Magnetic Motors, Pt. 1
Was the main reason you purchased a certain
guitar speaker because it had an Alnico mag-
net? Are you unwilling to purchase a ceramic
speaker for a vintage application? Are you
afraid to purchase a neodymium speaker
because of negative generalizations you’ve
heard about them, even though your back
won’t allow you to pack your
current gear to your gig? If
you can answer yes to any
of these questions, or if you
are curious about the topic of
magnets in general, then you
will surely be “attracted” to
this was common in the seventies and eighties. The assemblies were bolted together, but
adhesives are used today. Ceramic magnets
will vary in diameter and thickness even when
used on the same plates; the plates will also
vary in diameter and thickness. Using various
magnet and plate thicknesses affects the effi-
The core (or pole-piece) is attached to the
back plate. The core and back plate can be
tooled as one piece or tooled separately, in
which case the core is welded or staked to
the plate. It will vary in diameter relative to
the voice coil diameter and height. Guitar
speakers typically feature a solid core, but
in some cases it may be vented.
The core extends through the
centers of the magnet and top
plate to become even with the
top plate. Many pro audio and
bass speakers use extended
poles for longer excursions.
Three magnet materials are
commonly used in guitar
ceramic (or ferrite), Alnico
and neodymium. Each material has different magnetic
properties and cost. Ceramic Figure 1: Cross section view of a ceramic motor
is the cheapest and most
commonly used material.
Alnico is a rare composite of
aluminum, nickel and cobalt,
and is the most expensive.
Alnico is commonly thought
to produce a “vintage”
tone and has a reputation
for sounding compressed.
Neodymium seems to be the
wave of the future. It offers
reduced weight and overall
costs are coming down. It
produces the most magnetic flux per ounce. A neo- Figure 2 represents an Alnico
Figure 2: Cross section view of Alnico motor using ring magnet
dymium magnet exhibiting motor. The same design char-
the same strength as a given ceramic magnet ciency of the speaker, as well as impacts the acteristics previously explained for the ceramic
will have only 10-15 percent of the ceramic Qts parameter, which is an indicator of low-end magnet ring apply here. Alnico is produced
magnet’s weight. This makes neo ideal for response. Varying the diameter of the magnet in several different grades and strengths. The
use in multiple speaker cabinets to maintain has a much greater impact than increasing the most expensive grades provide more magnetic
performance while reducing handling and thickness, due to an increase in surface area. flux per ounce than ceramic magnets.
shipping weight. I will demonstrate how each An interesting fact is that a smaller magnetic
magnetic motor differs, briefly describe each motor will have more bass response than a
motor part, its contribution to the tone, and larger magnetic motor (assuming everything
maybe even challenge how you think about else is the same) because it raises the Qts.
choosing your speakers.
The magnetic gap is formed
between the core and the
top plate’s inner diameter.
This is where the voice coil
is positioned. The magnet is
positioned around the voice
coil on the ceramic magnet
motor. Manipulating the width
and height of the magnetic gap
affects the sensitivity and the
breakup characteristics of the
speaker. A tighter gap lends
more focused energy, which
correlates to greater efficiency.
Some manufacturers use tighter
gaps, offering significantly high-
er efficiency; however, failures
from voice coil rubs are much
more likely in this case. A bal-
ance of gap width and efficiency
is a must for all manufacturers.
Well, it looks like we’re about out of space
this month. We will continue this discussion in
next month’s article and will show you another
Alnico motor and a neodymium motor.
Figure 1 depicts a ceramic magnet motor. As
previously stated, this is the most common
type you will see on today’s guitar speakers.
The magnet is a ring positioned between a
top plate and back plate. You might see older
speakers that used square ceramic magnets;
The top plate has an inner diameter that will
vary relative to the voice coil diameter. It can
also vary slightly within the same voice coil
diameter to accommodate multiple wire sizes.
In some cases, speaker designers may vary the
inner diameter to manipulate the performance.
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.