KEF R&D

R Series 2018

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Contents
Introduction 
5Philosophy5
Objectives5
The cabinet
6
Visual design
6
Minimising panel sound radiation
6
Directivity and diffraction
8
Ports9
Resonance inside the cabinet
10
Organ pipe resonances
10
Turbulence10

Drive units (Drivers)

10Bass drivers
10
Diaphragm11
Magnet system
11
Spider (Damper)
12
Uni-Q array
12Filling in the gaps
13
Midrange13
Cone and surround13
Magnet system
14
Tweeter 
14Diaphragm assembly
14
Tangerine waveguide
15
Rear loading
15
Crossovers16

Model Range

18

R318
R519
R720
R1121R2c22R8a23

Summary24
Acknowledgements24
References24

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R11 in Black Gloss finish

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Introduction

Objectives

It is 7 years since the first R Series products were introduced.
In that time, KEF’s Research and Development work has
continued and its best technology has improved. The fruits ofthis work can be seen in the 2014 Reference Series and theBlade 2. Behind the R series 2018 are a wealth of technicalupdates and improvements – some trickle-down from theReference and Blade projects and some specific to the newseries. The result is a significant step up in performance anda purity of sound that’s clear from the first listen.

Loudspeakers have a difficult job, not always fully appreciated.
They must take a 1-dimensional signal and turn it into a
3-dimensional experience.

Philosophy“Of all art, music is the most indefinable and the mostexpressive, the most insubstantial 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 rightful habituation, your ears andmind, they aim to do so in the most natural way they can…
without drama, without exaggeration, without artifice.”
Raymond Cooke, KEF founder
These words were penned shortly after KEF’s founding in
1961. Cooke was an avid music aficionado 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 the live
performance through recordings.
Cooke realised that this could only be achieved by a scientificunderstanding of sound and its reproduction, concentrating
his efforts on loudspeakers – the last link in the recording
chain and probably the most difficult in that they have towork in an unknown 3-dimensional environment. Neverafraid to employ the most capable engineers and provide
them with the latest and most effective tools, Cookeestablished a philosophy that still exists today.
KEF engineers continue to pursue this scientific endeavour,
today using tools and technology that were unavailable to
Cooke in those early days, never forgetting that all the sciencein the world is useless unless it brings better performancesand greater pleasure to the user. The listening experience isstill the final arbiter in deciding whether or not the scienceis effective, but without it progress cannot be measured andthe way forward cannot be understood.

Stereo – the simplest common configuration – uses justtwo loudspeakers to create a complete sound stage for thelistener. Imagine throwing a handful of gravel into a pond andtrying to recreate the same ripple pattern by dropping justtwo pebbles into the pond spaced by about 2 metres. It can’t
be done and we haven’t even considered height! Stereo isan illusion that, fortunately, seems to work remarkably well.
Nevertheless, there are some clearly defined attributes that
separate good loudspeakers from mediocre ones:
The sound must come from the drivers and no other part ofthe structure. The most common form of unwanted added
sound is radiation from the cabinet. The panels are relatively
large in area and don’t have to move much to colour thetrue sound.
A loudspeaker should have controlled dispersion. If thecharacter of sound varies too much with angle, the3-dimensional sound stage cannot be accurately formed
and an acceptable listening position is very restricted. But
omni-directionality is not desirable either. The acoustics ofthe listening room become more intrusive and can swamp
any attempt to recreate the acoustic environment of theoriginal performance.
The sound quality should be the same at all levels. The
dynamic range of music (the quietest to the loudest sounds) ishigh and, in order to reproduce it accurately, the loudspeakermust not compress the sound nor to come to life only at
relatively high levels.
There should be no harmonic or intermodulation distortion– the creation of sounds at frequencies that are not present
in the original.
There should be no time or temporal distortion. This cancome from resonances, where the sound hangs on at
discrete frequencies, or diffraction, the re-radiation of soundat discontinuities in the structure that is delayed relative tothe direct sound from the drivers.
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Now, it’s all very well to say that there must be none ofanything, but we don’t live in a perfect world. The job of
the engineer is to minimise all these undesirables, ideally toa level below which they are audible. However, sometimes
there are conflicting issues and, in this case, the engineer
has to gauge the lesser of two evils. For example, low bass
at high volumes is extremely difficult for smaller cabinets toachieve. However, smaller cabinets may be preferred fortheir visual quality.
This brings us to the subject of visual design. Loudspeakers
must be considered a legitimate piece of furniture in theliving environment. They should, in short, look good.

The cabinet

Visual design
All R Series cabinets are designed to look good. They haveclean lines, there is a choice of finishes – black gloss, whitegloss, walnut – and the frontal aspect is slim (more of thatlater when we discuss temporal accuracy). The quality of
fit and finish is high and, for those who prefer not to see
drivers, there is the option of magnetically attached grilles.
The loudspeakers are optimised for when the grilles are
not fitted and they are ideally removed for serious listening,
even if they are fitted when the loudspeakers are not in use.
However, the grilles are designed to have the minimum effecton sound quality.

Minimising panel sound radiation

We have seen that the objectives of the cabinet are:
• To look good.
• To emanate a minimum amount of sound.
• To maintain temporal accuracy.
• To help control directivity.
There is a fifth attribute that the cabinet should also have andit is stability. The most obvious criterion is that it should not
be easy to knock the loudspeaker over. Loudspeakers canbe heavy and could cause discomfort or injury should theyfall on anyone, especially a small child. There are furniture
standards that cover this and the device should return to an
upright position if it is leaned over up to 10°.
Also important to the audiophile is that the cabinet should
not be allowed to rock when the drivers operate. If thishappens, the bass quality is impaired. It may be described as“softening” and the effect is most noticeable on percussive
bass, often referred to as “slam”.
All the floor-standers in the range are equipped with spikefeet that pierce through carpeting and prevent any rocking.
These spike feet are fitted to outriggers that extend beyond
the boundaries of the cabinet and provide the necessary
static stability.
Stand-mount systems should always be mounted on a standthat affords similar stability.

The panels of the cabinet may be excited by two mechanisms:• Reaction forces from the drivers – as the driver
diaphragm moves back and forth there is a reaction
force on the chassis (basket) that is them transmitted tothe cabinet.
• Forces from pressure in the air contained inside
the cabinet.
If cabinets suffer these problems, it is worth asking why weneed them at all. The answer lies in the drivers themselves.
They radiate the same amount of sound energy to the rearas they do to the front. Unfortunately, the rear radiation
is out of phase with that from the front and, if the twoare allowed to mix, they cancel each other out. The
ability of sound waves to go around corners depends on
the wavelength relative to the size of the object. A goodanalogy is to consider waves in the sea. They readily wrap
round small rocks but are reflected from cliffs. So it is witha loudspeaker driver. The lower the frequency the more thesound will wrap around the driver. A driver suspended freely
in air will have no bass. We need a mechanism for blocking
this interaction and the cabinet serves to do this.
There are two common ways of reducing panel output:• Adding bracing between the panels to increase stiffness.
• Adding damping pads to the panels.
The idea behind method 1 is that the increased stiffness
reduces panel motion. What it does in fact is to raise
the frequency of any resonances. A similar effect would
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occur if the panel thickness were to be increased. Thismight be of some benefit if the loudspeaker system has
separate enclosures for different drivers, like in R Series.
The resonances in any one enclosure may be pushed outof the working frequency range of the driver housed init. However, there is a drawback to this method. The bass
resonances may be pushed into the midrange, where the ear
is more sensitive.
The addition of damping pads to the panels does work andwas used on the R Series 2011. The pads are usually madeof some bituminous material that converts deformation into
heat and thus reduces the vibrational movement of the basepanel. Unfortunately, most commercially available pads aredesigned to damp resonances in the thin metal panels usedon cars. Their efficacy depends on matching the mechanical
impedances of panel and damping pad and the match
between the pads and wood is not ideal.

Figure 1
Driver in free air

KEF Engineers developed a technique, originally for the
LS50 loudspeaker system but applied to all R Series 2018
systems, that combines the two approaches and is more
effective than both. The technique, called damped bracing,consists of strategically placed wooden braces that, insteadof being rigidly coupled to the panels are joined via a lossy
interface. The braces are placed where the panel movement
would otherwise be greatest and the lossy layer, like normal
damping pads, converts the energy into heat.
Additional damped braces are situated at the back of eachdriver, which reduce the level of reactionary vibrationtransmitted from the driver to the cabinet.
The reduction in sound output from the cabinet varies
model by model, depending on the exact geometry, but
is typically of the order of 20dB, as shown in figure 3.
Cabinet sound radiation is highly distorted and, at theresonance frequencies, causes time-smearing as the output
hangs on. The increase in clarity wrought by the damped
bracing is readily noticeable, is probably best in class for
single-enclosure multi-way loudspeaker systems and iscomparable to the best high-end systems.

Figure 2
R11 Cabinet construction

Figure 3
Panel vibration — without damping — with damped bracing

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Directivity and diffraction
Both these attributes are affected by the geometry of thecabinet and so are lumped together here.
It is not often appreciated that the directivity of any
loudspeaker system is controlled as much by the geometryof the cabinet as by the size of the drivers. In this, theengineer is helped by the fact that listeners may occupy
positions that vary in the horizontal plane, but ear height
differs little. It follows that it is much more important to keep
the horizontal dispersion of the loudspeaker wide than is it
the vertical.
It was stated above that each of the R Series cabinets is slim.
Not only does this minimise the cabinet’s visual impact, but
it allows the wide horizontal directivity required to createa stable sound stage for all listeners. The larger systems in
the range employ multiple small-diameter drivers rather thata single larger diameter one precisely so the cabinet canremain slim and maintain the desired horizontal directivity.
There is a small reduction in vertical directivity – but this is
not a problem at bass frequencies, where the wavelength
is long compared to the driver spacing. In the more criticalmidrange, the Uni-Q array gives far better vertical directivity
than can be had from multiple vertically-displaced drivers.
Diffraction – the bending of wave motion around corners –can cause time smearing. Some of the sound energy generated
by the drivers travels along the front surface or baffle of thecabinet and, if it reaches an edge, re-radiates as shown by
figure 5. It’s rather like having small secondary drivers spreadalong the edge of the cabinet. But this radiation is delayedcompared to the sound coming directly from the driver,approximately by the time it takes to travel from the driverto the edge. There is thus a degree of time smearing that
impairs the clarity of what the listener hears. Not only
that, but there is perturbation of the frequency response
because this secondary radiation goes in and out of phasewith the direct sound as the frequency and commensurate
wavelength vary.
The classic way of dealing with this effect is to round the
sharp corners of the cabinet. In simple terms, this can be
regarded as creating a multitude of edges, each with much
lower levels of secondary radiation. The total level is much
lower with the result that the time smearing becomes

Figure 4
Acoustic Impedances after Beranek - Note that it is the 20dB/decadeslope of the real part that affects the driver response in its working range

inaudible and the driver’s response smoother. However,
the radius of curvature must be of the same order as the
wavelength of the sound, the overall width of the cabinet
increases, making it more visually intrusive, and the cost of
manufacture increases considerably. So, whilst this technique
is often found on the most expensive systems, it is not
appropriate to the R Series.
With most manufacturers, the diffraction effect is something
to be suffered and not talked about. But the fact that weare talking about it here indicates that KEF engineers havesuccessfully tackled the problem by a bit of lateral thinking
made possible by the fact that Uni-Q drivers are usedthroughout.
It has been known for many years, and is illustrated by Leo
Beranek in his book “Acoustics”, published in 1954, that the
acoustic impedance of a driver (the interface between thediaphragm motion and radiated sound) is smooth when
mounted both in an infinite baffle (as expected) and at theend of a long tube the same diameter as the diaphragm.
This latter situation comes about because the dispersion of
the driver is such that it doesn’t acoustically “see” the edgesof the tube. The physics of this situation can be bent a little
and it can be shown that a driver mounted in a cabinet notmuch wider than itself has a very low level of diffraction. It’s
not perfect, but the level of diffraction is much lower than it
would otherwise be.

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PortsPorts are used to augment bass response. They have thedual advantage that they enable a lower cut-off frequency
than closed-box systems for the same size of enclosure and,within the operating range of the port, the bass driver moves
less, thus lowering distortion.
Ports are not without their drawbacks, however.

Figure 5
Simulation of tweeter output
top – tweeter mounted directly in baffle
bottom – tweeter in Uni-Q array with shadow flare— direct — diffracted• They can be a window to resonances in the air cavity
inside the cabinet.
• At midrange frequencies, they act rather like an organ
pipe and exhibit a series of resonances.
• They can be a source of noise if turbulence is allowed
to happen as air vents out at each end of the port
(so-called chuffing).
KEF engineers have tackled each of these potential problems.

If we look at the drivers used in the R Series systems, we
can see that the bass drivers follow this requirement, but the
midrange and tweeter drivers do not. Normally, the tweeter
would be the most to suffer because it is much smaller than
the width of the cabinet, which is determined by the diameterof the bass drivers. However, in the Uni-Q driver (more ofwhich later), the tweeter fires into an acoustic waveguide
formed by the midrange diaphragm and it is the diameter ofthis relative to the width of the cabinet that determines thelevel of diffraction.
There is more work to be done, because the diameter of themidrange (for other reasons) is smaller than that of the bass
drivers, and it is here that the lateral thinking comes into play.

Figure 6
Simulation of bass driver + port — and port only —when the port isplaced close to the node of internal resonances.

Around the Uni-Q driver array is what is normally called atrim ring. Not simply a decorative piece, the R Series trim
ring is specially shaped and engineered. It increases theeffective diameter of the waveguide such that the driverarray “sees” the edges of the cabinet much less than if itwere mounted normally. Hence the name “Shadow Flare”.
The level of secondary radiation is decreased, resulting inless time-smearing, greater clarity and a smoother response.
This technique was originally developed for the Reference
Series products and illustrates our trickle-down philosophy.
Figure 7
Simulation of bass driver + port — and port only —when the port isplaced close to the antinode of internal resonances.

9Organ pipe resonances
These resonances within the port itself can be drasticallyreduced if the walls of the port have a degree of flexibility.
Instead of the normal rigid plastic, the walls are fabricatedfrom closed-cell foam. Originally developed for the LS50
loudspeaker, this technique dramatically reduces the pressure
variations inherent in these higher frequency resonances andrenders them less audible. Figure 8 illustrates this techniqueusing Finite Element Analysis (FEA), where the colour scalefrom green through to red indicates the level of air pressure.

Turbulence

Figure 8
Pressure of the first “organ pipe” resonance in a port Top –rigid walls and no end flareBottom – flexible walls and end flares

Resonances inside the cabinet
The port(s) are positioned on the rear panel of the cabinet.
This has two advantages:• Resonances inside the cabinet are at relatively highfrequencies and the sounds are not directed towards thelistener.
• There is more freedom on the rear panel comparedto the front in positioning the port so it is placed close
to antinodes (nulls) of the resonances, so less energy istransmitted through the port.
To illustrate this last point, figures 6 & 7 show the differencein output when the port is placed at a node and an antinodeof an internal resonance. For the first of these measurements
there is no wadding inside the cabinet, so the effect is madeclearer. 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 and cannot be
detected in the overall response.

This occurs at both ends of the port as the air exits theport either to the open air or the air enclosed within thecabinet. The solution is to flare both ends of the port. The
progressive expansion of the air afforded by the flaring
reduces turbulence and thus audible chuffing. It should be
noted that, if turbulence is allowed to develop, not only is
it audible, but the performance of the port changes withsound level, impairing the dynamic range of the loudspeaker.

Drive units (Drivers)
All but one of the systems are 3-way – they have one ormore bass drivers, a midrange driver and a tweeter. The
midrange driver and tweeter are combined into a single
array known as Uni-Q. The only 2-way in the series utilises a
bass-midrange driver instead of a dedicated midrange driverin its Uni-Q array.

Bass drivers
Whether the system contains one, two or four bass drivers,
the design remains the same, save for the impedance ofeach individual driver. The number of bass drivers simply
determines the maximum loudness of the system for a given
level of distortion. Note that, when there is an even numberof bass drivers, these are symmetrically arranged above andbelow the Uni-Q array to preserve a virtual point source
over the full range.

10DiaphragmThis is a 2-part structure, comprising a shallow concave
aluminium front skin connected to the driving voice coil by amore conventional paper cone (see figure 9).

Figure 10— Response of non-optimised bass driver — Response of optimised bass driver Panelis not excited.
Figure 9
Cross-section of bass driver moving parts

The aluminium skin has a shallow profile to minimise
perturbation of the response of adjacent drivers as a resultof diffraction.

Magnet system
With the new series, the magnet system has been totallyredesigned. One of the key objectives is to keep the magnetic
flux density seen by the voice coil as constant as possible
with coil position.

The cone itself is vented to allow free movement of air as
the diaphragm assembly moves back and forth. The static
centre pole of the magnet structure causes the volume of
air between the paper cone and the front aluminium skinto vary with diaphragm position and this must not becometrapped, otherwise the air will be pumped through thenarrow apertures of the magnet system and cause noise.
The front aluminium skin provides the stiffness necessary toprovide pistonic radiation and therefore temporal accuracy
within the working range of the driver. However, like all stiff
materials, the stiffness is not infinite and at some frequency
diaphragm breakup sets in and the Q (sharpness) of theresonances is high – the response peaks at resonance canbe as much as 20dB above the pistonic response. Such
resonances are not readily suppressed by the relatively
gradual attenuation of the crossover network and, if leftunchecked, will cause colouration.
To reduce the intrusion of these higher-frequency resonances,the paper cone is joined to the aluminium skin at a nodal
ring, not at the periphery. The nodal ring is an antinode ofthe lowest resonance and, when driven there, the resonance

Figure 11
Finite Element Analysis (FEA) of
Top – 2011 R Series bass driver magnet system
Bottom – 2018 R Series bass driver magnet system11

To this end, the old magnet system featured an extended
centre pole that balanced the solid material that connectedto the back plate. However, this design is still somewhat
asymmetric, with the result that the force on the voice coil
was also asymmetric, introducing more distortion than isnecessary. Further, the asymmetric nature of the metalwork
caused the coil’s inductance to vary with position, altering thetermination, and therefore the equalisation, of the crossover.
The new design is more complex and features a so-called
T-shape pole (see figure 11).
This is more symmetric than the older design and, with thebulk of the centre pole being further from the voice coil,lowers both the coil’s absolute inductance and its variation
with position. The result is a useful reduction in even-orderharmonic distortion.
Note, too, the undercut in the back plate that allows greater
coil movement before the coil hits the back plate.
Another feature of the new design can be deduced bylooking at the magnetic permeability in the metalwork.
The darker the colour, the lower the permeability and, where
it is almost black, the metalwork is saturated. This has two
advantages for the performance:• The saturation limits the flux density in the coil gap andso this remains constant regardless of the inevitableproduction tolerances in magnet strength.
• Saturation further reduces the absolute level of inductanceof the voice coil and, with it, further educes the variationin flux density that would normally occur as the coilmoves back and forth in the gap.

Spider (Damper)
The spider, or rear suspension, contributes to the linearity
of coil movement in addition to the magnet system. In thiscase, as we’re dealing with symmetry, it’s the odd-orderharmonics that are mostly affected. The new spider design
allows greater linear movement and a commensurate
reduction in distortion, clearly illustrated in figure 12.

Figure 12
Suspension compliance as a function of diaphragm displacement— Old spider — New spider

Uni-Q array
The Uni-Q array has been the mainstay of virtually all KEF
loudspeakers since its introduction in 1988. It delivers theholy grail of loudspeaker design in that all sound appears toemanate from the same point in space. Coaxial loudspeakers
had been around for many years before the introductionof Uni-Q, but the tweeter was never time aligned with themidrange or bass/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 wellbehind. With Uni-Q, both drivers have their acoustic centre
at the same point and the larger driver acts as a waveguidefor the tweeter. The result is that the blend between the
two units is virtually seamless in terms of both responseand dispersion. The two units together can be regarded asa single driver without the performance shortfall that would
be suffered by a true single driver covering such a wide
frequency range.
Over the intervening years, the Uni-Q concept has been
progressively refined. The midrange cone shape has been
optimised to create just the right amount of dispersion at all
frequencies and innovations such as the tangerine waveguide
have improved the performance of the tweeter. This is the
12th generation of the array and brings an extra degreeof refinement to the performance, but it is worthwhile todescribe everything that goes into this remarkable array, notjust the latest refinement.

12One of the requirements of building this driver combinationis the need to maintain tight tolerances. Some of the gaps
between parts are necessarily small and without robotic
assembly methods it would be impossible to achieve
consistency of performance. There is a place for handbuilding, but this is not it.

Filling in the gaps
One of the problems with constructing a combination driverarray like Uni-Q is dealing with the gaps that separate theconstituent parts. There is a narrow channel – an annulargap – between the moving midrange voice coil and thestatic start of the tweeter waveguide. This channel acts asan organ-pipe-like resonator, 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.

Figure 15
Simulation of tweeter response— In perfect waveguide — With open gap

Obviously, this annular gap is necessary to allow the midrange
cone and voice coil to move, so the solution was to createa cavity between the midrange and tweeter magnets towhich this annular gap connected. Adding damping to this
newly-created cavity was found to be effective in taming
the resonances in the annular gap and the removal of
the response glitches was immediately apparent as an
improvement in detail clarity.
Figure 16
Actual tweeter response— Original — With cavity damping

Midrange
Cone and surroundFigure 13
Uni-Q array with no gap between HF and MF magnets

Figure 14
Uni-Q array with opened cavity between HF and MF magnets and damping added

The cone is formed from a magnesium/aluminium alloy. Like
the front skin of the bass driver, it serves to provide thenecessary stiffness to give pure pistonic motion over thedriver’s working range. The stiffness is increased by the radialembossing in the cone profile, but nevertheless it is prone
to high-Q breakup in the frequency range covered by thetweeter.

Figure 17
Uni-Q drive exploded view

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The KEF Blade uses nodal drive (as described on page 11,
where the technique is used on the bass drivers) to tamethese resonances. But that technique, when applied to a
Uni-Q array, cannot use an intermediate cone and instead
requires an expensive large diameter voice coil which was
not appropriate to the R Series.
Instead, there is a lossy interface where the voice coil joins
the cone. This interface serves to decouple the drive at highfrequencies. The high-Q resonances are tamed to the extent
that they may be properly attenuated by the crossover andnot break through the tweeter output.
Figure 19 illustrates the improvement in the unit’s response
as a result of using the lossy interface.
The surround is particularly problematic. It has to allow
the cone to move freely, but it is also a critical part of thetweeter waveguide. Surrounds also tend to break into
complex vibrational behaviour at higher frequencies, often
moving in the opposite direction to the cone. The so-called“surround dip” in a driver’s response is quite common.
The new R Series driver features an improved surroundgeometry, achieved through extensive computer simulation,
optimisation and practical experimentation. It allows greater
movement of the cone and gives a smoother response forboth midrange and tweeter.

Figure 19
Response of midrange driver measured without crossover in 2π baffle— without interface — with interface

Magnet system
The design of the magnet system follows that of the bass
drivers. alongside the addition of aluminium de-modulation
rings. The reduction in harmonic distortion as a result of
the new magnet system and improved surround geometry
is shown in figure 20.

Figure 20
Midrange driver distortion (1m, 5.6V RMS)
— 2011 R Series — 2018 R Series

Tweeter
Diaphragm assembly
The dome diaphragm itself is aluminium. More exotic
materials – diamond and beryllium for example – aresometimes used in an effort to increase stiffness and push thepistonic region of the tweeter to the limits of human hearing.
But this can be done with aluminium at far lower cost,providing some ingenuity is used in designing how the dome
is constructed.
Figure 18
lossy interface between voice coil and midrange cone

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The optimum dome shape for waveguide loading is aspherical cross-section, but the optimal shape for stiffness
is an elliptical cross-section. Both were combined into thepatented KEF Stiffened Dome. An elliptical dome attaches
to the voice coil former and the centre is removed. This iscapped by a spherical dome and a very stiff triangular section
is formed where the two parts join.

Figure 23
Simulated response of tweeter— without tangerine waveguide — with tangerine waveguide

Rear loading
Figure 21
3D CAD sectional views of the tweeter dome and extended former
that meet to form a triangular stiffening member at the dome edge.

Tangerine waveguide

Figure 22 Tangerine waveguide

The interface between the radiating dome of the tweeter andthe waveguide formed by the midrange dome is extremely
critical. Ideally, the diaphragm should 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 overthe surface of the dome. To compensate for this non-ideal
situation and taking a leaf out of the design of pressure drivers,
the Tangerine Waveguide was developed to restore correct
coupling between the dome and the whole waveguide.
The improved coupling at high frequencies above 5kHz
brings with it a useful increase in sensitivity and a reduction in
the height of the first resonance peak, illustrated by figure 23.

Like any drive unit, the tweeter radiates as much energy
backwards as it does forwards. Not only does this energy
have to be absorbed, but there must be sufficient volume
behind the dome to prevent a build-up of excessive pressure.
This pressure can impair the suspension of the dome andcause displacement of the ferrofluid that sits in the magnet
gap to cool the voice coil.
To that end, the dome vents through the centre pole ofthe magnet system to a cavity filled with absorbent wadding.
The cavity is tapered to that, when the wadding is inserted,
it becomes more dense towards the rear of the cavity,
gradually increasing absorption.
All the rearward energy is absorbed and cannot re-radiatethrough the dome after being reflected at the back of
the cavity.

Figure 24
Front and rear view cross sections of tweeter showing rear loading15

Crossovers
All 3-way crossovers, which covers all bar the 2-way Atmos®
enabled R8a, follow the same general format shown in figure
25.
Each system in the range has been carefully voiced using awide variety of musical genres to ensure that no artificial
enhancement is introduced.
All crossovers have a reduced component count. This has
been made possible due to the decoupler in the Uni-Q
midrange as well as the improved tweeter, both of whichhave inherently smoother responses and require lesscorrection from the crossover. The series resistors in the
MF and HF sections are merely to change the output level ofthese drivers to match that of the bass drivers.

All cored inductors in the new series crossovers use alaminated core material that substantially lowers distortion
compared to the outgoing series, as shown in figure 27.
Mention should be made regarding the two parallel
capacitors C3 and C4. The total value is large and low-loss
types would be prohibitively expensive, so the bulk of thevalue is made up of an electrolytic type and around 10% of
the total value is a polypropylene type, wired in parallel - theso-called bypass configuration.
This configuration delivers almost the same low loss andquality that would be achieved by having the full value solely
supplied by one or more polypropylene capacitors wired inparallel, but at a much lower cost.

The HF section features a tuned circuit (L1/C2) in parallelwith the driver. This tuned to the fundamental resonancefrequency of the driver and allows the series capacitor tohave the correct attenuation. Without it, there would be
excessive movement of the diaphragm and an unwanted
hump in the response.
There are two tuned circuits in the LF section. One (L6/
C6/R4) is tuned to the upper peak of the reflex impedance,and prevents the system impedance falling too low should
the 2nd-order low-pass formed by L4 and C5 be allowed
to interact with it. The inductor L5 is small and tunes with
C5 to attenuate a residual high Q resonance in the driverresponse to prevent it colouring the overall sound.

Figure 26
Inductor distortion at 100W
— 2011 R Series — 2018 R Series

Figure 25
General schematic of R Series 3-way crossovers

16

17

Model Range
R3
Description
This is the smallest of the ‘regular’ loudspeakers in the range,
being a compact 3-way stand-mount model. It is suited tothe smaller room or closer listening, where less power isrequired to reach realistic sound levels.

Technical Specifications
DesignThree-way bass reflex

Drive units

Uni-Q Driver Array:
HF: 25mm (1in.) vented
aluminium dome
MF: 125mm (5in.)aluminium cone
Bass Driver:LF: 165mm (6.5in.)hybrid aluminium

Crossover frequency

400Hz, 2.9kHz

Frequency range (-6dB)
Typical in-room bass
response (-6dB)

38Hz - 50kHz
30Hz

Frequency response (±3dB) 58Hz - 28kHz
Harmonic distorition<0.3% 130Hz - 20kHz

Maximum output

110dB

Amplifier power(recommended)

15 - 180W

Nominal impedance8Ω (min.3.2Ω)Sensitivity (2.83V/1m)

87dB

Weight*13.5 kg (29.8 lbs.)

Dimension (H x W x D)
with terminal*

422.2 x 199.6 x 335.5 mm(16.6 x 7.9 x 13.2 in.)Finishes

Black Gloss / White Gloss/ Walnut* Measurement per unit

18

R5
Description
This is the smallest of the three floor-standers in the range.
It’s twin 130mm diameter bass drivers are positioned eachside of the Uni-Q array in d’Appolito configuration, topreserve an effective point source delivery throughout thewhole frequency range.

Technical Specifications
DesignThree-way bass reflex

Drive units

Uni-Q Driver Array:
HF: 25mm (1in.) vented
aluminium dome
MF: 125mm (5in.) aluminium
cone
Bass Driver:LF: 2 x 130mm (5.25in.)hybrid aluminium

Crossover frequency

400Hz, 2.9kHz

Frequency range (-6dB)
Typical in-room bass
response (-6dB)

38Hz - 50kHz
29Hz

Frequency response (±3dB) 52Hz - 28kHz
Harmonic distorition<0.3% 120Hz - 20kHz

Maximum output

110dB

Amplifier power(recommended)

15 - 200W

Nominal impedance8Ω (min.3.2Ω)Sensitivity (2.83V/1m)

87dB

Weight*27.3 kg (60.2 lbs.)

Dimension (H x W x D)
with terminal*

1025 x 175 x 343.5 mm(40.4 x 6.9 x 13.5 in.)Dimension (H x W x D)
with terminal with Plinth*1071.4 x 271.6 x 343.5 mm(42.2 x 10.7 x 13.5 in.)Finishes

Black Gloss / White Gloss /Walnut* Measurement per unit

19

R7
Description
Like the R5, this is a floor-standing loudspeaker featuring twinbass drivers positioned above and below the Uni-Q array.
However, the larger diameter of the bass drivers enables ahigher sound level to be delivered and the larger cabinet
affords a greater bass extension.

Technical Specifications
DesignThree-way bass reflex

Drive units

Uni-Q Driver Array:
HF: 25mm (1in.) vented
aluminium dome
MF: 125mm (5in.) aluminium
cone
Bass Driver:LF: 2 x 165mm (6.5in.)hybrid aluminium

Crossover frequency

400Hz, 2.9kHz

Frequency range (-6dB)
Typical in-room bass
response (-6dB)

33Hz - 50kHz
27Hz

Frequency response (±3dB) 48Hz - 28kHz
Harmonic distorition<0.3% 120Hz - 20kHz

Maximum output

111dB

Amplifier power(recommended)

15 - 250W

Nominal impedance8Ω (min.3.2Ω)Sensitivity (2.83V/1m)

88dB

Weight*31.4 kg (69.2 lbs.)

Dimension (H x W x D)
with terminal*

1062 x 200 x 383.5 mm(41.8 x 7.9 x 15.1 in.)Dimension (H x W x D)
with terminal with Plinth*1108.9 x 310.6 x 383.5 mm(43.7 x 12.2 x 15.1 in.)Finishes

Black Gloss / White Gloss /Walnut* Measurement per unit

20

R11
Description
This, the largest loudspeaker in the range, features four
165mm (6½ inch) diameter bass drivers, enabling a enoughsound level output to drive large rooms without distress.
The large cabinet affords the most extended bass response.

Technical Specifications
DesignThree-way bass reflex

Drive units

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

Crossover frequency

400Hz, 2.9kHz

Frequency range (-6dB)
Typical in-room bass
response (-6dB)

30Hz - 50kHz
26Hz

Frequency response (±3dB) 46Hz - 28kHz
Harmonic distorition<0.3% 120Hz - 20kHz

Maximum output

113dB

Amplifier power(recommended)

15 - 300W

Nominal impedance8Ω (min.3.2Ω)Sensitivity (2.83V/1m)

90dB

Weight*37.7 kg (83.1 lbs.)

Dimension (H x W x D)
with terminal*

1249 x 200 x 383.5 mm(49.2 x 7.9 x 15.1 in.)Dimension (H x W x D)
with terminal with Plinth*1295.5 x 310.6 x 383.5 mm(51 x 12.2 x 15.1 in.)Finishes

Black Gloss / White Gloss /Walnut* Measurement per unit

21

R2c
Description
This is the first of two special application loudspeakers. It’sa dedicated centre-channel model and, like the R5 features
two 130mm (5¼ inch) diameter bass drivers each side ofa centrally placed Uni-Q array. Normally, the horizontallydisposed drivers of centre-channel loudspeakers suffer a
restricted horizontal dispersion in the crossover region
between midrange and tweeter, or throughout the midrange
band if two drivers are used to cover that frequency range.
However, the point-source Uni-Q array avoids this and offers
better horizontal dispersion than any other horizontallydisposed configuration.

Technical Specifications
DesignThree-way closed box

Drive units

Uni-Q Driver Array:
HF: 25mm (1in.) vented
aluminium dome
MF: 125mm (5in.)aluminium cone
Bass Driver:LF: 2 x 130mm (5.25in.)hybrid aluminium

Crossover frequency

400Hz, 2.9kHz

Frequency range (-6dB)
Typical in-room bass
response (-6dB)

64Hz - 50kHz
49Hz

Frequency response (±3dB) 74Hz - 28kHz
Harmonic distorition<0.3% 120Hz - 20kHz

Maximum output

110dB

Amplifier power(recommended)

15 - 200W

Nominal impedance8Ω (min.3.2Ω)Sensitivity (2.83V/1m)

87dB

Weight*16.9 kg (37.3 lbs.)

Dimension (H x W x D)
with terminal*

175 x 550 x 308.5 mm(6.9 x 21.7 x 12.1 in.)Finishes

Black Gloss / White Gloss/ Walnut* Measurement per unit

22

R8a
Description
This compact model is the only 2-way loudspeaker in therange. It features a single Uni-Q array, this time having a bass/
midrange driver surrounding the centrally placed tweeter
unit. It’s angled front baffle makes it the perfect solution for
either a wall-mounted surround loudspeaker or a speakertop-mounted Dolby Atmos® module.

Technical Specifications
DesignTwo-way closed Box

Drive units

Uni-Q Driver Array:
HF: 25mm (1in.) vented
aluminium dome
MF/LF: 130mm (5.25in.)aluminium cone

Crossover frequency

2 kHz

Frequency range (-6dB)
Typical in-room bass
response (-6dB)

96Hz - 19.5kHz-

Frequency response (±3dB) 105Hz - 18.5kHz
Harmonic distorition<0.3% 200Hz - 20kHz

Maximum output

106dB

Amplifier power(recommended)

25 - 150W

Nominal impedance8Ω (min.4.2Ω)Sensitivity (2.83V/1m)

86dB

Weight*4.45 kg (9.8 lbs.)

Dimension (H x W x D)
with terminal*

173.8 x 174.6 x 259 mm(6.8 x 6.9 x 10.2 in.)Finishes

Black Gloss / White Gloss* Measurement per unit

23

Summary

References

The R Series sits immediately below the Reference Series
and borrows much of that range’s technological attributes.
It follows our belief that a great sounding loudspeaker is theresult of solid, careful and effective engineering. This involves
utilising every computer simulation and measuring techniqueavailable, backed up by practical experimentation andverification.

O J Pedersen, S Bech and K Rasmussen, Final Report on
Activities of Eureka Project ARCHIMEDES. Univ., 1995.

But loudspeakers, like any other component of the
reproduction chain, are there to communicate more thanmere measurements and specifications. As RaymondCooke realised from the start, emotion and involvement
are also part of what must be delivered to the listener. For
that reason, engineers and non-engineers alike audition
prototypes at every stage of development to make surethat the engineering is relevant and improves the overall
performance.

“The Reference (white paper)”, KEF R&D, May-2014.

More than that, it is the same team that guides every KEF
product through its development, with the result that there
is a consistency of what constitutes quality throughout
the company’s portfolio. The products deliver outstanding
performance whatever the musical genre and regardlessof whether the programme is 2-channel or multi-channel
based.
The R Series incorporates many technologies that were
pioneered by KEF - the Uni-Q point source array is probably
the best-known and certainly one of the most effectiveof these. But this paper illustrates how every aspect ofloudspeaker performance has been assessed and taken to thelimit, within the imposed budgetary constraints, sometimes
involving a new and different way of addressing the problem.
Some of the techniques described here are dealt with ingreater detail in the KEF Reference Series white paper andthe interested reader is urged to peruse that in addition tothe other publications featured in the list of references on
the next page.

Acknowledgements

L R Fincham, A Jones and R H Small, “The Influence ofRoom Acoustics on Reproduced Sound, Part 2: Design ofWideband Coincident-Source Loudspeakers”, in The 87th
Convention of the AES, 1989.

K Kessler, D A Watson and M Cooke, “KEF 50 Years ofInnovation in Sound” – A Limited Edition Book, GP Acoustics
International Limited, Maidstone, UK, 2011.
M Dodd and J Oclee-Brown, “A New Methodology for the
Acoustic Design of Compression Driver Phase Plugs with
Radial Channels”, presented at the Audio Engineering Society
Convention 125, 2008.
M Dodd, “Optimum Diaphragm and Waveguide Geometry
for Coincident source Drive Units”, presented at the AudioEngineering Society Convention 121, 2006.
M Dodd and J Oclee-Brown, “Design of a Coincident Source
Driver Array with a Radial Channel Phase-Plug and Novel
Rigid Body Diaphragms”, presented at the Audio Engineering
Society Convention 127, 2009.
M Dodd, W Klippel, and J Oclee-Brown, “Voice Coil
Impedance as a Function of Frequency and Displacement”,
presented at the Audio Engineering Society Convention 117,
2004.
L L Beranek, “Acoustics”, McGraw-Hill, 1954 revised 1986
G Perkins, “Modelling loudspeaker cabinet diffraction”,
presented at the IOA Reproduced Sound conference, 2014.
M Dodd, “The Development of the KEF LS50, a Compact
Two-way Loudspeaker System”, presented at the AudioEngineering Society Conference: 51st International
Conference: Loudspeakers and Headphones, 2013.

The KEF logo and Uni-Q is are registered trademarks of GP
Acoustics (UK) Ltd. Dolby and Atmos are trademarks of
Dolby Laboratories.
24