KEF R&D

LS50 Meta
LS50 Wireless II

CONTENTS CONT’D

CONTENTS

1Introduction

17

LS50 Wireless II

1

Philosophy

18

Overview

1

LS50 Meta

18

Inputs1

Tweeter18

Connection between Primary and Secondary

2

Metamaterials

19

Streaming support2

Tweeter Metamaterial Absorption Technology

20

DSP Processing

4

Coupling the Absorber to the Tweeter Dome21

EQ Settings

4

Tangerine Waveguide

20

Desk & Wall Modes

5

Tweeter Gap Damper

21

Treble Trim

6

Motor Design

21

Phase correction

7

Bass/Midrange

23

Bass Extension

23

Adding a Subwoofer
Syncing with Vision

7

Motor Design

9

Crossover24

9Industrial Design24

Firmware10

Specification - LS50 Meta

24

Power Amplification

11

Appendix 1 - Cabinet

25

Summary11

Diffraction

26

Specification - LS50 Wireless II

11

Rigid Bracing/Constrained Layer Damping

12

Appendix 2 - Ports

12

Resonances inside the cabinet

12

Organ pipe resonances

13

Turbulence

13

Appendix 3 - Uni-Q

13

Tweeter

13

Diaphragm

14

Tangerine Waveguide

14

Bass/Midrange driver

14

Diaphragm

15

Z-flex Surround16

Summary16

References

Introduction“If I have seen further, it is because I stand on the shoulders
of Giants.”
This famous quotation, attributed to Sir Isaac Newton,
illustrates how progress is made in virtually all
scientific disciplines - progress is evolutionary, rather
than revolutionary. So it is in the development of
speakers and techniques developed for one model are
carried through to subsequent designs and added to.
That small loudspeakers could be serious hi-fi
reproducers was proved in the 1960s with Laurie
Fincham’s Maxim design for Goodmans. Grossly
inefficient, with a cone tweeter and rudimentarycrossover, this speaker was outdated by the 1970s,
but nevertheless inspired the BBC to design thelarger, but still diminutive LS3/5. Originally designed
for ¼ scale auditorium acoustic modelling, this design
was soon recognised for its suitability as a monitor in
the cramped surroundings of BBC Outside Broadcast
vans. The design was subtly changed to become thelegendary LS3/5A when the KEF drive units, withwhich the speaker was equipped, were modified.
As time went by and loudspeaker design progressed,
it was again time to review the capabilities of theLS3/5A and in 2012, to mark the 50th anniversary of
KEF, the original LS50 was born. As the name suggests,
the LS50 Meta is a development of the original LS50,
and again redefines what a small monitor loudspeakercan do.

Philosophy“Of all art, music is the most indefinable and themost expressive, the most insubstantial and themost immediate, the most transitory and the mostimperishable. Transformed to a dance of electrons
along a wire, its ghost lives on. When KEF returns
music to its rightful habituation, your ears and mind,
they aim to do so in the most natural way they can…
without drama, without exaggeration, without artifice.”
Figure 1
LS50 Meta - Carbon Black

These words set the company’s philosophy fromday one and they are as relevant today as they haveever been. Continuing research into materials andengineering has always driven KEF’s quest for
innovation, with the added element of ensuring thatthe resulting designs work within a typical domestic
environment. Everybody deserves great sound.

LS50 Meta
Both LS50 Meta and LS50 Wireless II feature a 12th
Generation Uni-Q driver array featuring an industryfirst - Metamaterial Absorption Technology (MAT).
This particular Uni-Q driver, whilst being inspired bythat of the R Series, has been engineered with regards
to the overall system design.
Firstly, the addition of MAT has necessitatedconsiderable redesign of the driver structure, allowing
KEF engineers to further refine technologies such as
the tweeter gap damper.
Secondly, where R Series features 3-way designs, LS50
Meta is a 2-way loudspeaker, meaning the midrange
driver must cover a much wider bandwidth, down into
bass frequencies. Whilst excellent for integration, thisdoes provide challenges in regards to performance.
With this in mind, KEF engineers made a number ofchanges to the motor system, suspension, surroundand cone of the midrange driver to deliver as smooth aperformance as possible.

TweeterWith any drive unit, as much sound is generated at therear of the unit as at the front (see figure 2) and thisradiation is unwanted and needs to be absorbed.

Raymond Cooke, KEF founder
Figure 2 - Generic driver

1

The most important innovation in this loudspeakeris the near-perfect absorption of the unwanted rearsound generated by the tweeter dome. One might
legitimately ask, “Why is this important?”
It’s true that, if the tweeter is listened to on its own
without the contribution of the bass/midrange driver- and here we are talking of frequencies above about
3kHz - very little seems to come out of it. Morerevealing is to listen to any loudspeaker system
without the tweeter connected. The sound is muffled
and, although one can discern that it’s a piano or aviolin playing, it’s impossible to hear the difference
between a Steinway and a Bosendorfer, a Stradivarius
and an also-ran violin.
The true music lover or audiophile wants to hear thisamount of detail. More than that, one should be able
to imagine the striking of the keys or the way the bow
is stroking the strings. In short, it should be the closest
one can get to a live performance without actuallyturning up.

the mathematical expression between the target
absorption spectrum and the physical realisation,
which has a minimum thickness only 1/10th of the
wavelength at the lowest frequency (620Hz in thiscase).

Tweeter Metamaterial
Absorption Technolology
Figures 3 and 4 respectively show a computer modelof the absorber and the real thing. Its design borrows
from a room acoustics absorber in that it is basedaround ¼ wavelength resonators. Looking rather
like a maze, what you can see are 15 separate tubes,
folded up to make a circular object. In fact, there are 2
layers, so there are, in total, 30 tubes, each tuned to adifferent frequency.

Each tube has a very high Q, which means highabsorption over a very narrow band, and the only
damping, which is the way the energy is dissipated,
comes from the friction between the air moving in
the tubes and the tube walls. The frequency spacing isdesigned so that, when the effects of all the tubes are
combined together, the absorption is uniform over awide band.
Figure 5 shows the pressure response, measured at
the closed end of each of the 30 channels, together
with the absorption spectrum of the whole absorber.

This sort of detail comes from the tweeter andthe better the tweeter, the more the enjoyment oflistening to the music.

Metamaterials
Metamaterials are probably most familiar in the fieldof optics, where synthetic materials may be realisedthat have properties that cannot be found in nature.
For example, a base material may be infused withanother in varying density such that the refractive
index varies throughout the material. Thus things likeflat lenses may be constructed that are much easier toproduce than grinding glass to a precise shape.

Figure 5
Top: pressure response at closed end of each separate absorber channel.
Bottom: absorption spectrum of whole absorber.
Figure 7
Top: Amplitude and Phase responses of transfer functions
V1 (green), V2 (blue) and V1+V2 (red).
Bottom: Amplitude and Phase responses of input impedance

Figure 3
Computer model of absorber

The term “meta” has since gained the more generaldescription of any material that exhibits characteristics
foreign to the solid form. In this case, ABS has beenmoulded into a shape that is an almost ideal broadband absorber.
The design of the absorber was a joint project between
KEF and AMG (Acoustic Metamaterials Group).
With the help of Professor Ping Sheng (a worldrenowned expert on metamaterials), AMG derived

Figure 6
Generalised 3rd-order Butterworth constant-resistancecrossover. Units are Ohms, Farads and Henries. fc is cut-off

Figure 4
Actual absorber

2

These are steady-state measurements and anyone
who is familiar with filters will know that, although
the amplitude response of narrow-band filters may
well add to unity, this usually hides the presence of allpass phase responses and some time smearing occurs.
The system is said to be non-minimum phase. This is
the case with the summing of the transfer functionsof the separate channels of graphic equalisers andthe outputs of two or more drivers fed by a passive
speaker crossover network. Such time smearing canreadily be shown by impulse measurements, where achange in profile between the exciting signal and thatgenerated by the device(s) indicates time smearing.
3

Coupling the Absorber to theTweeter DomeThe absorber sits at the rear of the Uni-Q™ driverand is coupled to the tweeter dome by a slightly
tapered conical duct, which acts as a waveguide. This
waveguide passes through the centre poles of both thetweeter and bass/midrange drivers and has involved acomplete redesign of the tweeter magnet assemblyto accommodate the wider diameter required for theduct to work properly. The difference in the motor
assemblies is shown in figure 9.

Figure 8
Impulse response of tweeter dome velocity (red), overlaid with
ideal response (dotted blue)

A small amount of porous material is placed in the duct,
which has the dual effects of reducing the amount ofripple at high frequencies and fine-tuning the kneeof the absorption spectrum. Figure 10 shows theabsorption spectrum immediately behind the dome.

However, it must be realised that what is being usedhere is the acoustic impedance, not a transfer function.
To take an electrical analogy, consider a passive,
constant-resistance network such as that of figure 6.
While the individual low- (V1) and high-pass (V2)transfer functions are minimum phase, the sum of thetransfer functions (V1+V2) is not. It is a 2nd-order allpass. However, the total input impedance (Zin), is apure resistance, R. As such, it is minimum phase. Thereis no all-pass (figure 7).

Figure 11
FEA simulation of exaggerated deformation of tangerine
waveguide and surround support at 12kHz

Strengthening ribs were added to both components,
which reduced the deformation. Figure 12 shows
the actual modified parts (viewed from the rear) andfigure 13 illustrates the reduction in displacement.
It should be noted that the modified parts have an
area comparable to the tweeter dome itself and any
movement will add audible colouration to the overall.
For the basic operation of the tangerine waveguide,featured in previous models, see Appendix 3.

Figure 9LS50 (left) and LS50 Meta (right) tweeter motor assemblies.

To illustrate that the absorber is indeed minimumphase and introduces no time smearing, figure 8 shows
the impulse response of the tweeter dome movement
overlaid with an ideal, minimum phase response.
There is virtually no difference.
The tweeter dome movement was chosen because it
can be measured close-to, which increases the signal
to noise ratio. As it is only valid well below the firstbreak-up mode of the dome and the absorber only
works above a certain frequency, both signals wereequally band limited to avoid errors.

not being totally absorbed. It exposed a lesser, but still
important layer of residual colouration. It turned outthat the tangerine waveguide and the dome surroundsupport - both plastic mouldings - were physically
deforming at high frequencies. (figure 11)

Tweeter Gap Damper
One of the problems with constructing a combination
driver array like Uni-Q is dealing with the gaps that
separate the constituent parts. There is a narrow
channel – an annular gap – between the moving
midrange voice coil and the static start of the tweeter
waveguide. This channel acts as an organ-pipe-likeresonator, and is excited by the tweeter output. The
resonances modify the response of the tweeter, adding
a series of glitches that are not present if the gap isclosed off – simulating a perfectly smooth waveguide.
Obviously, this annular gap is necessary to allow the
LF/MF cone and voice coil to move, so the solution wasto create a cavity between the midrange and tweeter
magnets to which this annular gap connected (figure14). Adding damping to this newly-created cavity was
found to be effective in taming the resonances in theannular gap and the removal of the response glitches
was immediately apparent as an improvement indetail clarity (figure 15).
This feature - the tweeter gap damper - was first
introduced through the R Series (2018), and wasa major development that led to birth of the 12th
Generation Uni-Q. The addition of MAT and the

Figure 12
Modified surround support and tangerine waveguide viewed
from the rear to show added ribs
Figure 10
Absorption at the entrance of the conical duct, immediately
behind the dome diaphragm.
Figure 14
Uni-Q driver with damped, optimised gap

Tangerine Waveguide
Many acoustic engineers would be so pleased at
designing the almost perfect tweeter absorber thatthey would have rested on their laurels. But speakerdesign is rather like peeling an onion - remove one
layer and there is another one exposed. So it is with theremoval of colouration caused by the rear radiation
4

change in tweeter system structure for LS50 Meta,
however, necessitated a redesign of the tweeter gap
damper. Additional work was carried out in regards

Figure 13
Simulated displacement of tangerine waveguideto the shape of the cavity and the placement of thewadding material - now comprising of two rings - tofurther improve performance.
5the back of the dome is directed into the metamaterial
absorber. but also reduces non-linear distortions
caused by air pressure changes behind the tweeter.
As a tweeter moves in and out, it compresses anddecompresses the air behind it. This negatively
affects the excursion characteristics and linearity ofthe tweeter The larger the air volume, the lesser theeffect.
Also notable is the increased saturation of the steel
components of the motor system. The image colour
is indicative of the magnetic strength throughout thesteel, and dark blue indicates saturation.
Figure 15
Tweeter response
With damped cavity (blue)
Without cavity (red)Motor Design
In addition to the tweeter motor system being
modified to accommodate the wider duct through its
centre pole, other changes were made to provide amore consistent drive to the voice coil.
Figure 16 illustrates the difference in tweeter motor
design between the original LS50 and the LS50 Meta.

Normally the magnetic field in the gap is perturbed by
the electric current in the voice coil, which changes
the force on the voice coil during every cycle ofmovement. Perturbation cannot occur if the motor
system is saturated and so the force on and therefore
the motion of the voice coil more closely follows the
input signal.
Saturation did result in a drop of flux density in the gap.
This was compensated by changing the tweeter voicecoil from one to two layers, which would normally leadto an increase in coil inductance, with a progressive
loss of sensitivity at very high frequencies. A copper
sleeve on the centre pole, which couples inductively
to the voice coil, reduces the inductance, even belowthat of the LS50, as is shown by the blocked coilmeasurements of figure 17.

The design of the motor used in the bass/midrange
driver of the Uni-Q driver has also been modified toreduce modulation by the movement of the voice
coil, but this time the approach is different from thatused for the tweeter, partly because the driver has an
overhung voice coil (coil longer than the magnet gap)
as opposed to the underhung coil (coil shorter thanthe magnet gap) of the tweeter, which is always fullyin the steel gap.
The inductance of a voice coil presents a couple ofchallenges. Higher levels of inductance create a larger
net magnetic field as the signal passes through thevoice coil. This magnetic field causes distortion - thelower the inductance of the voice coil, the lower the
net magnetic field is produced, thus lower distortion.
The inductance of the voice coil also tends to change
as the coil moves in the gap (modulation). The steel
acts as a core to increase the coil’s inductance andthe amount of steel (and therefore the inductance)varies as the coil position changes. This variance is an
issue, as it introduces non-linear distortions into theloudspeaker performance.
As a result, more extensive use is made of auxiliary
aluminium parts in LS50 Meta than was the casewith the original LS50. The aluminium, although nonmagnetic, is conductive and couples inductively to thevoice coil. Being a virtual short circuit, it reduces thevalue of the coil’s inductance, which in turn reduces
modulation of the flux of the motor system - reducing
non-linear distortions.
The LS50 and LS50 Meta motor designs are comparedin figure 18. The aluminium parts are coloured lightgrey.

Figure 16
Tweeter motor systems of LS50 (top)and LS50 Meta (bottom).

One major change is the sheer size difference ofthe duct - 2.5x the surface area of that found in theoriginal LS50, increasing the air volume available tothe tweeter.

Bass/Midrange
Motor Design

Where the original LS50 features a small aluminium
ring placed between the magnet and voice coil,
LS50 Meta sports a much larger version. In addition,
another ring has been added to the top. The red
fringing in the otherwise blue steel is a measure of thecoil modulation and is seen to be much lower in the
LS50 Meta.

Figure 17
Tweeter blocked impedanceThis ensures as much of the sound as possible from
6

Figure 18
Bass.midrange motor systems of LS50 (top)and LS50 Meta (bottom).

The LS50 Meta motor system also features anundercut pole design to minimise the steel volume
and focus the magnetic field more precisely on thevoice coil. Also instructive is to measure the voicecoil inductance at different frequencies and compare
different designs.
For the measurements in figure 19, the voice coil isblocked (glued in place) at the centre (rest) position.
LS50 Meta - with the shorting rings - exhibits much
lower inductance than the other designs.
Inductance modulation was also measured at two
different frequencies (200Hz and 2kHz) against
excursion. Figure 20 clearly shows that the LS50
Meta design, with the shorting rings, exhibits the least
variation of inductance against excursion - creatinga frequency response that stays balanced, with very
little influence from the excursion of the driver.

7

Crossoverthe HF filter is on a separate board, placed well awayfrom the LF filter.

The topology of the crossover of the LS50 Meta isshown in figure 24. The smoother response of the
new tweeter has resulted in fewer components in the
HF filter and the remaining series capacitor (C1) is ofhigher quality.

Figure 19
Bass/midrange blocked voice coil inductance against frequency.

Figure 20b
Bass/midrange voice coil inductance against displacement
at 200Hz (figure 20a) and 2kHz (figure 20b).

Figure 25
Current consumption of inductors L2 and L4
for an input of 2.83V.

Industrial Design

The change in magnet design has had two further
advantages. The undercut centre pole gives higher
flux density in the gap itself and lower magnetic
fringing (figure 21). This results in higher sensitivity ofthe driver and more linear drive at higher excursions(figure 22).
Looking at figures 18-22 together, we see that the
new design gives less modulation of the magnet flux
density in the gap, more linear drive, lower voice coilinductance and virtually no change in inductance withvoice coil position. The upshot of all this analysis andredesign is a significant reduction in total harmonic
distortion (THD) in the frequency range where the ear
is at its most sensitive (figure 23).

Little has changed from the original LS50, but there
are a few detailed changes to the back of the cabinet
to improve performance and enhance looks (figures 1& 26):• The port opening is now flush with the rearsurface, which further reduces turbulence.

Figure 21
Comparison of flux density in the gap.

Figure 24
LS50 Meta crossover topology

The LF filter is similar to that in the original LS50, but
includes an extra low-Q parallel resonance branch (C3R3-L4) to compensate for the higher sensitivity in thedriver’s upper midrange due to its lower inductance.
The inductor L4 is the only one in the crossover tohave a core. It is vital that only the resistor R3 controls
the Q of the branch. To that end, the resistance ofL4 (which is subject to change as it heats up when
passing current) is kept as low as possible. As the coreis a possible source of harmonic distortion if it nears
saturation, measurements were taken to relate thecurrent passing through L4 with that passing throughL2. Figure 25 shows that it will be seen that the current
passing through L4 is much lower and the chances of
its core saturating are virtually nil, a fact borne out by
the THD measurements of figure 23.

Figure 22Comparison of bass/midrange force factor

Figure 20a

To minimise any coupling between inductors, thethree in the LF filter are arranged orthogonally and

Figure 23
THD, measured as a percentage of the fundamental level
90dB SPL at 1m for LS50 (red) and LS50 Meta (blue).

8• The 4 cavities in the corners for the baffle
retention bolts have been eliminated.
• The rear surface is now slightly domed.
• A step transition between the rear panel andthe sides of the cabinet has been introduced.
• There is now a soft notch around eachbinding post.

Figure 26
Rear views of LS50 (left) and LS50 Meta (right)

9

Specification - LS50 Meta

Appendix 1 - Cabinet

Description

Bookshelf Loudspeakers

The cabinet construction follows closely that of theoriginal LS50.

Drive Units

Uni-Q™ Driver Array
LF/MF: Nominal dia. 130mm (5.25 in) magnesium/aluminium alloy cone
HF: Nominal dia. 25mm (1 in) aluminium alloy dome vented with
Metamaterial Absorption Technology (MAT)

Frequency Range (-6dB)

47Hz - 45kHz

Frequency Response (±3dB)

79Hz - 28kHz

Sensitivity (2.83V, 1m)

85dB SPL

Maximum SPL at 1m

106dB

Harmonic distortion<0.4% 175Hz - 20kHz (90dB SPL, 1m)

Crossover frequency

2.1kHzNominal Impedance

8Ω

Minimum Impedance

3.5Ω

Dimensions (inc. terminals)

H: 302mm (11.9 in)
W: 200mm (7.8 in)
D: 278mm (10.9 in)

Weight7.2kg (15.8 lb)

Written specifications like these, however useful for comparison, do not fully describe the listening experience.
However good the component parts, voicing the system with the crossover is all-important.
The following graphs, which include additional directivity and early reflection information in line with theresearch work of Floyd Toole [9] [10], show how smooth the response of the LS50 Meta is, indicating a superb
listening experience.

Diffraction
Diffraction is the secondary radiation that happens
when the wavefront from the driver meets a sharpedge. This reradiation causes what might be described
as a mini-echo that time-smears the driver’s output.
The effect varies with angle, but needs to be minimised
to maintain clarity.
At very low frequencies, rather like a sea wave
wrapping round a small pebble, the sharp edges of
the cabinet have little effect - the sound simply wraps
around the object in a smooth manner. At very highfrequencies, only a minimal proportion of the energy
travels along the front baffle, due to the increased
directivity of the radiating diaphragm, and thereradiation level is low. In between these extremes,
the shape of the cabinet, especially the front baffle,
has a marked effect.
Traditionally, it was thought that the edges of the baffle
need to be rounded, preferably with a radius similar
to the wavelength of the sound. However, detailed
research has shown that, if the baffle is curved, theangle at the edge does not have to reach 90 degrees.
for the majority of the diffraction effect to beeliminated. This is extremely fortunate, because fullyrounding the corners with a large radius is wasteful of
space and not very nice to look at. The front baffle iscurved in both directions enough to make diffraction
inaudible.

Rigid Bracing/Constrained
Layer Damping
The ultimate goal in the design of any speaker is tohave all the sound energy coming from the drivers
and none from anything else. The cabinet walls are aparticular problem as they have a much larger area
than the driver diaphragms and their output can beproblematic with relatively little flexure. Not only is
the sound delayed relative to the direct sound fromthe drivers, but it’s highly distorted and the resulting
colouration can be extremely unpleasant.
Vibration of the cabinet walls is excited both byvariation in air pressure inside the cabinet and by thereaction force on the driver motor as the diaphragmmoves back and forth. This latter force is transmitted
via the driver chassis to the cabinet panel.
That the cabinet needs to be there at all may berealised by going back to figure 2. The bass/midrange
driver radiates energy to the rear and that energy
has to be contained, otherwise the rear radiation
will progressively cancel the front radiation as thefrequency decreases, and absorbed - which effectively
means turning it into heat.
This constraint has traditionally been done by adding
bracing. On its own, this has less effect than may be
expected from simply considering static stiffness.
What it tends to do is shift the problem to higher
frequencies. However, the simple expedient of addinga lossy layer between the braces and the cabinet
walls absorbs far more energy - Constrained Layer
Damping. This lossy material is sandwiched between
the bracing and the driver/panel, converting vibration
into heat.

Figure 27
Low-diffraction Baffle1011• There is more freedom on the rear panel
compared to the front in positioning the port so it
is placed close to nodes (nulls) of the resonances,
so less energy is transmitted through the port.
To illustrate this last point, figure 29 shows thedifference in output when the port is placed at an
antinode and at a node of an internal resonance. For
the first of these measurements there is no wadding
inside the cabinet, so the effect is made clearer. The
resonance can clearly be seen in the total response.
When the ports are optimally placed and the wadding
added, the resonances are greatly depressed andcannot be detected in the overall response.

Organ pipe resonances
Figure 28
Constrained-layer damped bracing

Appendix 2 - Ports
Ports are used to augment bass response. They havethe dual advantage that they enable a lower cut-off
frequency than closed-box systems for the same
size of enclosure and, within the operating range ofthe port, the bass driver moves less, thus lowering
distortion.

These resonances within the port itself can be
drastically reduced if the walls of the port havea degree of flexibility. Instead of the normal rigid
plastic, the walls are fabricated from closed-cell
foam. Originally developed for the original LS50
loudspeaker, this technique dramatically reduces thepressure variations inherent in these higher frequency
resonances and renders them less audible.

Figure 30 illustrates this technique using Finite
Element Analysis (FEA), where the colour scale fromgreen through to red indicates the level of air pressure.

Figure 30
Comparison of the Finite Element modelled pressure
magnitude in the port at the first standing wave with rigid
walls (top) and flexible walls (bottom).

TurbulenceThis occurs at both ends of the port as the air exits theport either to the open air or the air enclosed within
the cabinet. The solution is to flare both ends of theport (figure 31). The progressive expansion of the air
afforded by the flaring reduces turbulence and thus
audible chuffing. It should be noted that, if turbulenceis allowed to develop, not only is it audible, but theperformance of the port changes with sound level,
impairing the dynamic range of the loudspeaker.

Ports are not without their drawbacks, however.
Figure 31 - Flow pressure contour of straight port (top)and flared port (bottom).

• They can be a window to resonances in the air
cavity inside the cabinet.

Figure 32 shows the final design of the port locatedinside the LS50 Meta cabinet.

Appendix 3 - Uni-Q™
The Uni-Q array has been the mainstay of virtually
all KEF loudspeakers since its introduction in 1988.
It delivers the holy grail of loudspeaker design in thatall sound appears to emanate from the same pointin space. Coaxial loudspeakers had been around formany years before the introduction of Uni-Q, but thetweeter was never time aligned with the midrange orbass/midrange driver it was partnered with. Either
the tweeter was in front of the larger driver, whichbrought the added disadvantage that the tweeter
impaired the response of the driver it was in front of,or it was well behind. With Uni-Q, both drivers havetheir acoustic centre at the same point and the larger
driver acts as a waveguide for the tweeter. The result
is that the blend between the two units is virtually
seamless in terms of both response and dispersion.
The two units together can be regarded as a single
driver without the performance shortfall that would
be suffered by a true single driver covering such awide frequency range.
Over the intervening years, the Uni-Q concept has
been progressively refined. The midrange cone shapehas been optimised to create just the right amountof dispersion at all frequencies and innovations
such as the tangerine waveguide have improved
the performance of the tweeter. Uni-Q is simple inconcept, but tricky to implement in practice. Hereare some of the techniques previously developed andcarried over to this model.

Tweeter• At midrange frequencies, they act rather like anorgan pipe and exhibit a series of resonances.

Diaphragm• They can be a source of noise if turbulence isallowed to happen as air vents out at each end of
the port (so-called chuffing).

The dome diaphragm itself is aluminium. More exotic
materials – diamond and beryllium for example – aresometimes used in an effort to increase stiffness andpush the pistonic region of the tweeter to the limits ofhuman hearing. But this can be done with aluminium
at far lower cost, providing some ingenuity is used indesigning how the dome is constructed.

Resonances inside the cabinet
The port is positioned on the rear panel of the cabinet.
This has two advantages:• Resonances inside the cabinet are at relatively
high frequencies and the sounds are not directed
towards the listener.

Figure 29
Simulated comparison of the port and total speaker outputs
with unoptimised (top) and optimised (bottom) port location.

Figure 32 - Offset flexible port located inside the LS50 Meta

12

13

The optimum dome shape for waveguide loading
is a spherical cross-section, but the optimal shapefor stiffness is an elliptical cross-section. Both werecombined into the patented KEF Stiffened Dome. A
one-piece elliptical dome and voice coil former is deep
drawn and the centre removed. This is capped by aspherical dome and a very stiff triangular section isformed where the two parts join.

Being within the human audio range, the breakup has to be tamed, rather than putting it out of
hearing range. This is done by introducing a flexible,
decoupling material between the cone neck and thevoice coil former.

It will be seen that the outer vertical wall of thesurround is another possible source of discontinuity,
so the outer trim ring is part of the whole design,
giving a smooth transition all the way from the tweeter
diaphragm to the front baffle.

The high-Q resonances are tamed to the extent thatthey may be properly attenuated by the crossoverand not break through the tweeter output. Figure
37 illustrates the improvement in the unit’s response
as a result of using the lossy interface. Mechanicalcorrection is far better than equalising the peaks withthe crossover. The latter does not subdue peaks in the
THD at sub-multiples of the fundamental.

Figure 34
FEA Simulation of tweeter output
without tangerine waveguide (red)with tangerine waveguide (blue)

Figure 33
3D CAD sectional views of the tweeter dome and extended
former that meet to form a triangular stiffening member
at the dome edge

Figure 37
Effect of lossy interface on response of LF/MF driver

Tangerine Waveguide
The interface between the radiating dome of thetweeter and the waveguide formed by the midrange
dome is extremely critical. Ideally, the diaphragmshould be a pulsating dome, which would involve
the radius of curvature changing. This is not possible
and the motion is in the same direction all over thesurface of the dome. To compensate for this nonideal situation and taking a leaf out of the design ofcompression drivers, the Tangerine Waveguide wasdeveloped to restore correct coupling between thedome and the whole waveguide.
The improved coupling at high frequencies above
5kHz brings with it a useful increase in sensitivity anda reduction in the height of the first resonance peak,illustrated by figure 34.
Figure 35 shows the tangerine waveguide in place inthe Uni-Q array. Note that this is the basic operationof the tangerine waveguide that is carried over fromprevious models. It is enhanced by the extra stiffening
described in the new features section (figure 12).

Figure 38
Z-Flex surround - line drawing

Figure 35
Tangerine waveguide in position on top of tweeter dome

Figure 36
Cone Neck Control interface between voice coil former and driver cone.

Bass/Midrange driver

Z-Flex Surround

Diaphragm
The cone is formed from a magnesium/aluminium
alloy. Like the tweeter dome, it serves to provide thenecessary stiffness to give pure pistonic motion overthe driver’s working range. The stiffness is increased by
the radial embossing in the cone profile. Neverthelessthe cone is still prone to high-Q break-up, but this timein the frequency range covered by the tweeter.

14

The midrange cone and outer surround form awaveguide for the tweeter and the normal rollsurround is not ideal in this respect, especially close to
the driver’s axis, where symmetry has a large effect.

Figure 39
Cross section of complete Uni-Q driver and trim ring

The Z-Flex surround has been specially designed toallow the necessary movement of the bass/midrange
cone, but provide a low profile to allow the cone andsurround to act together more accurately as a tweeter
waveguide.
15

References[1] M. Yang, S. Chen, C. Fu and P. Sheng, “Optimalsound-absorbing structures,” Mater. Horiz., vol. 4, pp.
673-680, 2017.
[2] S. Degraeve and J. Oclee-Brown, “Metamaterial
Absorber for Loudspeaker Enclosures,” 148th Audio
Eng. Soc. Convention, 2020.

LS50 Wireless II[3] KEF Research and Development, “R Series White
Paper,” 2018.
[4] KEF Research and Development, “The Reference
White Paper,” 2013.
[5] M. Dodd and J. Oclee-Brown, “New methodology
for the Acoustic Design of Compression Driver Phase
Plugs with Concentric Annular Channels,” J. Audio
Eng. Soc., vol. 57, no. 10, pp. 771-787, Oct 2009.
Figure 40
FEA simulated tweeter impulse response with half-roll surround (top)and Z-Flex surround (bottom).
Note the improved response, especially close to the unit’s axis

Summary
As can be deduced from reading this paper, there isa lot of technology packed into the LS50 Meta, both
new and existing - a culmination of many years of KEF
innovation and acoustic expertise.
All this technology, although tackling legitimatesound reproduction problems in a scientifically
logical manner, is of little use if the listener is notbrought ever closer to sound Utopia - getting more
and more enjoyment and fulfilment from the listening
experience.
This is probably the best compact passive monitor
that is available today. This is a bold claim, but valid
not just because of the engineering expertise packed
into it, but also because the engineers responsible forits design, often frustrated musicians, are fully awareof what sounds good, what sounds bad and whatshortcomings the ear is able to gloss over.

[6] M. Dodd and J. Oclee-Brown, “A New Methodology
for the Acoustic Design of Compression Driver Phase
Plugs with Radial Channels,” 125th Audio Eng. Soc.
Convention, 2008.
[7] M. Dodd, “The development of the KEF LS50, a
compact two way loudspeaker system,” International
Conference, Helsinki, Finland, 2013.
[8] A. Salvatti, A. Devantier and D. J. Button,“Maximizing Performance from Loudspeaker Ports,” J.
Audio Eng. Soc., vol. 50, no. 1/2, pp. 19-45, Jan. 2002.
[9] F. E. Toole, “Loudspeaker Measurements and Their
Relationship to Listener Preferences: Part I,” J. Audio
Eng. Soc., vol. 34, no. 4, pp. 227-235, Apr. 1986.

Figure 41
Primary Speaker Rear Panel

Figure 42
Secondary Speaker Rear Pane[10] F. E. Toole, Sound Reproduction, The Acoustics
and Psychoacoustics of Loudspeakers and Rooms,
Amsterdam; Boston: Elsevier, 2008.

16

17

Overview

4.

Analogue stereo - Aux 3.5mm stereo jack socket

Streaming support

The active LS50 Wireless II incorporates all thetechnology described for the passive LS50 Meta, plus
a few extras that either add to the presentation of themusic - making it even more accurate - or add to thefunctionality.

5.

Network - Wireless (2.4GHz or 5GHz) and Wired(RJ-45 socket and CAT 6 cable) supporting up to
384kHz/24bit, DSD256, MQA

Wireless streaming supports the following:

The electronics, mounted on the back, includes twochannels of power amplification (one channel for eachdriver and each of higher power than the original LS50
Wireless) and the KEF Musical Integrity Engine (MIE).
This is essentially custom Digital Signal Processing(DSP) that allows the acoustic engineer to do far morethan is possible in any passive design.

6.

Bluetooth 4.2

Selection is made using the smartphone app, thecontrols on the top of the Primary speaker or the
supplied remote control.

The following music streaming services are also
supported (dependent on territory):

AirPlay 2
Google Chromecast
ROON Ready (via future firmware update)
UPnP compatible
Bluetooth 4.2

Spotify (via Spotify Connect)
Tidal
Qobuz
DeezerAmazon MusicQQ Music via QPlay
Internet Radio
PodcastEach stereo pair comprises a Primary and a Secondary
speaker. The former incorporates adjustments
applicable to both speakers, such as bass extension,
room acoustic compensation, source selection andplacement/EQ.
Bass may be extended to lower frequencies than islimited by the usual passive cabinet-size/sensitivity
trade-off, the balance may be adjusted for speaker
position and room acoustics and the all-pass delay
inherent in passive crossovers may be eliminated.
All adjustments are made using a smartphone app.
This is available for both iOS and Android devices andis a free download either from the Apple App Store or
the Google Play Store as appropriate.

InputsAll inputs are fed to the Primary speaker (figure 43).
The six choices available are:1.

eARC PCM at up to 192kHz/24 bit - HDMI (TV)socket2.

S/PDIF at up to 96kHz/24 bit - TOSLINK Opticalsocket3.

S/PDIF at up to 192kHz/24 bit - RCA Phono (coax)
socketFigure 44 - Block diagram of Primary Electronics

Figure 43
Primary Speaker Inputs

Connection between Primaryand Secondary
The two speakers may be connected together either
wirelessly or hard-wired.
LS50 Wireless II is pre-paired at the factory for easy
setup in wireless mode. The wireless transmission
is totally independent of the local Wi-Fi signal. The
miniscule delay (2.9ms) between the primary andsecondary speakers in this mode is automticallyaccounted for, through an equal delay applied to theprimary speaker. Finally, the wireless connection
resamples all audio to 96kHz/24 bit.

Figure 45 - Block diagram of Secondary Electronics

The wired interspeaker connection uses CAT-6 cable,
plugged into the relevant sockets on the rear of the
speakers. Cable mode must also be enabled in the KEF
Connect app settings menu. In this mode, all audio
signals are resampled to 192kHz/24 bit.

Figure 46 - DSP Details

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19

DSP Processing

Phase correction

Figures 44 & 45 show block diagrams of the electronics
in the Primary and Secondary speakers respectively.
All settings are controlled from the smartphone app
and applied equally to both speakers.

As stated before, the basic crossover introduces anallpass into the speaker’s response. This does not
impinge the magnitude response, which may still
be smooth and flat, nor does it affect the directivity,
but it does introduce some phase distortion with its
accompanying time delay.

DSP allows much finer control of the signals reaching
the drivers than with a passive crossover, and theLS50 Wireless II crossover has been redesigned fromthe passive version to take advantage of this. Bass
cut-off, although still 4thorder, has a less rounded
corner because there is more level adjustment. By
themselves, these filters still result in an all-pass
response. However, there is no interaction between
the crossover and the driver impedance in the case ofactive crossovers, leading to better consistency fromsample to sample. The all-pass characteristic may beeliminated by the Phase Correction stage and this will
be discussed in greater detail in a subsequent sectionof the paper.

Figure 48
Recommended wall mode settings

EQ Settings

Figure 49
Recommended desk mode settings

The KEF Connect app allows the listener to make
EQ adjustments (Figure 47) based on the listening
environment and speaker position.

However, it is possible, with an active crossover, tocompensate for this phase distortion and, if the Basicmode setup is used, this phase compensation is appliedby default; the toggle switch is only provided in Expert
mode. This enables listeners to hear the difference which is subtle, but there. There is also a small amoutof delay involved with the correction. This extra delay
is unlikely to be a problem (see the later section Syncing with Vision), but the overall latency can be
reduced if necessary by switching the correction off.
At this point, it is worth emphasising just how well the
Uni-Q principle lends itself to this type of correction better even than physically separated drivers that are
staggered to correct for time delay on axis.
Here is a conventional speaker with the drivers
mounted on a stepped baffle so that the distances tothe measuring microphone on axis (red for the bass
driver, black for the tweeter) are equal.

EQ settings are available in both Basic and Expert
modes. Both feature largely the same settings, withthe main difference being the way parameters arepresented - generally distances for Basic and decibels
for Expert.

Desk & Wall Modes
Figure 50
Recommended combined wall & desk mode settings

These two modes are used in conjunction to equalise
for the method of mounting. Below the on/off toggle
is a slider showing the selected value. The line below
the slider shows the applicable range. However, themechanism of nearby reflections is very complicatedand simple level settings are only an approximate
solution. We would always recommend standmounting, well away from walls for optimum results.
Figures 48-50 show some recommended settings.

Treble Trim
This is used to compensate an over-lively (lots of glass/flat surfaces) or over-dead (lots of soft furnishings,
curtains) room acoustic. The former requires a
modicum of treble cut, whilst the latter requires boost.
Figure 47
Expert EQ Settings page (KEF Connect)

Best results are found by ear.

20

Figure 51
Conventional speaker, mic. on axis.

Now, move the microphone up or down and it isapparent that the distances of the two drivers to themicrophone are no longer equal.

21

The result is that any phase correction in the crossoveris only valid directly on axis. In fact, any blending at allof the two drivers suffers phase distortion off axis.

This is how the output of the speaker would look withthe phase correction switched OFF:Not so with a Uni-Q combination; the two drivers
remain at the same point in space and have the same
distance to the microphone, whatever the angle ofmeasurement.

Should the user prefer to be unbounded by thelimitations of a small bass driver, there is always theoption of adding one or two subwoofers.

Figure 52
Conventional speaker, mic. above axis.

Adding a Subwoofer

Figure 57
Speaker output - Phase Correction OFF

And this is how it looks with the phase correction
switched ON:

Figure 53
Conventional speaker, mic. below axis.

Even measuring off axis in the horizontal plane resultsin unequal path differences once the angle is large
enough that the radiation from the bass driver does
not travel directly to the microphone, but has to travel
first to the front of the cone.

In addition, there is a dynamic bass stage that limits
the driver excursion whatever the setting. This
prevents excessive distortion and protects the driverfrom possible damage. The application is practicallyunnoticeable - however the effect is creating aloudspeaker that sounds larger than it actually is.

Having established just how suitable a Uni-Q speakeris for phase correction, let’s see how effective theprocess is. Here is an input signal - a square wave of
1kHz:
Figure 58
Speaker output - Phase Correction ON

Bass Extension
There are three settings, which should be chosen
in conjunction with the room acoustics The “Less”
setting is similar to the natural bass extension of thepassive version, giving a cut-off frequency of 46Hz(-6dB). The “Standard” and “Extra” settings give cutoff
frequencies of 43Hz and 40Hz respectively.
Normally, the “Extra” setting will be chosen, unlessthere is a particularly bad room resonance around30- 40Hz, where one of the higher cut-off settings willcompensate.

Figure 54
Conventional speaker, mic. horizontally off axis.

22

Each speaker (Primary or Secondary) has a Subwoofer
Out socket (RCA Phono). The signal sent to bothsubwoofer outputs is a summed mono signal.
A crossover between the LS50 Wireless II and thesubwoofer is provided and the crossover frequency is
set within the app. There is no choice of filter shape - itis always a 4th-order Linkwitz-Riley giving -6dB at the
cut-off frequency and a flat in-phase total response.
If there is any low-pass filter supplied with thesubwoofer, it should be bypassed if possible or, if not,the frequency should be set to the highest possible.

Figure 55
Uni-Q speaker on & off axis

Figure 56
Input signal - 1kHz square wave

Either one or two subwoofers may be added toreinforce the bass of the audio system.

The choice of crossover frequency is something of acompromise. The higher it is, the less the excursion
demands on the LS50 and the louder the wholesystem can play. However, if a mono subwoofer output
is selected and the crossover frequency is too high,some of the stereo information is lost. 80Hz is usuallyconsidered the highest to avoid any perception of lossof stereo information and the lower it is the better inthis respect.
In Basic setup mode, the cut-off frequencies of thehigh-pass to the LS50 Wireless II and the low-pass
to the subwoofer(s) are always the same and thisis normally the course to follow. However, some
subwoofers do not have a flat response and different
cut-off frequencies may give better results. Having
different frequencies requires the user to use Expert
mode for setup.

23

Should the subwoofer have an inverting amplifier(rare) or be sited well away from the LS50 Wireless
II such that the output is not in-phase with that of
the LS50 Wireless II at crossover, there is the option
to invert the signal to the subwoofer. In this case theappropriate toggle should be set to ON. More usually
it is set to OFF. The correct setting will provide a fuller
sound.
If no subwoofer is used the high-pass filter to the LS50
Wireless II should be bypassed by switching the toggle
in the app to OFF.
The correct sub gain setting depends on several
factors such as the sensitivity of the subwoofer andhow many are used. The correct balance may be set byear.

Syncing with Vision
Should the speakers form part of a home theatre
installation, where the programme content isaudiovisual, the thorny question of the sound syncing
with the picture (so-called lip-syncing) must be
addressed.
The all-pass characteristic of the bass/midrange totweeter crossover adds delay to the tweeter, so any
correction, if it is to be causal, must delay the whole
signal. Additionally, wireless transmission adds somedelay. It is important that all this inherent delay
does not lead to the perception of mis-syncing, even
if theoretically it is impossible to keep everything
absolutely together for all settings.
The Advanced Television Systems Committee (ATSC)
recommends that the sound to vision delay tolerance
is between -15ms to +45ms if lip-syncing is to beunnoticeable. The maximum delay through the speakeris comfortably less than 6ms and so lip-syncing should
not be a problem, whatever the settings chosen in theapp.

Firmwarefirmware, to improve security, add functionality or
correct bugs.
Fortunately, the user needs to do nothing. If there is an
update available, it will be automatically downloaded
from our servers overnight, if the devices are either
fully on or in standby.
If preferred, users can manually download updates at atime to suit them using the app. It is also not necessary
for the interspeaker cable to be connected.
The KEF Connect app will also receive updates, theprocess of updating/setting automatic updates will bedependant on the smart device.

Summary
All too often, active speakers simply add power amplifiers and active crossovers to a passive design, but there ismuch more that active drive can offer.
This speaker takes an already excellent passive design and benefits from all the acoustic technology packed into
it. The inherent benefits of Uni-Q are improved upon even further through the inclusion of Phase Correction,whilst the lack of a physical crossover eliminates what can be a significant source of distortion.
LS50 Wireless II also represents a loudspeaker system equally driven by the concept of ‘Let You Be You.’
LS50 Wireless II does not aim to change the way someone experiences their music, but is designed to elevate theindividual’s unique situation through access to a plethora of music services and personalised EQ settings. Further
features and improvements, where developed, will be available through easy to perform firmware updates.

Power Amplification
As a true active loudspeaker, LS50 Wireless II features
four individual amplifiers - one for each drive unit in
the system. These sit after the crossover, meaning thateach amplifier receives only the frequencies destinedfor the corresponding driver. This differs from a
passive or powered system, where the crossovercomes after the amplification.
A significant benefit from a design standpoint is thatdifferent amplifiers can be explored for each drive
unit. Through intensive computer modelling andlistening tests, KEF engineers were able to develop
amplification that is perfectly matched for thefrequencies - and drivers - at hand.
LS50 Wireless II features the same amplifier classes as
the original LS50 Wireless - Class AB for the tweeter
and Class D for the midrange/bass driver. However, anumber of changes have been carried out, owing to
improved R&D and driver improvements.
The Class AB high frequency amplifier sports a totallynew topology - not only providing a higher output,
but distortion and dynamic capabilities have beenfurther enhanced over the original. The Class D mid/
low frequency amplifier has also seen a significant
increase in power output.

From time to time, like any software-controlled device,
it will be necessary to update the LS50 Wireless II
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25

Specification - LS50 Wireless II
Description

Wireless HiFi Speakers

Drive Units

Uni-Q™ Driver Array
LF/MF: Nominal dia. 130mm (5.25 in) magnesium/aluminium alloy cone
HF: Nominal dia. 25mm (1 in) aluminium alloy dome
vented with Metamaterial Absorption Technology (MAT)

Frequency Range (-6dB)

47Hz - 45kHz (Less)
43Hz - 45kHz (Standard)
40Hz - 45kHz (Extra)

Frequency Response (±3dB)

53Hz - 28kHz (Less)
48Hz - 28kHz (Standard)
45Hz - 28kHz (Extra)

Maximum SPL at 1m

108dB (full range, no subwoofer)

Harmonic distortion<0.4% 175Hz - 20kHz (90dB SPL, 1m)

Crossover frequency

1.9kHzPower Input

100-240VAC 50/60Hz

Minimum Impedance

3.5Ω

Power Consumption

200W (operating)< 2.0W (standby)

Power Amplifiers

LF/MF: 280W Class D
HF: 100W Class AB

Input Impedance

50kΩ

Dimensions

H: 305mm (12.0 in)
W: 200mm (7.9 in)
D: 311mm (12.2 in)WeightPrimary: 10.1kg (22.2 lb)
Secondary: 10.0kg (22.0 lb)

Supported Formats (all inputs)

MP3, M4A, AAC, FLAC, WAV, AIFF, ALAC, WMA, LPCM and Ogg
VorbisSupported Formats(extra on network inputs only)

MQA
DSF: DSD64, DSD128, DSD256
DFF: DSD64

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