KEF R&D

BLADE
THE REFERENCE

CONTENTS

2

Introduc�on2

Philosophy

2

BLADE

3

THE REFERENCE

3

High End 12th Genera�on Uni-Q with MAT

3High Frequency Driver

4

Metamaterial Absorp�on Technology

4

Coupling MAT to the tweeter dome

4

Tangerine Waveguide S�ffening Ribs

5

HF Motor System

5

Tweeter Gap Damper

6Midrange Driver

6

Ultra-Low Distor�on MF Motor System

8

MF Surround and Spider

9

Flexible Decoupling Chassis

11

Crossover12

Summary

13

Appendix (Blade)13

Single Apparent Source

13

Force Cancelling

13

Cabinet Design

14

References

15

BLADE - Model Informa�on, Specifica�ons and Measurements

18

THE REFERENCE - Model Informa�on, Specifica�ons and Measurements

₁

Introduc�on
The 12th Genera�on of KEF’s signature drivertechnology – Uni-Q – is amongst one of the mostsignificant in the company’s history. Each itera�on up to
this point has sported considerable new technologies
leading to improved clarity, detail, and performance.
The debut of this genera�on arose with R Series (2018),
and the introduc�on of the Tweeter Gap Damper. This
dealt with the annular channel resonances created by
the gap formed between the midrange cone and thetweeter housing. This was soon followed by the world’s
first use of a metamaterial in a loudspeaker – KEF’s
patented Metamaterial Absorp�on Technology (MAT) –first developed for the LS50 Collec�on (LS50 Meta andLS50 Wireless II).
With KEF celebra�ng its 60th anniversary, and a�er the
success experienced in implemen�ng MAT in the LS50
Collec�on, it naturally followed to incorporate thistechnology into KEF's high-performance ranges.
However, the engineering brief for the new Blade and
The Reference upgrades went beyond simply
incorpora�ng MAT into their exis�ng Uni-Qs. As is
explained in this document, this project reassessed alltechnologies u�lised by KEF to date and introduces
fresh innova�ons, further cemen�ng Blade andReference as leading loudspeaker designs.
The result is the most advanced, highest performing
speakers KEF has ever produced, where the approachhas never been closer to Raymond Cooke’s original
design philosophy.
This paper will explain these new features. An appendix
is provided to discuss Blade’s Single Apparent Source
technology in more depth. Interested readers can also
refer to the Reference 2014 White Paper [1] for more
informa�on on Reference technologies (LF drivers,
cabinet, and port design), which also serves as an
excellent general discussion on loudspeaker design.

Philosophy“Of all art, music is the most indefinable and the mostexpressive, the most insubstan�al and the most immediate,
the most transitory and the most imperishable. Transformed
to a dance of electrons along a wire, its ghost lives on. WhenKEF returns music to its righ�ul habitua�on, your ears andmind, they aim to do so in the most natural way they can...
without drama, without exaggera�on, without ar�fice.”
Raymond Cooke OBE, KEF Founder

The first commercially available itera�ons, Blade andlater Blade Two, were thus a feat in electroacous�ctechnological advancement, which incorporated a fis�ulof revolu�onary technologies that deserve to be listed:

These words were penned shortly a�er KEF’s founding
in 1961. Cooke was an avid music lover and his mission
from the outset was to enable a wide audience to derive
the same pleasure from music as he did. He sought todeliver to his customers the immersive experience of alive performance through recordings.

•
•
•
•

Cooke realised that this could only be achieved throughscien�fic understanding of sound and its reproduc�on.
He concentrated his efforts on loudspeakers - the lastand probably most difficult link in the chain, in that theymust work in an unknown three-dimensional
environment. Never afraid to employ the most capable
engineers and provide them with the latest and mosteffec�ve tools, Cooke established an engineering
philosophy that s�ll exists today.

•

world's first Single Apparent Source speakers [2]powerful vented tweeter with s�ffened dome andTangerine Waveguide [3]op�mised midrange cone shape with smoothsurround, hybrid s�ffened cone and nodal drive [4]LF drivers with vented coupler, diaphragmdecoupling, and force-cancelling configura�onultra-low diffrac�on cabinet made of a s�ff glassreinforced composite material

Uni-Q is now in its 12th genera�on, which began with R
Series (2018). The new defining technology was the
Tweeter Gap Damper. LS50 Meta soon followed, adding
Metamaterial Absorp�on Technology. For the new Bladeand The Reference series, KEF engineers took theopportunity to redefine the state of the art and provide
a flagship technological example of the latest andgreatest 12th genera�on Uni-Q (Figure 1).

The latest Blade range consists of two floorstanding
models, the Blade One Meta, and its smaller sibling theBlade Two Meta.

THE REFERENCE

KEF engineers con�nue to pursue this scien�fic
endeavour, using tools and technology that were
unavailable to Cooke in those early days. The listening
experience is s�ll the final arbiter in deciding whether or
not the science is effec�ve, but without scien�fic
method and analysis, progress cannot be measured, andthe way forward cannot be understood.

The first KEF Reference speaker, Model 104, waslaunched in 1973. As a direct result of KEF’s earlyadop�on of computer aided digital design andmeasurement, Model 104 represented a new level of
performance that could be achieved within the confinesof normal rooms. Over the following decades, The
Reference would generally be where newest
technologies were introduced. As with all KEF ranges,
updates and redesigns are only considered when asignificant improvement in performance can bedelivered.

BLADE
Blade was originally a ground-up development, with theaim of producing a loudspeaker that could be theul�mate expression of KEF’s ideal. The result was acoherent sound source that acous�cally resembled asingle point across the en�re frequency spectrum.

The Reference range consists of a standmount model(Reference 1 Meta), a pair of floorstanding models(Reference 3 Meta, Reference 5 Meta) and two centre
channels (Reference 2 Meta, Reference 4 Meta), making
it a flexible range for both two-channel and hometheatre applica�ons.

Making its debut at the 2009 Munich High End Show,
Concept Blade was the world’s first speaker to feature
KEF’s Single Apparent Source technology – an extensionof the Uni-Q concept across the full bandwidth,
ensuring matched direc�vity over a wide area across thefull frequency range.

High End 12th Genera�on UniQ with MAT

However, with its carbon fibre/balsa wood cabinet
construc�on, Concept Blade would have been too
expensive to consider as a produc�on model. During thesearch for a cabinet material that could allow Blade to bea feasible proposi�on, further research was conducted
into how to improve the system beyond theachievements of Concept Blade.

Over the last 30 years, KEF has developed over 50
examples of the Uni-Q driver array, designed specificallyfor the range or models in mind. In addi�on, these 50+
itera�ons have been grouped into various genera�ons.
Within each genera�on, different Uni-Qs present
various levels of performance, with a common
technological thread.

₂

Figure 1. Cut-away of the bespoke high end 12th Genera�on Uni-Q with MAT

This itera�on of Uni-Q presents an advancement inevery single aspect of the driver array and every single
component has been revised without compromise. Bothmidrange and tweeter have been designed from theground-up for this applica�on, ensuring seamlessintegra�on of both drivers into one, both acous�callyand as a physical package. To do this, several exis�ng
and new technologies have been developed andsuccessfully incorporated, achieving levels of
performance not previously available.
More informa�on on the concept of Uni-Q, as well as anaddi�onal discussion on various common technologies
can be found in the 2014 Reference White Paper [1].

High Frequency Driver
The tweeter of this new Uni-Q has undergone
significant work. In similar fashion to the LS50 Meta, theintroduc�on of MAT prompted the requirement andopportunity to fully redevelop the tweeter geometry
and its components. This not only allows for the
acous�cal ven�ng of the rear of the tweeter into the₃

MAT disc, but simultaneously improves motor linearity,
as well as the mechanical and acous�cal behaviour of its
waveguide and its interface with the midrange driver.

Metamaterial Absorp�onTechnology

To reduce this deforma�on and the resultant distor�on,strengthening ribs were added to the rear of theTangerine Waveguide (Figure 6). The resultant decreasein displacement is shown in Figure 7.

Si�ng at merely 11mm deep, it is comparable inperformance to a well-designed tapered tube measuring
50cm long. This allows its inclusion into loudspeakers ofany size, without requisi�oning excessive acous�cvolume from the cabinet [5][6].

Metamaterial Absorp�on Technology (MAT) was first
introduced with the LS50 Collec�on (2020). This mazelike disc (Figure 2) sits behind the tweeter, and its
func�on is to absorb the rear sound wave radiated by
the tweeter dome. This wave would otherwise be asource of distor�on when being reflected back into thedome.

HF Motor System
Determining the op�mal shape for the opening of theconical waveguide coupling the rear of the tweeter
dome to the MAT disc was accompanied by a complete
restudy of the tweeter motor system geometry and its
electroacous�c performance.
For a descrip�on of the steps involved in redeveloping atweeter motor system to accommodate a wider
waveguide through its central pole, while allowing
reduced inductance and maintaining similar sensi�vity,
the reader can refer to the 2020 LS50 Collec�on white
paper [6].

Tweeter Gap Damper
Figure 4. Absorp�on at the entrance of the conical duct, immediately behind thedome

Coupling MAT to the Tweeter Dome

Figure 5. FEA simulation of exaggerated deformation of tangerine waveguide andsurround support at 12kHz

Of equal importance to the applica�on of MAT is how
the back wave is shepherded into the disc. This requires
a complete redesign of the area behind the tweeter toform a waveguide with par�cular characteris�cs.

Figure 2. Metamaterial Absorp�on Technology (MAT)The MAT disc comprises 30 channels of differing
lengths, formed into tubes sharing an opening on one
end and closed on the other. The tubes act as quarterwave resonators, each tuned to a different frequency
with a high Q, which effec�vely absorb a narrowfrequency band and its harmonics (Figure 3). The
absorp�on of these channels is tuned to overlap infrequency, leading to almost 100% absorp�on acrossthe spectrum above 620Hz - well below the lower
threshold of the tweeter’s working bandwidth (Figure 4).

One of the problems with construc�ng a combina�ondriver array like Uni-Q is dealing with the cons�tuent
parts. There is a narrow channel - an annular gap between the moving midrange voice coil and thesta�onary tweeter waveguide. This channel acts as anorgan-pipe resonator when excited by the tweeter
output. The resonances modify the response of thetweeter, adding a series of glitches that are not present
if the gap is closed off - simula�ng a perfectly smooth
waveguide.

Essen�ally, the acous�c impedance of the waveguidemust match that of the opening of the MAT disc to avoid
a reflec�on of the wave within the waveguide back into
the tweeter dome. This led to designing a tapered duct,
which reduces in diameter towards the disc opening
with a conical profile. This inverted conical horn
eliminates said reflec�ons, but also fulfils the design
requirements of allowing for easier accommoda�on of
the absorber into the driver package and reducing thesize of the MAT disc itself.
Figure 6. Modified surround support and tangerine waveguide viewed from the rear toshow added support ribs

In addi�on, the sheer increase of acous�c volume
behind the tweeter increases the ven�ng and reduces
non-linear distor�on related to the spring effect ofcompressing this rear acous�c volume.
A detailed explana�on and the mathema�cs behind
MAT and the coupling technique can be found in theoriginal Audio Engineering Society paper [5].

The annular gap is necessary to allow the MF cone andvoice coil to move. Blocking it by means of a surround ora similar device can result in more detriments thanperformance benefits. The solu�on was to create an
acous�cal cavity packaged between the midrange andtweeter motors, to which this annular gap connected(Figure 8). Adding acous�c damping to this newly
created cavity was found to be effec�ve in taming theresonances in the annular gap. The removal of the
response glitches was immediately apparent as an
improvement in detail clarity (Figure 9).
Further performance improvements were gained
through careful shaping of the cavity, and the addi�on of
a second ring of damping material, as per LS50 Meta [6].

Tangerine Waveguide S�ffening Ribs

Figure 3. Pressure response at closed end of each absorber chamber

The Tweeter Gap Damper is an established technology
for the 12th Genera�on of Uni-Q – in fact, its
development is what defined the move from the 11th
Genera�on.

A�er applying MAT to the LS50 Collec�on Uni-Q, a lowlevel residual coloura�on caused by the deforma�on of
the plas�c components of the Tangerine Waveguide andthe dome surround support was detected (Figure 5) [7].

Figure 7. Simulated displacement of tangerine waveguide₄₅

Ultra-low
SystemDistor�on

MF

Motor

The motor force factor BL is the product of the flux
density B of the magne�c field crossing the voice coilgap, and the voice coil length L dipped in that magne�cfield. BL is a func�on of the voice coil’s displacement.
Typically, as the voice coil moves away from the gap, BL
decreases as the length of coil present in the gap isreduced. This modulates the force applied to the voicecoil and thus distorts the signal being reproduced – theway the voice coil reacts to the input signal changes
based on coil posi�on. This is a well-understood problem
and is one of the main sources of harmonic distor�on indrivers.

The MF motor system is a complete ground-up design. It
has been engineered with the specific goal of reducing
distor�on due to force factor modula�on and voice coilinductance. Addi�onally, it must now accommodate theconical waveguide connec�ng the rear of the tweeter
diaphragm to the MAT, as well as the Tweeter Gap
Damper in the same available space as before. This
makes for a much more ambi�ous packaging endeavour
than has previously been a�empted in a Uni-Q. Figure
10 shows a sec�on of the magne�c circuit of thetweeter and midrange motors.

The new split top plate design focuses the magne�c flux
available from the motor’s Neodymium ring magnet
away from the centre of the voice coil towards its ends.
This creates a magne�c flux density profile B(x) that
decreases around the voice coil’s rest posi�on and isshaped like an ‘M’ (Figure 12).

Figure 8. Line drawing showing the mechanism of the Tweeter Gap Damper

As AC current flows through the voice coil, an
alterna�ng magne�c field is produced. Its strength isdependent on the inductance of the voice coil, withhigher inductance producing a stronger field. This AC
magne�c field is conducted by the steel in the motor
system, superimposed on the DC field due to thepermanent magnet.
Steel is highly magne�cally non-linear, and thesuperimposed AC magne�c field causes some of themagne�c domains within the steel to reorient. This
results in a shi� in the permeability of the steel, leading
to modula�on of the DC magne�c field, and a
modula�on of the motor BL. These sudden shi�s in themagne�c domains are also picked up as induced voltage
signals in the voice coil, corrup�ng the music signal. Thisbehaviour is highly hysteric, and the generated
distor�on has a par�cularly unpleasant characteris�c.
This is not the only issue, however. Inductance itself
varies with the posi�on of the voice coil - where thevoice coil is constantly being pulled towards where
inductance is highest. This is known as ‘reluctance force’and is highly non-linear as it is propor�onal to the squareof the current flowing through the voice coil.
The new Uni-Q tackles these issues in two ways. Firstly,
it is possible to reduce the impact of the voice coil’s
magne�c field by using a stronger permanent magnet in
the motor. The stronger the magnet (all else being equal),
the lesser the effect of the voice coil’s magne�c field.

Figure 10. Uni-Q motor system magne�c circuit

The top plate of the MF motor, which together with thecentral pole forms a long gap in which an underhung
voice coil moves, is an unconven�onal design consis�ngof two sec�ons separated by an air gap. In the gap
between the two sec�ons, and slightly indented, sits acopper ring which is aligned with the centre of the voicecoil (Figure 11).

Figure 12. New motor’s B(x)The resul�ng BL(x) is thus fla�er along the voice coil’s
excursion of +/- 2mm compared to previous designs(Figure 13). A fla�er BL(x) means the force applied to thevoice coil will more closely be able to follow the applied
signal.

Figure 9. Improvement in frequency response due to the Tweeter Gap Damper

Midrange Driver

To achieve this, an extremely strong Neodymium ringmagnet has been u�lised. Neodymium magnets exhibit
a far higher energy product than Alnico magnets(sintered Alnico exhibits in the region of 10-88kJ/m³,
whereas sintered Neodymium magnets are in the regionof 200-400kJ/m³).
The motor geometry has been addi�onally op�mised toincrease satura�on of the steelwork to reduce its
magne�c permeability, thus decreasing its suscep�bility
to be magne�sed by the voice coil’s AC magne�c field.

The midrange driver has also received significant
upgrades and improvements over both the previous
Blade and The Reference Uni-Q drivers. As with the highfrequency driver, a major aim was to improve motor
linearity. This was done through a new motor system,
improved suspension and a redesigned spider.

Secondly, in addi�on to two Aluminium rings si�ng oneither side of the voice coil gap, a wide copper insert has
been placed within the air gap created by the split top
plate.

Work was also carried out on a novel, decoupling chassis
to reduce unwanted sound emana�ng from the rim of
the chassis.
Figure 11. Detail of the MF motor

Figure 13. Comparison of BL(x)₆

Conduc�ve regions like these couple to the voice coiland allow the flow of induced current through them.
This then produces an opposing magne�c field to the₇one created by the voice coil, further reducing the abilityof the voice coil to magne�se the motor steel.

reduc�on in Total Harmonic Distor�on in the midrange
between the previous Reference 1 and the new
Reference 1 Meta.

1.6

Figure 17 shows a comparison of the simulated
frequency response of the 2014 Reference MF driverand the new MF driver. The disappearance of the dip at
650 Hz caused by the spider resonance is clearly
apparent.

0.8

1.4
1.2
1

This arrangement is par�cularly advantageous, however.
Si�ng right in the middle of the two top plate sec�ons
and aligned with the voice coil’s centre along its length,
the conduc�ve copper ring’s effect on lowering
distor�on is much greater, and symmetrical withdisplacement. Careful determina�on of its dimensions
serves to maximise its effect with minimum use ofmaterial.
Figure 14 shows a visualisa�on of the flux density
modula�on regions present in the steelwork of the
motor near the voice coil gap at a representa�vefrequency. It is worth men�oning this performance isalready class leading. However, a considerable further
reduc�on can be observed when the op�mised
conduc�ve regions of Aluminium and Copper are added
to the motor.

width decreases the mass of the spider, which has theeffect of increasing the resonant frequency out of the
passband.

100

0.6
0.4
0.2
0
-2

(dB) Sound Pressure

-1.5

0.5
1.5
1
2
Displacement (mm)Mmd(x) of 2014 Reference MF
-1

-0.5

0

Mmd(x) of 2022 Blade/Reference MF

90
1.6

80

1.4
1.2

70

1

60

Figure 15. MF voice coil inductance improvement

0.5

50
20

(%) THD

0.8
0.6

50

100

200

500

1k

2k

5k 10k 20kFrequency (Hz)2014 Reference MF

0.4

2022 Blade/Reference MF

0.3

Figure 17. MF frequency response showing the spider ‘dip’ dele�on
0.2

One of the quintessen�al characteris�cs of Uni-Q is its
uninterrupted tweeter waveguide. To achieve this ideal,
the surround must be as flat as possible to allow forsmooth propaga�on of the sound wavefront travellingout along the tweeter waveguide. Any irregularity or
obstacle would simply interrupt and smear the
wavefront. At the same �me, the surround can be an
important source of non-linear distor�on. As thesurround moves with cone displacement, it deforms. As
its shape changes, its s�ffness, radia�ng area andmoving mass vary.

0.1
0
200

300

400 500600

Reference 1

800 1k

2k

3k
4k 5k
Frequency (Hz)
Reference 1 Meta

Figure 16. Comparison of THD (%) of Reference 1 (red) and Reference 1 Meta (blue)at 90dB SPL @ 1 metre

Finally, the copper ring efficiently dissipates heat awayfrom the voice coil. This reduces thermal compression of
the signal whilst improving efficiency.

MF Surround and Spider

For this design itera�on, the challenge was to increase
the midrange excursion whilst maintaining linear
behaviour. At the same �me, no obstacle should bepresented to the tweeter wavefront. The profile of thesurround was examined in detail to improve symmetricals�ffness along the cone’s displacement and, for the first�me, research was carried out to develop a surroundthat presented minimum varia�on of its mass andradia�ng area as it deforms with displacement.

For the new Uni-Q, both the surround and the spider
have been redesigned to improve their linearity andreduce harmonic distor�on.

Figure 14. MF flux density modula�on at 500Hz without conduc�ve regions (top)and with conduc�ve regions (bo�om)

As the spider (suspension) moves back and forth withthe voice coil, a dip in the driver frequency responsedevelops. This occurs at the spider’s resonant frequency,
which is dependant on its s�ffness and mass. It is very
easy for this resonance to sit in the passband of a
midrange driver. Shi�ing it out of this region ischallenging since it requires a very low mass while s�llallowing the driver to move its full linear excursion.

Figure 15 shows the overall reduc�on in voice coilinductance across frequency, whilst Figure 16 shows the

The Uni-Q redesign has allowed the reduc�on of the
spider’s width to merely 0.69cm (0.27”). Decreasing the

Figure 18 shows the normalised varia�on of thesurround’s radia�ng area Sd and moving mass Mmd withcone displacement x. The reduc�on in varia�on in the
new design is clear in both metrics.

₈

0.4
0.2
0
-2

-1.5

0.5
1.5
1
2
Displacement (mm)Sd(x) of 2014 Reference MF
-1

-0.5

0

Sd(x) of 2022 Blade/Reference MF

Figure 18. Varia�on of Mmd (top) and Sd (bo�om)

The design of both components together ensures thatthe cone excursion is linear in both direc�ons, reducing
distor�on of the signal even at high output levels.

Flexible Decoupling Chassis
Rigid affixing of the driver to the cabinet allows thedriver to directly excite the cabinet. Since cabinets areusually comprised of large flat sheets of material withlimited s�ffness and mass, this can result in the walls of
the cabinet being easily excited. Since the cabinet walls
have a large surface area, this excita�on can efficiently
transform into unwanted acous�c radia�on.
Much can be achieved by designing the cabinet to bes�ff, heavy, and well damped, to avoid transmi�ng thisvibra�on on as sound.
KEF has been experimen�ng with driver decoupling eversince Model 105.2 in 1979. For this loudspeaker, KEF
developed a decoupling method colloquially known as
the ‘KEF mount.’ It consisted of a metal apparatus with arubber grommet screwed into the cabinet. The driver
was in turn mounted to the apparatus. This decoupled₉the driver from the cabinet. Work was also carried out toensure the resonant frequency of the decoupling
matched the resonance of the speaker in the cabinet tolimit the effect on the frequency response.
A common decoupling method involves moun�ng thedriver onto the cabinet through a so� compliant element
under the chassis rim. Whilst this approach does reducetransmission of vibra�on into the cabinet, it comes with
its own drawbacks. Since the motor and chassis
assembly is suspended, when excited by the force of thevoice coil, it is allowed to displace substan�ally over thedriver’s
resonance
frequency. This
increaseddisplacement of the chassis rim, mul�plied by its large
area rela�ve to the driver’s diaphragm, results inconsiderable unwanted acous�c radia�on, which, tosome extent, defeats the purpose of the decoupling
method (Figure 19).

CrossoverFor the blue curve the new decoupled chassis is used but
no damping has been added. Finally, for the green curve
suitable damping has been added to the decoupledchassis, as implemented on the new Uni-Q.

Since the introduc�on of the original Blade, theengineers at KEF R&D have been working on thecon�nuous development of new measuring techniques
and in-house simula�on so�ware to further improvetheir understanding of how a speaker design performs.
In both Blade and The Reference, there was anopportunity to completely redesign the crossovernetworks that integrate the output of the different
drivers, taking full advantage of the increased
capabili�es of the 12th genera�on Uni-Q specificallydesigned for this applica�on.

The effect of the Flexible Decoupling Chassis on thevibra�on of the cabinet is readily apparent. Two
phenomena are taking place. Firstly, a peak in vibra�on
associated with the en�re mass of the driver resona�ngagainst the cabinet is 'split’ into two peaks out of themidrange frequency band, due to the sprung mechanism
decoupling the mass of the MF motor. Secondly, thepeaks in vibra�on are heavily reduced by adding
damping material in parallel with the sprung mechanism
and thus dissipa�ng energy. The net result is much lower
excita�on of the cabinet across the frequency range
covered by the midrange driver.

Firstly, the higher excursion and increased linearity of
the MF driver afforded the engineers more flexibility to
specifically improve its acous�c integra�on with the LF
drivers.
This prompted a re-study of the off-axis radia�on of the
LF and MF Blade drivers and their complex interac�onaround the crossover region, both in the horizontal andver�cal direc�ons. As a result, the MF driver polarity has
been inverted and the filter transfer func�ons have been
designed to priori�se smoothness in all direc�ons andreduce any troughs and peaks previously appearing inthe speaker’s off-axis response around the crossover
region. Figure 23 shows a comparison of the horizontal
dispersion of Blade Two and Blade Two Meta as acontour map, where the improvement in smoothness
and the direc�vity taper are apparent.

Figure 20. Chassis and upper motor sec�on showing Flexible Decoupling Chassis
arrangement with decoupling material highlighted in green

Figure 19. Illustra�ons of vibra�on caused as the voice coil moves when a driver iscoupled to (top) and decoupled from (bo�om) the cabinet

This new Uni-Q approaches the issue slightly differently,
building decoupling into the chassis itself (Figure 20).
The decoupling separates the motor assembly from therest of the driver, and thus the cabinet (Figure 21). This
reduces the sprung mass and allows for be�er control ofthe decoupling mechanism. The motor is suspended andconnected to the chassis through eight flexible springelements integrated into the chassis moulding. Where
the spring elements end the rigid chassis begins andpads of a suitable damping material provide energy
dissipa�on in between.
Figure 22 shows a comparison of measured point laser
velocity on a speaker baffle when excited by a sinusoidal
chirp fed into the midrange driver, using three different
chassis configura�ons. For the red curve the rigid
Aluminium chassis from the 2014 Reference line is used.

Secondly, the improved break-up behaviour of themidrange and the improved lower-end response andlower distor�on of the tweeter allowed the redesign of
the MF-HF crossover, to be�er integrate the radia�on ofboth drivers and further smooth the system’s off-axis
response.
In passive crossover design the crossover cut-off
frequency and filter slopes between the midrange andthe tweeter are typically chosen to protect the tweeter
from genera�ng high distor�on and overhea�ng. In a
speaker incorpora�ng a Uni-Q, however, the crossover isan integral part of matching the direc�vity of themidrange and the tweeter, so that the transi�on
between the two is seamless to the ears.
Figures 24 and 25 (following page) show an example
speaker with a Uni-Q and a crossover set at 2 kHz and 4
kHz, using the same filter slopes (acous�c 4th order
Linkwitz-Riley). The on-axis frequency response is flatand is the same in both cases. A second curve, the power
average, is calculated by averaging the speaker response
in all direc�ons. This curve loses its con�nuity andsmoothness in the case where the crossover frequency
is 4 kHz, as the midrange becomes more direc�onal than
the tweeter. The direc�vity of the midrange and thetweeter in the Uni-Q is not matched anymore. This
demonstrates the principle of direc�vity matching in a
Uni-Q driver array to provide a smooth dispersion acrossthe frequency spectrum. This emphasises how this ispossible only when both drivers already have a superbly
smooth and wide frequency response, as is the case withthis Uni-Q.
Lastly, research conducted during the development of
The Reference (2014), R Series (2018) and LS50 Meta
(2020) has provided evidence for an improved choice of
inductors, capacitors and resistors that directly results inlower system distor�on and improved signal integrity.

Figure 21. Illustra�on of the decoupling mechanism

All these improvements in the crossover together havehelped the engineers achieve a substan�ally smootherfrequency response in any direc�on, free of ar�facts anddiscon�nui�es and clean of resonances for all speakers
in the Blade and The Reference series. The result has
been a notable increment in stereo image stability,
realism and spa�al depth, and in preserving the true
�mbral quali�es of a musical signal within any listening
room in which the speakers are posi�oned.

Figure 23. Horizontal dispersion of Blade Two (top) vs. Blade Two Meta (bo�om)

Figure 22. Effect of chassis decoupling on baffle vibra�on

₁₀₁₁

Appendix (BLADE)
Summary

Single Apparent Source

Simply rebuilding a high-end Uni-Q around the new
12th genera�on technology addi�ons would havegarnered improvements, but this is not the way KEF
operates. Prior research and development help to inform
some of the ways to improve performance, but therequirements and expecta�ons of what a KEF
loudspeaker range should achieve must always be
central – and never has that been truer.

In tradi�onal loudspeaker designs, sound is radiatedfrom different sources covering the audible frequency
spectrum. As these sources are separated in space andplaced in different posi�ons, this causes issues in theway sound arrives at the listener - difference indirec�on, �mbral changes due to cancella�on andinterference between sources, and poor transientalignment.

Even a�er 60 years, the same guiding philosophy anddedica�on to excellence in engineering laid down by
Raymond Cooke con�nue to lead the march forward.
These new incarna�ons of Blade and The Reference are
the reward for maintaining that approach, a ground-up
redesign represen�ng the pinnacle of performance.

100(dB) Sound pressure

100

90

90

80

80

70

70

60

60

50
50

100

200

500

1k

2k

5k
10k 20k
Frequency (Hz)Uni-Q tackles these issues in the midrange and highfrequencies. As a coincident source, the sound from thetwo cons�tuent radia�ng elements travels the same
path to the ear - meaning sound emanated by both
sources at the same �me across the frequency region inwhich they crossover will not interfere destruc�vely andform regions of a�enuated output around the speaker.
According to Linkwitz, this arrangement enables the�mbral proper�es of reflected sound in most rooms tobe very similar to the direct sound, allowing the humanbrain to filter out the room much more effec�vely [8].

(dB) Sound pressure

50
50

100

200

500

1k

2k

Summed Acoustical Pressure

Summed Acoustical Pressure

MF

MF

HF

HF

100

Directivity Index (dB)(dB) Sound pressure

10k 20k
5kFrequency (Hz)Directivity Index (dB)

50

100

90

40

90

40

80

30

80

30

70

20

70

20

60

10

60

10

50
50

100

200

500

1k

2k

0
10k 20k
5kFrequency (Hz)

50
50

100

200

500

1k

2k

On AxisOn AxisPower Average

Power Average

Power Average Directivity Index

Power Average Directivity Index

With Blade, the LF drivers are securely bonded together,back-to-back, through a combina�on of threaded bolts
and the same bituminous material used in Constrained
Layer Damping, as found in many other KEF
loudspeakers. The reac�ve force of one is cancelled out
by the other member of the pair (Figure 26).
In fact, when demonstra�ng Blade, many KEF
employees will place a coin standing upright at the apexof the cabinet, followed by playing wide bandwidthmusic at high output. Due to the vanishingly low
resonance of the cabinet, the coin will con�nue to stay
upright.
Combined with the curved cabinet construc�on and theglass reinforced composite material used, Blade requires
minimum bracing, allowing for a maximum of available
acous�c volume.

With Single Apparent Source, the concept of Uni-Q isfurther extended to the LF drivers of a three-way systemdesign. It has been found that large public address horns,
given enough distance, create an observed point source
due to the wave-front curvature. Single Apparent Source
begins to u�lise this idea by posi�oning four iden�cal LF
drivers equidistant around the Uni-Q in both thehorizontal and ver�cal axes. At a distance, this creates anapparent acous�c source for the four LF drivers.

Figure 24. Effect of crossover frequency on Uni-Q driver array’s frequency
response when crossover is at 2kHz (le�) and 4kHz (right)(dB) Sound pressure

without a force-cancelling configura�on driver mo�oncreates an equal opposing force that acts upon thecabinet, causing colora�on.

Comple�ng the puzzle is the rela�onship between the
LF drivers and Uni-Q array. The LF-only apparent source
sits on the same axis horizontally and ver�cally as the
Uni-Q. As the axial posi�on of the two acous�c centres
are excep�onally close together, within a frac�on of the
wavelengths generated by the LF drivers, at a distancethese two sources appear to merge, crea�ng a single
apparent source.

50

0
10k 20k
5kFrequency (Hz)Force Cancelling
An addi�onal benefit of Single Apparent Source is that
the LF driver arrangement creates a force-cancelling
effect. The science behind force-cancelling is a major
topic in the KC62 white paper [9], but to summarise,

Figure 25. Effect of crossover frequency on Uni-Q driver array’s power average anddirec�vity index when crossover is at 2kHz (le�) and 4kHz (right)

₁₂

Figure 26. Illustra�on of reac�ve force on a tradi�onal cabinet and the lack thereof
in Blade’s force-cancelling arrangement

Cabinet Design
KEF has long worked to combine acous�cs and design insuch a way that the two become more than partners aesthe�cs and performance are not mutually exclusive
endeavours, and the shape can absolutely work to thebenefit of performance.
The performance of the Single Apparent Source concept
is directly related to how �ghtly the drivers are packed

₁₃together on all axes. This necessitates a slim cabinet
profile to posi�on the side-firing LF drivers as close to
the Uni-Q array as possible.
The original industrial design was carried out by KEF’s
R&D and Product Planning & Design teams, inconjunc�on with Eric Chan of ECCO Design in New
York. The parabolic shape was inspired by Bird in Space,a series of 1920s sculptures by Romanian-born
Constan�n Brancusi. The curved form is not only slim,
helping to maximise the poten�al of the Single Apparent
Source concept, but is also free of sharp edges.
Diffrac�on is related to the rela�onship between the
wavelength of a frequency and the distance between
sound source and an irregularity - say a cabinet corner. If
the wavelength is sufficiently longer than this distance,
diffrac�on is essen�ally minimal [10]. If Blade were justas slim but with a rec�linear box construc�on,considerable diffrac�on effects would occur, albeit at
higher frequencies. The fact that Blade is curved meansthat it has innumerable corners with infinitesimally small
distances between them, greatly reducing the nega�veeffects of diffrac�on.
KEF pioneered the use of separate enclosures fordifferent drivers when first employing the Total System
Design approach with Reference Model 105 in 1977.
The idea has certainly not been abandoned – instead,the compartmentalisa�on is internalised within thecabinet of Blade (and The Reference). Uni-Q sits within
its own can structure, with a finely tuned internal
volume for op�mal acous�c loading. The two pairs of LF
drivers also sit within their own compartments. These
are of equal volume, and each is served by its own bass
port. The size and shape of these compartments havebeen calculated to push standing wave resonances well
above the working bandwidth of the bass drivers.

References

BLADE[1] KEF Research and Development, “THE REFERENCE
White Paper”, 2014

Model Informa�on, Specifica�ons andMeasurements[2] M. Dodd and J. Oclee-Brown, “Design of a Coincident
Source Driver Array with a Radial Channel Phase-Plugand Novel Rigid Body Diaphragms”, 127th Audio Eng.
Soc. Conven�on, 2009[3] M. Dodd and J. Oclee-Brown, “A New Methodology
for the Acous�c Design of Compression Driver Phase
Plugs with Radial Channels”, 125th Audio Eng. Soc.
Conven�on, 2008[4] M. Dodd, “Op�mum Diaphragm and Waveguide
Geometry for Coincident source Drive Units”, 121st
Audio Eng. Soc. Conven�on, 2006

[5] S. Degraeve and J. Oclee-Brown, “Metamaterial
Absorber for Loudspeaker Enclosures”, 148th Audio
Eng. Soc. Conven�on, 2020

[6] KEF Research and Development, “LS50 Collec�onWhite Paper”, 2020

[7] KEF Research and Development, “R Series White
Paper”, 2017[8] S. Linkwitz, “Room Reflec�ons Misunderstood?”, 123rd
Audio Eng. Soc. Conven�on, 2007

[9] KEF Research and Development, “KC62 White Paper”,
2021[10] D.A. Barlow, “The Sound Output of Loudspeaker
Cabinet Walls”, 50th Audio Eng. Soc. Conven�on, 1975

₁₄

₁₅

Blade One Meta

Blade Two Meta

Three-way Single Apparent Source
Loudspeaker

Three-way Single Apparent Source
Loudspeaker

Blade One Meta is the flagship model of the range.
Featuring the Single Apparent Source driver layout,
Blade extends the benefits of Uni-Q across theen�re audio range.

Perfect for smaller rooms, but when nothing but theabsolute best will suffice. Blade Two Meta u�lises
the exact same sized Uni-Q as its larger sibling,
pairing it with four 165mm (6.5 in.) bass drivers

Owing to its large cabinet size and use of four
225mm (9 in.) bass drivers, it is capable ofeffortlessly producing high output levels into thelower frequencies, alongside a detailed top end andlifelike midrange. A true statement of sound.

Just as with Blade One Meta, Blade Two Meta
excels in both stereo and home theatre applica�ons- the aim is always the same: to reproduce therecorded sound as accurately as possible.

Technical Specifica�ons

Technical Specifica�ons

SystemThree-way bass reflex
Single Apparent Source

SystemThree-way bass reflex
Single Apparent Source

Drive units

Uni-Q Driver Array:

Drive units

Uni-Q Driver Array:

HF: 25 mm (1 in.) aluminium
dome with MAT
MF: 125 mm (5 in.)aluminium cone

HF: 25 mm (1 in.) aluminium
dome with MAT
MF: 125 mm (5 in.)aluminium cone

Bass Drivers:

Bass Drivers:

LF: 4 x 225mm (9 in.)aluminium cone, forcecancelling

LF: 4 x 165mm (6.5 in.)aluminium cone, forcecancelling

Frequency range free-field(-6dB)
Typical in-room bass
response (-6dB)
Frequency response(±3dB)
Crossover frequencies
Recommended amplifier
power
Sensi�vity (2.83V/1m)

27Hz - 45kHz

Harmonic distor�on
2nd & 3rd harmonics (90dB, 1m)

Maximum output

Frequency range free-field(-6dB)
Typical in-room bass
response (-6dB)
Frequency response(±3dB)
Crossover frequencies
Recommended amplifier
power
Sensi�vity (2.83V/1m)

30Hz - 45kHz<0.5% 40Hz and above<0.2% 200Hz - 2kHz
<0.1% 2kHz - 20kHz

Harmonic distor�on
2nd & 3rd harmonics (90dB, 1m)<0.5% 40Hz and above<0.2% 200Hz - 2kHz
<0.1% 2kHz - 20kHz

117dB

Maximum output

116dB

20Hz
35Hz - 35kHz
350Hz, 2kHz
50 - 400W
88dB(Peak sound pressure level at
1m with pink noise)

100(dB) Sound pressure

Directivity Index (dB)

50

90

40

80

30

70

20

60

10

50
20

50

100 200

500

1k

2k

0
5k 10k 20kFrequency (Hz)On AxisListening Window
Early reflections
Power Average
Early Reflections Directivity Index

25Hz

Power Directivity Index

33Hz - 35kHz
450Hz, 2.2kHz
50 - 400W
86dB(Peak sound pressure level at
1m with pink noise)

Impedance

4 Ω (min. 2.8 Ω)

Impedance

4 Ω (min. 3.2 Ω

Weight57.2 kg (126 lbs)

Weight35.3 kg (77.8 lbs)

Dimensions with plinth(H x W x D)

1590 x 363 x 540 mm(62.5 x 14.3 x 21.2 in.)Dimensions with plinth(H x W x D)

1461 x 338 x 475 mm(57.5 x 13.3 x 18.7 in.)Blade One Meta Spinorama (top) and dispersion plots - horizontal(middle) and ver�cal (bo�om)

₁₆Blade Two Meta Spinorama (top) and dispersion plots - horizontal(middle) and ver�cal (bo�om)

₁₇THE REFERENCE

Reference 5 Meta

Model Informa�on, Specifica�ons andMeasurements

Three-way Floorstanding Loudspeaker
Leading the range, Reference 5 Meta is KEF’s mostaccomplished speaker that uses a modified
D’Appolito layout, be�ered only by Single Apparent
Source. Selectable port tubes allow for the finetuning of the speaker’s bass performance
depending on room size and boundary proximity.

Technical Specifica�ons

SystemThree-way bass reflex

Drive units

Uni-Q Driver Array:
HF: 25 mm (1 in.) aluminium
dome with MAT
MF: 125 mm (5 in.)aluminium cone
Bass Drivers:
LF: 4 x 165mm (6.5 in.)aluminium cone

Frequency range free-field(-6dB)
Typical in-room bass
response (-6dB)
Frequency response(±3dB)
Crossover frequencies
Recommended amplifier
power
Sensi�vity (2.83V/1m)

Long port: 32Hz - 45kHz
Short port: 35Hz - 45kHz
25Hz

Harmonic distor�on
2nd & 3rd harmonics (90dB, 1m)<0.5% 40Hz and above<0.2% 200Hz - 2kHz
<0.1% 2kHz - 20kHz

Maximum output

116dB

40Hz - 35kHz
450Hz, 2.1kHz
50 - 400W
88dB(Peak sound pressure level at
1m with pink noise)

Impedance

4 Ω (min. 3.2 Ω)

Weight60.2 kg (132.7 lbs)

Dimensions with terminals(H x W x D)

1350 x 205 x 462 mm(53.1 x 8.1 x 18.2 in.)1402 x 323 x 467 mm(55.2 x 12.7 x 18.4 in.)Dimensions with plinth andterminals (H x W x D)

₁₈

Reference 5 Meta Spinorama (top) and dispersion plots - horizontal(middle) and ver�cal (bo�om)

₁₉

Reference 3 Meta

Reference 1 Meta

Three-way Floorstanding Loudspeaker

Three-way Standmount Loudspeaker

The smaller of the two Reference floorstanders,
Reference 3 Meta matches its Uni-Q with a pair of
6.5 in. bass drivers. Whilst less of a monolith thanits bigger brother, Reference 3 Meta is more thancapable of filling all but the largest of listening
rooms.

Thanks to the inclusion of Uni-Q, it is possible todevelop a standmount loudspeaker that can truly
compete with larger floorstanders. This three-way
design reproduces clean, accurate sound beyond its
size, throughout its bandwidth.

Technical Specifica�ons

Technical Specifica�ons

SystemThree-way bass reflex

SystemThree-way bass reflex

Drive units

Uni-Q Driver Array:

Drive units

Uni-Q Driver Array:

HF: 25 mm (1 in.) aluminium
dome with MAT
MF: 125 mm (5 in.)aluminium cone

HF: 25 mm (1 in.) aluminium
dome with MAT
MF: 125 mm (5 in.)aluminium cone

Bass Drivers:

Bass Drivers:

LF: 2 x 165mm (6.5 in.)aluminium cone

LF: 165mm (6.5 in.)aluminium cone
Frequency range free-field(-6dB)
Typical in-room bass
response (-6dB)
Frequency response(±3dB)
Crossover frequencies
Recommended amplifier
power
Sensi�vity (2.83V/1m)

Long port: 37Hz - 45kHz
Short port: 40Hz - 45kHz
30Hz<0.5% 40Hz and above<0.2% 200Hz - 2kHz
<0.1% 2kHz - 20kHz

Harmonic distor�on
2nd & 3rd harmonics (90dB, 1m)<0.5% 40Hz and above<0.2% 200Hz - 2kHz
<0.1% 2kHz - 20kHz

113.5dB

Maximum output

111dB

Frequency range free-field(-6dB)
Typical in-room bass
response (-6dB)
Frequency response(±3dB)
Crossover frequencies
Recommended amplifier
power
Sensi�vity (2.83V/1m)

Long port: 35Hz - 45kHz
Short port: 38Hz - 45kHz
28Hz

Harmonic distor�on
2nd & 3rd harmonics (90dB, 1m)

Maximum output

43Hz - 35kHz
450Hz, 2.1kHz
50 - 300W
86dB

45Hz - 35kHz
450Hz, 2.1kHz
50 - 200W
85dB(Peak sound pressure level at
1m with pink noise)(Peak sound pressure level at
1m with pink noise)

Impedance

4 Ω (min. 3.2 Ω)

Impedance

4 Ω (min. 3.2 Ω)

Weight51.3 kg (113.1 lbs)

Weight18.2 kg (40.1 lbs)

Dimensions with terminals(H x W x D)

1155 x 205 x 462 mm(45.5 x 8.1 x 18.2 in.)1207 x 323 x 467 mm(47.5 x 12.7 x 18.4 in.)Dimensions with terminals(H x W x D)

440 x 205 x 422 mm(17.3 x 8.1 x 16.6 in.)Dimensions with plinth andterminals (H x W x D)

Reference 3 Meta Spinorama (top) and dispersion plots - horizontal(middle) and ver�cal (bo�om)

₂₀

Reference 1 Meta Spinorama (top) and dispersion plots - horizontal(middle) and ver�cal (bo�om)

₂₁

Reference 4 Meta

Reference 2 Meta

Three-way LCR Loudspeaker

Three-way LCR Loudspeaker

The larger of the two LCR models, Reference 4
Meta is an ideal match for Reference 5 Meta or
Reference 3 Meta in a high-end home theatre
system. They can even be used as stereo pairs
when built into custom entertainment furniture.

Compact, yet extremely capable, especially within
dual-purpose spaces such as living rooms. Perfectly
at home as a centre/mul�channel system speaker,
or as le�/right stereo pairs.

Technical Specifica�ons

Technical Specifica�ons

SystemThree-way bass reflex

SystemThree-way sealed cabinet

Drive units

Uni-Q Driver Array:

Drive units

Uni-Q Driver Array:

HF: 25 mm (1 in.) aluminium
dome with MAT
MF: 125 mm (5 in.)aluminium cone

HF: 25 mm (1 in.) aluminium
dome with MAT
MF: 125 mm (5 in.)aluminium cone

Bass Drivers:

Bass Drivers:

LF: 4 x 165mm (6.5 in.)aluminium cone

LF: 2 x 165mm (6.5 in.)aluminium cone
Frequency range free-field(-6dB)
Typical in-room bass
response (-6dB)
Frequency response(±3dB)
Crossover frequencies
Recommended amplifier
power
Sensi�vity (2.83V/1m)

65Hz - 45kHz<0.5% 40Hz and above<0.2% 200Hz - 2kHz
<0.1% 2kHz - 20kHz

Harmonic distor�on
2nd & 3rd harmonics (90dB, 1m)<0.5% 40Hz and above<0.2% 200Hz - 2kHz
<0.1% 2kHz - 20kHz

116dB

Maximum output

113.5dB

Frequency range free-field(-6dB)
Typical in-room bass
response (-6dB)
Frequency response(±3dB)
Crossover frequencies
Recommended amplifier
power
Sensi�vity (2.83V/1m)

Long port: 40Hz - 45kHz
Short port: 43Hz - 45kHz
33Hz

Harmonic distor�on
2nd & 3rd harmonics (90dB, 1m)

Maximum output

48Hz - 35kHz
450Hz, 2.1kHz
50 - 400W
88dB

46Hz
80Hz - 35kHz
450Hz, 2.1kHz
50 - 300W
86dB(Peak sound pressure level at
1m with pink noise)(Peak sound pressure level at
1m with pink noise)

Impedance

4 Ω (min. 3.2 Ω)

Impedance

4 Ω (min. 3.2 Ω)

Weight45.2 kg (99.6 lbs)

Weight22.8 kg (50.3 lbs)

Dimensions with terminals(H x W x D)

205 x 1090 x 463 mm(8.1 x 42.9 x 18.2 in.)Dimensions with terminals(H x W x D)

205 x 630 x 335 mm(8.1 x 24.8 x 13.2 in.)

Reference 4 Meta Spinorama (top) and dispersion plots - horizontal(middle) and ver�cal (bo�om)

₂₂Reference 2 Meta Spinorama (top) and dispersion plots - horizontal(middle) and ver�cal (bo�om)

₂₃