|
Dear CF Cheuk,
The reccommendation for expensive material may be
for a reason other than what you assume.
It may not involve permeability (or reluctance)
considerations at all.
Here's how I think of the permeability
considerations, by the way: if I have a 1" air gap of a certain cross
section, the reluctance of that is equal to half that same air gap filled in
with a material of permeability=2 (twice that of air). In other words, the mu=2
filled air gap magnetically appears as a 1/2" gap of air. Likewise, a
mu=4 filled gap appears as a 1/4" air gap, all other considerations
aside.
Now, if you assume that steel has a mu of 1,000
and Vacoflux 50 (which I've never heard of before) has a mu of 10,000 let's
say that the magnetic path length from the magnet to the gap is 2".
Therefore, the
1000-mu steel now appears as a 1/500" air gap. The Vacoflux 50 appears as a
1/5,000" air gap. Since the actual air gap is (let's say) 3mm, the difference
between 1/500 and 1/5000" air gap additionally (represented by the pole piece
material) is indeed insignificant, probably below the physical tolerances of the
device.
In short, the airgap (where the permeable material
is to be placed) is has an actual value of the gap length divided by the
gap permeability.
(This is, of course, discounting nonlinear
considerations like saturation, where the small "air gap length" represented by
the permeable material suddenly begins to widen, eventually becoming an actual
piece-length "air gap" when fully saturated).
Although the air gap analogy (as well as FEMM
analysis) shows that material choice is relatively insignificant (at least above
a thousand mu or so) the conductivity of expensive,
high-permeability mu material is, on average, greater than cheap
steel.
Do remember that the pole pieces you are using
probably represent shorted turns, in transformer relation to the voice coil. As
shorted turns, their conductivity determines how "shorted" a "secondary"
they represent.
As the voice coil conducts amplifier current and
exhibits a varying magnetic field external to it, the flux cuts the mu material,
causing eddy currents to circulate around the material (which is not exactly
something FEMM was built to model). Lenz's Law dictates that these induced
currents serve to counter what induces them; therefore, the force between the
coil and the mu material (carrying induced current) is repulsive and the
impedance of the voice coil is closer to resistive, and less reactive
("imaginary").
This results in a better speaker,
performance-wise: the voice coil exhibits additional force from inductive
interactions, and the amplifier sees a load that is more resistive: current
and voltage have a less convoluted relationship.
If I owned a speaker company, I would do the
following (which may give you an advantage, if your company manufactures
speakers): use cheaper permeable materials, and coat the pole faces of these
materials (or any part of the pole piece that the voice-coil flux will cut)in
silver, with a final (thin) layer of gold. The silver gives the highest
conductivity (inductive repulsion) possible; the gold prevents oxidation.
(Polymer coatings could be used as well, provided they are thin enough).
This conductive-coating method may let you produce
a speaker whose pole pieces are less expensive than exotic permeables, with
better performance than those expensive components could offer
alone.
Regards,
Graham Gunderson
|