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more... GearAmpsGear BlogSpeaker Tone CenterAugust 2009

Magnetic Circuits

What causes a speaker cone to move?
The magnetic circuit and voice coil make up the motor of a guitar speaker. An alternating electrical current flowing through the voice coil generates an alternating magnetic field perpendicular to the flow of current through the coil. The magnetic circuit creates a strongly focused magnetic field in the air gap between the front plate and the pole piece on which the voice coil is centered. The voice coil is pushed and pulled through the air gap based on the interaction between these two magnetic fields. Since the speaker cone is connected to the voice coil, it now has a mechanical force with which to push air particles and make sound.

Cross-sectional side view of a ceramic magnet circuit indicating the location of the air gap (between front plate and pole piece).


Cross-sectional side view of a ceramic magnet circuit showing the centering of the voice-coil over the pole piece.


A cross-sectional side view of the speaker drawn without its basket to show the location of the magnetic circuit and voice-coil.
Permanent Magnets
Today, there are three permanent magnet materials commonly used in guitar speakers: alnico, ceramic and neodymium. Most guitarists can sense a characteristic sound and feel associated with each type.

Alnico
Alnico magnets came first and are usually associated with vintage sound. Alnico speakers are frequently described as smooth, harmonically rich and warm. They are usually preferred by blues, jazz, and rock players.

Cross-sectional side view of an alnico magnet circuit used for a low power speaker. (The lines represent the flow of magnetic flux through the circuit.)


Cross-sectional side view of an alnico magnet circuit used for a high power speaker. (The lines represent the flow of magnetic flux through the circuit.)
Ceramic / Ferrite
Ceramic magnets were introduced as the price of alnico increased and have become the most common magnet used for guitar speakers. Ceramic speakers are usually associated with a harder, more aggressive feel. They tend to be favored by rock, hard rock and metal players.

Cross-sectional side view of a ceramic magnet circuit. (The lines represent the flow of magnetic flux through the circuit.)

Neodymium
Neodymium magnets are a new development. A growing consensus suggests that neodymium speakers sound smooth and are highly responsive, with tight definition. They are versatile and seem to be enjoyed by players of all musical genres.

Cross-sectional side view of a neodymium magnet circuit. (The lines represent the flow of magnetic flux through the circuit.)

Physical Comparison
Neodymium magnet circuits are capable of generating extremely high levels of magnetic flux from a relatively small magnet. This allows the magnet to be placed on the inside of the voice coil in a manner similar to the alnico magnet circuits of lower-power speakers. The small neodymium magnet also makes for a lightweight speaker. Ceramic magnet circuits require a larger magnet to generate high levels of magnetic flux compared to alnico and neodymium, so the ceramic magnet is placed on the outside of the voice-coil.

There are many factors that affect the sound and feel of a speaker, and the magnetic circuit is mostly an indirect influence. The subjective tone descriptions and musical genres I have associated with each magnet are generalizations meant to invite guitarists to try each speaker type and discover which one best compliments their style.


Kurt Prange
Kurt Prange (BSEE) is the Sales Engineer for CE Distribution, LLC, in Tempe, AZ (jensentone.com). Kurt began playing guitar at the age of nine in Kalamazoo, MI. He is a guitar DIY’er and tube amp designer who enjoys helping other musicians along in the endless pursuit of tone.