9
KEF T Series Technologies Explained
Z-FLEXSURROUND
An impor tant feature of the T Series Midrange Driver is the new
Z-Flex Surround. The surround is a critical component of any
bass/midrange driver. The designer must carefully choose the
material and shape so as to avoid irregularities in the midband
response due to resonance in the moving parts whilst at the
same time allowing sufficient excursion of the cone in order to
reproduce bass frequencies. Most modern drivers use a half-roll
design typically moulded from butyl rubber. The half-roll design
can often perform well if carefully designed, however, KEF’s
computer modelling techniques have allowed us to investigate
some more adventurous possibilities. The Z-Flex surround is the
result.
The images below show how the Z-Flex surround changes
shape when the cone moves back and forth. This is quite unlike
the motion that is normally seen with a conventional half-roll
design.
Design Features
Unlike a conventional surround, the Z-Flex allows very fine
control over the surround behaviour. There are three main
features which are critical to the performance:
• The membrane sections which form the air seal
• The suppor ting blocks attached to the membrane
• The undulations on the front surface of the membrane
The membrane section is present to form an air seal, the size of
the front and side walls are chosen to allow the driver to move
to its full extent whilst minimising diaphragm edge mass loading.
The supporting blocks and the front surface undulations are
present to control the behaviour of the surround in the mid
band. The positions, shapes and sizes of these two features fine
tune both the mechanical impedance which the surround
presents to the cone edge and also the dynamic behaviour of
the surround itself. In essence, the Z-Flex will readily allow
simple deformations, such as those occurring at low frequencies
when the cone is moving some distance back and forward, but
greatly resist complex deformations such as those which can
cause problems in the mid-band. Conventional problems, for
• FEA model showing the Z-Flex surround
deformation as the cone moves forward.
• FEA model showing the Z-Flex surround
deformation as the cone moves backward.
example the surround termination dip, can then be completely
avoided by fine tuning the design.
Minimised Edge Mass Loading
A major benefit of the Z-Flex approach is that, for a given cone
excursion, less of the surround moves and hence the surround
contributes a much lower effective mass to the cone edge. The
effective mass decreases with increasing driving frequency. This is
impor tant in minimising mass-loading of the edge of the
diaphragm which would lower the frequency of the diaphragm
bending resonances. The Z-flex surround therefore works in
conjunction with the Twin Layered MF Driver technology to
achieve pistoning diaphragm motion throughout the working
band.
Perfect Baffle
Discontinuities in the surface of a loudspeaker’s baffle cause
reflections of the higher frequency sound waves. Features such
as a large roll surround can therefore colour the sound of the
tweeter. The smooth and low profile Z-Flex Surround, in
conjunction with the flat fronted diaphragm of the Twin Layered
MF Driver, have only a minimal effect on the acoustic waves
being radiated by the tweeter. Perfect Baffle was a technology
first introduced for the KEF Concept Blade Loudspeaker.
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General
Weight and Dimensions
