Tim Martin wrote:
"spudrick" wrote in message
...
The largest is 15" but it only has a 1 inch or so magnet.
This is to save money or is there another reason?

I imagine it's because the speaker was designed to handle only a few watts,
not the hundreds of watts of today's designs. It may have an alnico magnet,
too, which is I believe a more powerful magnetic material than modern
ceramic magnets.

This has been mentioned elsewhere in this thread, but there's a
misconception that needs to be cleared up.

The "strength" of the magnet, in term that are important to the
operation of the speaker, is directly related to the flux density
in which the wire of the voice coil is immersed. The relevant
parameter is referred to as the "Bl product" (that's "bee-ell").
It is the product of the flux density, B, measured in Tesla and
the length of the wire, in meters, that's immersed in that field.
The Bl product is measured in units of Tesla-meters, and is
equivalent to the amount of force per amount of current (actually
1 Tesla meter is equal to 1 Newton of force per ampere of current).

Speaker magnets using Alnico have traditionally been claimed as
being "more powerful" than magnets using ferrite ceramics, but
this is simply not the case.

WHat IS true of Alnico is that it is capable of storing more
total energy per unit volume than ceramics can. But the SIZE
of the hard magnet material is NOT what determines the important
factor here, and that is the flux density in the voice coil cap.
That's something the magnet material used cannot determine.

In BOTH the cases of Alnico- and ferrite ceramic-based magnet
structures, the material used to actually focus and concentrate
the magnetic field in the gap is almost always common, soft,
low-carbon steels, for several reasons:

1. They're inexpensive

2. They're ductile and malleable and thus can be shaped using
common metalworking methods (stamping, forging, diecutting)
into the desired shapes and configuration,

3. They are capable of supporting substantially flux densities
than most hard magnetic materials such as alnico or the
ceramic ferrites.

The first two speak to the manufacturability of the assembly,
especially since both of these material are VERY hard to machine.
Try to machine Alnico, and you'll wear you tools out much faster
than the alnico, and if you try to force the issue, it will shatter
like glass. And ceramics are like trying to machine brittle rocks.

But the last point, the issue of flux density, is VERY important
in the design of the magnet.

The problem is that while both Alnico and ceramic hard magnet
materials are capable of storing large amounts of magnet
energy permanently, they have a limit: the flux density the
can store is limited. HOwever, the flux can be concetrated and
directed by using the auxiliary soft-iron structure.

Fo, for example, the typical flux density at the surface of a
ceramic magnet might be on the order of only 0.2 Tesla. But if
we could concentrate that same total flux in a smaller area, the
flux density would, obviously, be greater. Look at the relative
cross-sectionaly areas of the magnet vs the surrounding magnet
structure, and you can see how the relatively low fux density
of the magnet is conctrated to a higher flux desnity at the gap.

However, there is a limit, and that is that the amount of flux
you can direct through these auxiliary structures is limited.
At best, soft, low-carbon steels "saturate" at flux densities
of about 1.2 Tesla. Any attempt to put any more flux into them
will not result in any increase in flux desnity, the extra
flux will simple "spill out" of the structure into the air.

Because, then, BOTH Alnico-based and ceramic-based magnets rely
the same low-carbon steels to concentrate and direct the magnetic
flux, these materials impose their own limits on the maximum
flux density in the gap. The result is that, all other things
being equal (same gap depth, same diameter, same voice coil),
a speaker magnet using Alnico will not be any more "powerful"
than one using ceramic.

In essence, the "strength" of the magnet field where it counts,
that is in the voice coil gap, is limited NOT by the magnet
material, alnico or otherwise, but by the soft iron.

So what's the advantage to Alnico? In this day and age, not a lot.
For any given Bl product, an Alnico-based magnet can be physical
smaller and lighter. But it will also be more expensive, because
of the ioncreased cost of the material and the difficulty in
handling it.

So, just because a speaker has an Alnico magnet, don't assume
that it's "more powerful." In all likelihood, it's not.