GUITAR SPEAKER TONE CENTER
Magnetic Motors, Pt. 2
This month we’ll continue discussing ceramic,
Alnico and neodymium magnetic motors. In
the early days of speaker manufacturing, Alnico
was the magnet of choice, but increased costs
forced manufacturers to seek cheaper alternatives. It was also not as critical to have as tight
a magnetic gap on a ceramic motor. This was
a big plus because it made manufacturing easier. Alnico has a much
lower coercivity; which is easier to
demagnetize and it requires a stronger field to keep it magnetized.
The biggest affect of this design is a decrease
in inductance (or Le) that lends a brighter
speaker, a bit more extended in the top range.
This is not intended to imply that all neodymium speakers are inherently bright; it is just
brighter in comparison to a similar design with
a ceramic magnet. There are plenty of design
tions nearly impossible. In order to make a fair
apples to apples comparison, you would have
to compare two speakers with the same cone,
spider, voice coil, dust cap, etc., but with different magnetic materials having the same flux.
The lower coercivity is also the
reason Alnico exhibits natural compression. As you push an Alnico, the
speaker heats up and temporarily
loses some magnetism; the result is a
compressed sound. Don’t worry; permanent damage to the magnet only
occurs beyond the temperature that
the voice coil and adhesives could
withstand. You will damage your
speaker from overpowering before
you harm the Alnico. Permanent
demagnetization can also occur from
a drop or by removing the magnet
from the magnetic circuit.
As we’ve discussed and seen, magnet placement can vary from one design to the
next. Also, magnets vary in strength
from one material to the next. They
even vary in size and possibly grade
when comparing two of the same
material. If you could compare speakers that have the same magnetic
flux but are generated from different
magnet compositions, you probably
wouldn’t notice a difference in tonality.
Differences in tonality that are often
attributed to the magnet material
probably have more to do with the
positioning of the magnet and resultant differences in magnetic flux within
the motor structure.
Figure 1 shows another commonly
used motor. This is an Alnico slug
housed inside a rectangular yoke.
The top and bottom of the yoke act
as the top plate and back plate. The
core, positioned above, is glued to
the magnet. Note that the magnet is
beneath the voice coil on this design.
A dust guard between the core and
the top of the yoke is used to keep
debris out of the magnetic gap.
options with other components one can use to
compensate for brightness.
Like Alnico, neodymium is also available in different grades. Manufacturers must consider
price, strength and operating temperature
when selecting the grade of neodymium.
A neodymium motor is shown in figure 2. The
neodymium magnet is a slug similar to an
Alnico motor. A shell pot houses the motor
assembly. In order to achieve higher power ratings, optimal heat dissipation must be achieved
when designing the shell pot. The core, also
referred to as a core tip, is a slug much like the
magnet, and is positioned above the magnet.
This design features the magnet inside the
voice coil.
One drawback of neodymium guitar speakers is that there are not a lot of component
options available. Neodymium is fairly new
and more expensive, and manufacturers must
weigh demand versus tooling costs. The
toolbox for neodymium designs in regards to
cones, voice coil diameters, magnet sizes, etc.
remains fairly small right now.
In conclusion, prices on Alnico speakers are already a lot higher; due to
recent and imminent price increases on
the materials used to produce Alnico,
especially cobalt, such speakers are
expected to increase further in price.
If you are convinced that you can only
achieve “that sound” from an Alnico
speaker, then be prepared to pay
dearly for it. You probably should not
consider a neodymium speaker unless
you are looking for weight savings. You
will spend a little more to get it and
your options will be a bit limited. Yes,
more designs with neo will certainly hit
the market, but I don’t think the feeling that
more horsepower takes more cubic inches will
ever go away. There’s still a pretty good market
for tube amps! Ceramic magnet speakers are
the most cost effective approach and there are
many more tonal options available.
One major point I want to convey is when
you compare speakers using different magnetic materials, there are likely several other
significant differences that attribute to the
overall sound as well. This makes generaliza-
Anthony “Big Tony” Lucas
is a guitarist and Senior Lab Technician at Eminence
Speaker LLC, where he specializes in guitar-speaker
design and customer support. Big Tony has been with
Eminence for over 10 years and is responsible for many
well-known guitar speaker designs.
