8
The basket is also made of magnesium, for several very good reasons: Firstly it
makes it possible to create a very open basket – even behind the spider – thus
ensuring high airfl ow at low frequencies, lessens refl ections at high frequencies
and prevents under/over-pressure causing mechanical losses.
Secondly, as magnesium is a non-magnetic material, the magnetic force from
the magnet does not spread out in the basket but stays concentrated where it is
needed… around the voice coil.
The ingenious magnet system is one that’s been patented by Seas. Unlike
conventional ferrite ring-type magnets this arrangement employs a combined
pole-piece / back-plate, that reduces eddy-currents and thus distortion in the
magnetic circuit.
Further, the voice coil is completely ventilated which means there’s no compression
and so excellent heat dissipation.
However, the most conspicuous feature of this drive unit has to be the matt-
chromated, solid brass centre-plug.
It replaces the traditional dust cap in the centre of the loudspeaker cone thereby
making the cone lighter and subjectively faster sounding.
This also eliminates the potential break-up points that would result as a consequence
of the dust cap being glued to the cone.
There’s more. It also prevents over and under pressure developing behind the dust
cap when the cone moves back and forth. The advantage here is a more dynamic
and precise sound quality.
Turned from solid metal the centre plug also has a cooling effect on the voice coil,
providing greater power handling capability and most importantly, extra sonic
stability when playing at high volume levels.
When a voice coil heats up, the impedance increases (it can actually double) which
means that the output will decrease and the cross-over points to the other drivers
will move. That’s why maintaining a stable temperature in the voice coil is essential
for optimum sound reproduction.
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