User Manual
Table Of Contents
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Appendix 2 - Ports
Ports are used to augment bass response. They have
the 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 of
the port, the bass driver moves less, thus lowering
distortion.
Ports are not without their drawbacks, however.
• 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 chufng).
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.
• 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, gure 29 shows the
difference in output when the port is placed at an
antinode and at a node of an internal resonance. For
the rst 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 and
cannot be detected in the overall response.
Organ pipe resonances
These resonances within the port itself can be
drastically reduced if the walls of the port have
a degree of exibility. 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 the
pressure 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 from
green through to red indicates the level of air pressure.
Turbulence
This occurs at both ends of the port as the air exits the
port either to the open air or the air enclosed within
the cabinet. The solution is to are both ends of the
port (gure 31). The progressive expansion of the air
afforded by the aring reduces turbulence and thus
audible chufng. It should be noted that, if turbulence
is allowed to develop, not only is it audible, but the
performance of the port changes with sound level,
impairing the dynamic range of the loudspeaker.
Figure 32 shows the nal design of the port located
inside 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 that
all sound appears to emanate from the same point
in space. Coaxial loudspeakers had been around for
many years before the introduction of Uni-Q, but the
tweeter was never time aligned with the midrange or
bass/midrange driver it was partnered with. Either
the tweeter was in front of the larger driver, which
brought 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 have
their 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 a
wide frequency range.
Over the intervening years, the Uni-Q concept has
been progressively rened. 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. Uni-Q is simple in
concept, but tricky to implement in practice. Here
are some of the techniques previously developed and
carried over to this model.
Tweeter
Diaphragm
The dome diaphragm itself is aluminium. More exotic
materials – diamond and beryllium for example – are
sometimes used in an effort to increase stiffness and
push the pistonic 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 29
Simulated comparison of the port and total speaker outputs
with unoptimised (top) and optimised (bottom) port location.
Figure 30
Comparison of the Finite Element modelled pressure
magnitude in the port at the rst standing wave with rigid
walls (top) and exible walls (bottom).
Figure 31 - Flow pressure contour of straight port (top)
and ared port (bottom).
Figure 32 - Offset exible port located inside the LS50 Meta
Figure 28
Constrained-layer damped bracing