KEF R&D

KC62

CONTENTS

2

Introduction

2

Philosophy

2

KC62

2

Compact Subwoofers2

Hofmann’s Iron Law

3
3

Technology
Uni-Core Driver Array

3

Force Cancelling/Dual Voice Coils

4

Matching Driver Behaviour

5

Inductance6

P-Flex Surround7

Smart Distor�on Control Technology

7

Hybrid Control Systems

8

Aluminium Cabinet

8

Vibra�on Control

8

Thermal Management

8

Frequency Response

9Conclusion

9

References

10

Specifications

KC62 front (Mineral White finish, top) and side (Carbon Black finish, bo�om)

₁Introduc�on
Designing a small, high-performance subwoofer isdifficult. There are compromises to be made in regards
to output and extension.

With the KC62 Uni-Core subwoofer, KEF engineers
have developed three new patent-pending technologies
that work together to deliver a higher level of
performance in all aspects at the form factor.

This paper discusses these new technologies, and otherfeatures, as well as giving insight into KEF’s philosophy
on subwoofer and loudspeaker design.

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

unavailable to Cooke in those early days. The listening
experience is s�ll the final arbiter in deciding whether or
not the science is effec�ve, but without it progresscannot be measured and the way forward cannot be
understood.

KC62
KC62 is the smallest force-cancelling subwoofer KEF
has ever developed. It aims to push the idea of
Hofmann’s Iron Law to its limits. Brand new technologies
and features work together to produce a novel systemthat delivers on all three fronts - high output, low
extension and small cabinet size. When combined with
KEF’s thorough understanding of driver design, KC62
provides all of this with finesse, delivering detail andimpact precisely when required.

Compact Subwoofers
There are four main reasons why somebody may preferto choose a compact subwoofer:•

Smaller listening area•

Placement in a dual-purpose room, for example aliving room•

Aesthe�c considera�on•

Easier to accommodate mul�ple subwoofers

Raymond Cooke OBE, KEF Founder

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

Cooke realised that this could only be achieved by ascien�fic understanding of sound and its reproduc�on,concentra�ng his efforts on loudspeakers - the last linkin the chain and probably the most difficult, in that they
have to work in an unknown three-dimensional
environment. Never afraid to employ the most capable
engineers and provide them with the latest and mosteffec�ve tools, Cooke established an engineering
philosophy that s�ll exists today.

KEF engineers con�nue to pursue this scien�fic
endeavour, using tools and technology that were

However, the size of compact subwoofers introduces
certain compromises.

Hofmann’s Iron Law
Josef Anton Hofmann described the rela�onshipbetween three parameters in loudspeaker design cabinet volume, low frequency extension and efficiency.
His Iron Law states that, of these three, one must becompromised in order to achieve favourable results inthe other two. Various papers have further discussed therelated loudspeaker parameters [1][2][3].

₂Consider the modern string family of musicalinstruments - the violin, viola, cello and double bass.
Table 1 shows the lowest notes that these instruments
will play in their standard tuning, along with thefrequency of that note - the violin being the highest
pitch, through to the double bass as the lowest.

It should also be no�ced that the size of the instrument
increases as the frequency of the lowest note it can
reproduce decreases (Figure 1). It would be possible to
make a double bass with a smaller body - but it would be
far, far quieter.

The same occurs with subwoofers.

Instrument

Lowest Note

Frequency

ViolinG3

196 HzViola

C3130.8 Hz

Cello

C2

65.4 Hz

Double Bass

E1

41.2 Hz

Table 1. Lowest frequencies typically produced by members of the modern string
family of instruments

To create a subwoofer that can deliver the maximum
performance for the minimum of space, KEF R&D
worked to push the compromises as far as possible.

A�er all, Iron Laws cannot be broken, but they cancertainly be bent.

Technology
Uni-Core Driver Array
Uni-Core is a brand new, patent-pending driver designthat combines high excursion and force cancelling whilst
reducing size - ideal for pushing the boundaries ofultracompact subwoofer performance.

Force Cancelling/Dual Voice Coils
KEF has considerable experience with force cancelling,having released subwoofers such as KF92, andReference 8b, as well as the world’s first Single Apparent
Source loudspeaker, Blade. What may be less wellknown, however, is that KEF actually launched the firstcommercial loudspeaker with force cancelling all the way
back in 1984 - the Reference 104/2 (Figure 2).

Figure 1. Violin and Double bass

When designing a small subwoofer, the choice has to bemade between high efficiency and low frequency
extension. Both are very desirable features for a
subwoofer.

Why the term ‘Iron Law’? Goethe, in the poem Das
Gö�liche, refers to ‘great, eternal iron laws.’ Iron isstrong, everlas�ng, and never to be broken.
Figure 2. Reference 104/2 - the internal twin bass drivers were braced using a metalrod, allowing for a force cancelling arrangement₃Force cancelling provides two major benefits - firstly, as
driver mo�on exhibits Newton’s Third Law -every ac�on
has an equal and opposite reac�on - the driver’s mo�on

To achieve force cancelling, one voice coil in the UniCore arrangement has been wired in inverse polarity due
to the shared magne�c flux direc�on.

causes mechanical vibra�on in the cabinet. Having an
iden�cally performing driver placed in opposi�on
cancels out the reac�on force of each driver - hence theterm ‘force cancelling.’ This greatly reduces cabinet
vibra�on, and in turn reduces sound colora�on.
Secondly, the doubling of cone area increases the
maximum output of the subwoofer by up to 6dB.

Force cancelling can be achieved by moun�ng, or
bracing, two opposing drivers back to back. This will
dictate the depth of the cabinet. To achieve reduced
depth, driver excursion must be reduced, compromising

Figure 4. Schema�c of the Uni-Core driver motor

maximum output.

Uni-Core, however, is not comprised of two drivers
mounted back to back. Instead, the two voice coils arearranged within a single motor system (Figure 3).

Figure 3. Cutaway of the Uni-Core driver array

Figure 5. Magne�c FEA simula�on of the flux paths of two opposing
motor systems (top) and Uni-Core with a single motor system andtwo gaps (bo�om)

The two voice coils have different diameters, allowing
one to be placed concentrically within the other. This
arrangement allows the voice coils to travel within their

Matching Driver Behaviour

own gap without colliding. With increased maximum
excursion, higher sound pressure levels can be achieved(Figure 4).

Clearly, the two component drivers of Uni-Core are notphysically iden�cal, but, for force cancelling to work,
both have been designed to perform equally.

Figure 5 compares the magne�c flux paths in aconven�onal back-to-back arrangement of two driver
motors against the KC62 Uni-Core motor. In the
tradi�onal arrangement, two paths can be seen, one foreach voice coil gap, whilst with Uni-Core, a single pathcrosses both voice coil gaps.

From the onset, three areas were highlighted - the BL(motor force factor), the s�ffness of the suspension, andthe moving mass must all be consistent across bothdrivers. The first two are also func�ons of cone
displacement.

₄

BL is the product of the magne�c flux density in thevoice coil gap (B) and the length of wire in the gap (L). In
Uni-Core, the larger diameter voice coil has a larger
amount of wire in the gap. However, the structure of
Uni-Core naturally accounts for this.

In the single motor system design of Uni-Core, bothsides have approximately the same magne�c flux
passing radially through the motor gap. However, due tothe different coil diameters, the area through which theflux passes is different. This means that the flux density(B) in the small gap is greater by approximately the ra�oof the coil diameters. This allows the BL of both drivers
to match across the whole driver excursion (Figure 6).

Inductance is a cri�cal aspect of all high performance
loudspeaker drivers. The self-inductance of the voicecoil determines how much magne�c field is generated aselectrical current flows in the windings. Excess
inductance results in a strong magne�c field being
generated by the voice coil, and this can result inmodula�on of the permanent magne�c field of the
motor leading to distor�on. In addi�on, with a typicalmotor system the coil inductance varies depending on
the cone posi�on. This results in an unwanted force on
the coil, called the reluctance force, which pulls theloudspeaker cone towards the loca�on where theinductance is highest [5]. The reluctance force is
propor�onal to the square of the electrical current andis hence highly non-linear.

(N/A) BL
12
10
8
6
4
2
0
-15

-10

-5

0

5

10
T

15
mm

Smaller diameter voice coil BL(x)Larger diameter voice coil BL(x)Figure 6. Comparison of BL(x) for the two voice coils in Uni-Core

In regards to the suspension s�ffness, KEF engineers
carried out Finite Element Analysis (FEA) [4] to develop
two different suspensions which behave iden�cally,
even though they are a�ached to voice coil formers ofdifferent diameters.

Addi�onally, the driver with the smaller diameter voicecoil has a small mass added to it to compensate for thehigher mass of the larger diameter voice coil in the
opposing driver.

In addi�on to the self-inductance, in the Uni-Core themutual inductance of the two coils must be considered
too. Mutual inductance measures the ability of current in
one voice coil to generate a magne�c field whichinteracts with a second coil. In a sense, it is a measure of
the cross-talk between the coils. Since Uni-Core isdesigned to be operated with both coils driven inparallel, signal cross-talk is not the primary concern, butrather that if the magnitude of mutual inductance varies
depending on the separa�on of the coils then this could
give rise to addi�onal non-lineari�es.

At the outset of the technology development theinductance behaviour of Uni-Core was one of the major
concerns. To understand the inductance behaviour indetail, a series of numerical simula�ons were carried out.
This study revealed some surprising, and unexpected,
characteris�cs of the arrangement, which aresummarised in Figure 7. Shown are three curves of
inductance versus frequency for three different motor
arrangements. These results are not specifically for theKC62 driver but they are comparable with one anotherand typify the characteris�cs of the motor geometries.

Inductance
The Uni-Core arrangement places two voice coils inclose proximity to one another, and with the same
current orienta�on during use. In addi�on, as the voicecoils move, their rela�ve separa�on is constantly
changing. A major concern during the development of
the technology was the effect this would have on boththe self and mutual inductance of the two coils.
Figure 7. Inductance comparison of Uni-Core with an equivalent
conven�onal motor

₅The red curve shows the inductance of a pair of parallelconnected back-to-back conven�onal drivers with asimple ring ferrite motor and no aluminium or copper
conduc�ve rings, and with the coil held at the rest
posi�on. The inductance is plo�ed against frequency
and is maximum at low frequencies, then reduces as
frequency increases due to the effect of eddy currents
flowing in the motor steelwork. The conven�onal result
may be compared with the blue curve, which shows theinductance for a simple Uni-Core configura�on, again
without any conduc�ve rings.

The surprising result is that the Uni-Core inductance islower. The reason that this is surprising is because
ins�nc�vely, a loudspeaker engineer would expect the
total inductance to be higher because of the mutual
inductance in the Uni-Core, which is not present in thetwo conven�onal drivers. Other dual gap motor systems
are in use by other manufacturers and one of the mostcommon arrangements is the “Differen�al Drive”
arrangement invented by JBL shown in Figure 8. This
arrangement is known to have favourable inductancecharacteris�cs and it is commonly commented that thisresults from the cancella�on effect of the two voicecoils, which in this case carry current in opposite
direc�ons. In fact, the improved inductance is largely
due to the influence of the second magne�c gap in the
motor system [6].

inside both coils and a suppor�ng member is needed tohold this in place (Figure 4). The Uni-Core uses a large
aluminium cylinder placed between these coils to serve
this purpose. The material and geometry of the member
is an important design parameter. From an inductanceperspec�ve it acts as a large, low-resistance, shorted
turn in close proximity to both coils. The result is todrama�cally drop the total inductance and the totalinductance varia�on with the loca�on of the coils. The
green line in Figure 7 shows this effect, and compared tothe conven�onal motors the inductance is verysignificantly reduced. A�er more detailed analysis it was
found that this aluminium cylinder can be used todecrease the mutual inductance of the two coils.

The aluminium cylinder also has an important role in the
power handling of the Uni-Core motor, adding vital
thermal mass near the voice coils and helping to conduct
heat away from the interior of the motor.

P-Flex SurroundIn a sealed cabinet, when a loudspeaker diaphragmmoves in and out, the internal air volume compresses
and expands. The number of air molecules within does
not change, however.

What does change is air pressure - increasing when thedriver moves inwards, and decreasing when the driver
moves outwards. This is because an enclosed volume of
air is always seeking to achieve equilibrium with thesurrounding environment. As a result, the en�re interior
surface of the cabinet and drivers experience a pressure
force in opposite polarity to the driver mo�on.

Figure 8. Lumped circuit representa�on of a single driver, dual voicecoil system

Uni-Core has lower inductance for the same reason –the magne�c circuit of the motor system has a higher
reluctance than a conven�onal motor due to theaddi�onal gap for the second coil. This reduces theinductance amplifying effect of the motor systemsteelwork.

In a Uni-Core motor the central steel pole is locatedAs the driver moves inwards, the air pressure insideincreasingly pushes against the driver. The opposite
occurs when the driver moves outwards. In a very smallbox, the restoring force of the air has a non-linear effect,with the outward force when the driver moves inward
exceeding the force when the driver moves outward.
This results in nonlinear distor�on.

Another important considera�on is how this pressure
force affects the performance of the driver surround.
With a conven�onal half roll surround, high pressure candeform the surround causing it to buckle (Figure 9). This
leads to gross distor�on.

₆

Smart Distor�on Control Technology
There are numerous mechanisms whereby distor�on is
introduced into the radiated sound. An ideal loudspeaker
would exhibit the following:•

BL(x) stays the same at all coil posi�ons•

The s�ffness of the suspension is consistent acrossthe excursion•

The s�ffness of the internal air volume does notchange as the driver moves back and forth•

Voice coil inductance should not modulate with coilposi�onFigure 9. Cross-sec�on of half roll type surround buckling at highexcursion

A conven�onal solu�on to this problem is to use a very
thick surround. However, this has two nega�ve effects.
Firstly the surround mass is substan�ally increased
leading to a lower efficiency. Secondly, the s�ffness ofthe surround is increased, drama�cally compromising
the low frequency extension of the driver.

The P-Flex surround is a patent-pending, pressure
resistant surround consis�ng of a pleated design (Figure10). The shape is op�mised to resist the deforma�oncaused by the changing internal air pressure of thecabinet, whilst allowing extended, linear movement of
the driver, without the issue of buckling at higher
excursion.

During the component design stage of KC62, thevaria�on in these parameters has been reduced as far as
possible. To achieve further reduc�on of distor�on,
KC62 incorporates a patent pending ac�ve control
system. This is a key contributor to its performance, anda major reason for the breakthrough for this ultracompact subwoofer.

Hybrid Control System
Smart Distor�on Control Technology (SDCT) is a hybrid
system combining DSP pre-correc�on with indirect conemo�on sensing and feedback. These two component
systems are specifically designed to work together, eachtackling different distor�on mechanisms (Figure 11).

Figure 10. Cross-sec�on of P-Flex surround at highexcursion showing folded detail and no buckling

Figure 11. Representa�on of the Hybrid Control System

₇

The DSP por�on of the system uses an inverted
numerical model of the loudspeaker, pre-programmedwith the driver parameters, which pre-corrects the DSP
output signal such that the added distor�on counteracts
distor�on generated by the drivers. Simultaneously the
instantaneous cone mo�on is indirectly sensed from theelectrical current in the driver coils, and this signal isused in a feedback loop within the amplifier electronics.

In addi�on, the Uni-Core driver is specifically designed
to maximise the performance of SDCT - achieving a
reduc�on in THD of up to 75% (Figure 12).

The addi�onal strength afforded also eliminated theneed for internal bracing, further increasing the available
air volume inside the cabinet.

Thermal Management
A small cabinet needs an excep�onal amount of power
in order to help deliver bass depth and output - a�er all,Hofmann’s law cannot be broken. The trade-off,however, is through heat build-up. Both amplifiers andloudspeakers are not 100% efficient, meaning any power
not converted to radiated sound is converted into heat.
So while KC62 needs to have a significantly powerfulamplifier, considera�ons must be made to avoid heatbuildup.

Power is provided by a pair of 500W RMS Class-D
amplifiers - each one driving a single voice coil. Class-D
is a highly efficient topology, ideal for ac�ve subwoofers
as less heat is produced. The heat dissipated by thedrivers is also carefully managed by the cabinet aluminium makes for an excellent heatsink, and has been
carefully designed to ensure effec�ve heat dissipa�on.
Figure 12. THD(%) comparison (right axis) against frequency of KC62
with and without SDCT

Frequency Response

Aluminium Cabinet
Vibra�on Control
The issue of internal air pressure has already been
discussed in the P-Flex surround sec�on of this paper.
However, there is another mechanism wherein thesubstan�al pressure forces within the cabinet willa�empt to equalise - by flexing the walls. As a vibra�ng
surface this would cause sound colora�on.

Through these new technologies, KC62 exhibits an
excep�onally wide and deep frequency response,
especially for a subwoofer that is barely larger than afootball. Figure 13 shows the frequency response at 1
metre, at several listening levels. Also included are
addi�onal do�ed lines, showing the in-room frequency
response. This takes into account the low frequency
enhancement in a typical room.
(dB)
110

100

With this in mind, an extruded aluminium cabinet was
designed. As aluminium has a higher elas�c modulus
than wood for a given thickness, the use of a thinner wall
was possible. This provides a larger internal acous�cvolume for the given external dimensions.

90

80

70

60

The aluminium extrusion is shaped as a curved shell.
Curved surfaces are significantly more rigid than flat
ones, and having a monocoque construc�on ensures
there are no stress points, which would otherwise
compromise the cabinet integrity.

10

20

50

100

200

500

1k
(Hz)

Frequency Response at low listening level
Frequency Response at low listening level with avg. room gain
Frequency Response at moderate listening level
Frequency Response at moderate listening level with avg. room gain
Frequency Response at high listening level
Frequency Response at high listening level with avg. room gain

Figure 13. Frequency Response for low, medium and high listening
levels, both anechoic and with room gain

₈Conclusion
Throughsignificant

Referencesnewtechnologicaladvancements and considered engineering, KC62
delivers an unprecedented combina�on of depth,
detail and output from an ultracompact form. By
pushing the boundaries of what has been considered[1] J. Wright, ‘Finite Element Analysis as a Loudspeaker
Design Tool’, presented at the Audio Engineering
Society UK 13th Conference; Microphones andLoudspeakers, Mar. 1998. [Online] Available: h�p://www.aes.org/e-lib/browse.cfm?elib=7992

possible, KEF offers a way for almost anybody withany system, in almost any room to benefit from whata well-designed and integrated subwoofer can bringto both music and film - the complete experience,just as the ar�st intended.

[2] R. Small, ‘Closed-Box Loudspeaker Systems-Part
One: Analysis’, JAES Volume 20 Issue 10 pp. 798808 Dec. 1972 Available: h�p://www.aes.org/e-lib/
browse.cfm?elib=2022[3] R. Small, ‘Vented-Box Loudspeaker Systems-Part 1:
Small-Signal Analysis’, JAES Volume 21 Issue 5 pp.
363-372
Jun.
1973
Available:h�p://
www.aes.org/e-lib/browse.cfm?elib=1967[4] J. Oclee-Brown, ‘Loudspeaker Compression-Driver
Phase-Plug Design’, University of Southampton,
Faculty of Engineering and the Environment, Nov.
2012,
PhD
ThesisAvailable:h�ps://
eprints.soton.ac.uk/348798/1/Jack%2520OcleeBrown%2520PhD%2520Thesis.pdf[5] L. Risbo, F. T. Agerkvist, C. Tinggaard, M. Halvorsen,
and B. Putzeys, ‘Force Factor Modula�on in Electro
Dynamic Loudspeakers’, presented at the AudioEngineering Society Conven�on 141, Sep. 2016,
Accessed: Oct. 26, 2017. [Online]. Available: h�p://www.aes.org/e-lib/browse.cfm?elib=18411[6] F. Kochendörfer and A. Voishvillo, ‘Compara�ve
Sta�c and Dynamic FEA Analysis of Single and Dual
Voice Coil Midrange Transducers’, presented at the
Audio Engineering Society Conven�on 139, Oct.
2015, Accessed: Jan. 22, 2021. [Online]. Available:h�ps://www.aes.org/e-lib/browse.cfm?elib=17971

₉

Specifica�on

Model

KC62

DesignUni-Core Force Cancella�on

Drive units

2 x 6.5 in.

Frequency response

11Hz - 200Hz

Max output

105dB

Amplifier type

Built-in Class-D

Amplifier power

1000W RMS (2 x 500W RMS)

Variable low pass filter

40Hz - 140Hz, LFE

Input

RCA Phono sockets
Speaker level inputs

Line output

RCA phono sockets

Line output high pass filter

40Hz - 120Hz, Bypass

Power requirements

100 - 240V ~50/60HzPower consump�on

1000W

Weight14kg (30.9lbs)

Dimensions (H x W x D)
with rear panel and feet

246 x 256 x 248 mm(9.68 x 10.07 x 9.76 in.)₁₀

Control panel of KC62

KC62, Carbon Black

₁₁