommended amplifier power: 40 W to 100 W
- Frequency response: 47 Hz to 45 kHz (at -6 dB)
- Sensitivity: 85 dB (for 2.83 V at 1 m)
- Impedance: 8 ohms (3.2 ohms minimum)
- Connection frequency: 2.1 kHz
- Tweeter section: 25 mm aluminium dome with MAT damper
- Woofer section: 130 mm anodised aluminium cone
