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• + Surround Sound with Fewer Speakers
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• + Optimum Diapragm Waveguide Geometry
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•   Pod Technology
• + Uni-Q White Paper
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Magnetic Design

Towards the end of the 1980s neodymium iron boron (NeFeBo) became available in commercial sample quantities. Initial samples of this new permanent magnet material had an energy product BHmax nearly ten times greater than typical ferrite magnets. In particular, the remanence of NeFeBo exceeded 1 Tesla compared to 0.4 Tesla of ferrites. This meant that for comparable (equal total flux, F ) loudspeaker magnet designs, which often operate close to remanence, the magnet area Am could at least be halved since, from simple magnet theory, neglecting leakage,
where Bm is the flux density in the magnet and Bg is the flux density in the air gap. The coercivity H of neodymium iron boron was 17 times that of Alnico (50 kA/m) and so, depending on the actual operating point, the magnet could also be very short, from,

where Lm is the magnet length, Hm is the magnetising force in the magnet, at the magnet, operating point and F is the magnetomotive force across the magnet.

The volume of magnet is related to the energy product by the relation,

and so, it can be seen that to minimise the magnet volume the operating point should be at (BH)max.

However, in general, we are more interested in the gap flux density, rather than the magnet operating flux density, and so, if the magnet assembly is inefficient, it is of little interest that the available magnet volume is being optimally used to generate external leakage.

The most common loudspeaker magnet structure is an open ferrite ring configuration as shown in Fig.9. With this arrangement it is not unusual for half of the available magnet flux to be wasted as external leakage.

If a more efficient pot structure, as in Fig.10. having less leakage than open external ferrite ring designs, was used, then the magnet area could be reduced still further. This would make the placement of a usably sensitive tweeter within the voice coil of the mid/low frequency unit a real possibility.

There followed a program of magnet design work in order to establish the feasibility of producing a powerful and small enough tweeter magnet. A series of in-house trials using magnet manufacturers' samples and suggested designs led to the production of many prototypes. Steel properties were analysed and further magnet simulations were performed with the help of the Small Motors department of Sheffield University. Magnet designs were refined still further by later in-house work using a boundary element simulation package [14] running initially on a 386 PC.

Two tweeter designs were produced. A 19 mm unit which would fit inside a 32 mm voice coil and a 25 mm unit which would fit inside a 39 mm voice coil. The typical layout is shown in Fig.11.

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