JAMO D 4 - Manual (Page 10)

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However, the anodised aluminium centre plug is probably the most
conspicuous feature of the D 4 woofers.
It replaces the traditional dust cap in the centre of the loudspeaker
cone at the place where the dust cap would normally be mounted -
thereby making the cone lighter and thus faster.
Also this prevents potential break-up points where the dust cap
would have been glued to the cone.
In addition, it prevents over- and under pressure behind the dust cap
when the cone moves back
and forth. The advantage is
more dynamic and precise
sound reproduction.
The aluminium centre plug also has a cooling
effect on the voice coil providing greater power
handling capability and very important, a stable
sound reproduction when playing loud.
The mid-woofers all feature lightweight 2-layer voice coils in order to achieve maximum
“attack” (the ability to start and stop precisely) and minimum distortion – these acoustic
properties are of utmost importance for optimum voice reproduction.
In the designated woofer for the D 450 there is a small but significant difference.
It is supposed to work only at the lowest frequencies. By using a heavy 4 layer voice coil,
we can reduce sensitivity at high and mid frequencies while gaining efficiency at low
frequencies.
This works in 2 ways: A high B*L product** will increase port output and reduce bandwidth
around port resonance. The increased weight will counteract the increased B*L at higher
frequencies. The result is a gain of about 3 dB at low frequencies compared to the 2 layer
type in the same cabinet.
**The combination of magnet size/force and length of the voice coil wire results in the so-called B*L Product. In popular
you could call this the size of the “engine” in a loudspeaker.