Magnetic material and magnetic fields surround us in our day-to-day lives, from the refrigerator magnets to the Earth's magnetic field to the speakers in our homes, on the stage, in our cars, etc.
Why and how do speakers use magnets? Speakers are transducers of energy that convert electrical energy (audio signals) into mechanical wave energy (sound waves). Many speaker drivers utilize electromagnetism to turn the AC voltage of the audio signal into diaphragm movement within a magnetic field to produce sound.
In this article, we'll have a closer look at how and why speakers use magnets and discuss the various speaker-types that use electromagnetism to convert audio into sound.
The Role Of Magnets In Loudspeaker Design
Magnets are a crucial component in the design of most loudspeakers because most speakers rely on electromagnetism to act effectively as transducers. Without magnets, many loudspeaker designs would be impossible.
The loudspeaker driver is the key transducer element that converts audio into sound. Most loudspeaker drivers work on principles of electromagnetism and rely on magnets to work.
Typically, permanent magnets are designed into the driver to interact with the speaker diaphragm's induced magnetic field as an audio signal is passed through the driver.
The diaphragm is either made of a conductive material or is attached to a conductive material. As electricity flows through the conductor, electromagnetic induction states that a coinciding magnetic field will be produced in and around the conductor.
What Is Electromagnetic Induction?
Electromagnetic induction refers to the production of a voltage across an electrical conductor in a changing magnetic field.
It also states the opposite: that a voltage across a conductor will cause a magnetic field around that conductor.
So by sending an alternating current (audio signal) through a conductive material, we produce a varying magnetic field around it. If this coil is placed in proximity to a magnet, the two magnetic fields will interact and cause the conductor and magnet to repel or attract one another.
When it comes to speaker drivers, combining permanent magnets and some sort of conductive diaphragm design would allow us to turn audio signals into sound waves. It is possible to send an electrical audio signal through a conductor and have it move an electromagnetic diaphragm to produce sound.
This brings us to loudspeaker driver design.
The Electromagnetic Loudspeaker Transducers
Moving-coil drivers are often considered the most common speaker transducer types. They are the most popular driver we'll find in loudspeakers, studio monitors, car audio systems, external computer/smartphone speakers, etc.
These moving-coil drivers utilize electromagnetism to transduce energy, thereby making electromagnetic loudspeaker transducers commonplace.
Moving-coil drivers are also used in some mobile and computer devices, though these speakers are also often of the piezoelectric-type or are made with MEMS technology. It's difficult, therefore, to state with complete accuracy what the most common speaker type is.
The main point is that loudspeakers utilize drivers to convert audio into sound. The drivers that utilize electromagnetism and, therefore, magnets are as follows:
Let's describe each type in more detail in the following sections.
Magnets In Moving-Coil Dynamic Loudspeaker Drivers
The moving-coil dynamic loudspeaker driver resembles the following diagram:
The magnet of the moving-coil driver, along with its pole pieces, possesses an odd shape.
The conductive coil is cylindrical in shape and must be suspended within a cylindrical cutaway in the magnetic structure.
To achieve maximum efficiency, the magnet must have opposite magnetic poles to the interior and exterior of the coil. This allows for a concentrated magnetic field around the coil.
As the electrical audio signal is passed through the voice coil, a magnetic field is produced in the coil, and the coil/diaphragm move within the permanent magnetic field that is provided by the magnets.
This is ultimately how the moving-coil dynamic loudspeaker acts as a transducer. The diaphragm movement mimics the waveform of the audio signal and produces sound that represents the electrical audio signal.
Since the particular shape of the magnetic structure is practically unobtainable via a single magnet, pole pieces are required.
One strong central ring-shaped magnet is used with a few pole pieces to extend its poles to the interior and exterior of the coil. A cross-sectional diagram is provided below:
Note that the diagram above is not to scale.
So the main magnet (in red) is shaped like a ring (or a thick washer). It has its south pole facing upward and its north pole underneath.
A thicker ring-shaped pole piece extends this south pole and provides the boundary just to the outside of the moving coil.
A disc-shaped pole piece is laid underneath the central magnet to extend the north pole. The north pole is furth extended with a cylindrical pole piece the reaches upward and provides the boundary to the inside of the moving coil.
The idea is to get the opposite poles as close to the coil as possible, with the north pole to the interior and the south pole to the exterior. This causes the greatest number of magnetic flux lines through the coil and, therefore, the greatest amount of coil movement.
The moving-coil driver design applies to headphones (on a smaller scale) and to microphones as well (only in reverse). To learn more about these other moving-coil dynamic transducers, check out the following My New Microphone articles:
• Complete Illustrated Guide To Moving-Coil Dynamic Headphones
• The Complete Guide To Moving-Coil Dynamic Microphones
Magnets In Magnetostatic/Planar Magnetic Loudspeaker Drivers
The magnetostatic (planar magnetic) loudspeaker driver has been made famous by the brand Magnepan. A basic cross-sectional diagram of a planar magnetic loudspeaker is shown below with the appropriate magnetic field lines:
As we can see, there are magnet arrays to either side of a movable diaphragm. The magnetic field strength is concentrated at the diaphragm.
This diaphragm has a printed/embedded conductive wire that covers its area in a serpentine fashion. As the AC voltage audio signal passes through the conductive element of the diaphragm, a varying magnetic field is produced in/around the diaphragm.
This varying magnetic field interacts with the permanent field of the magnetic arrays. It causes the diaphragm to move (and produce sound) in accordance with the waveform of the applied audio signal.
Note that the magnetic arrays used in magnetostatic loudspeaker drivers have space between their carefully positioned magnets. This is to allow the sound waves produced by the diaphragm to “escape” the driver and be heard by the listeners.
The planar magnetic driver design also extends to headphones. For more information, check out my article Complete Guide To Planar Magnetic Headphones (With Examples).
Magnets In Ribbon Loudspeaker Drivers
Let's have a look at the basic design of a ribbon loudspeaker driver in the simplified diagram below:
With the ribbon driver, we have a magnetic structure with opposite magnetic poles to the left and right of the ribbon (rather than to the front and back like the aforementioned planar magnetic driver).
These magnets must be very powerful to provide the magnetic field strength required to move the ribbon effectively.
The ribbon itself is made of a conductive material and is often corrugated to improve durability and efficiency. The extreme thinness and low mass of the ribbon allow it to move very accurately. However, strong permanent magnets are required to produce a decent level of sound.
As the audio signal passes through the conductive ribbon, a voltage is applied across the ribbon, producing a varying magnetic field. The ribbon's field then interacts with the permanent field and causes diaphragm movement.
The movement of the diaphragm produces sound waves that effectively mimic the audio signal.
Ribbon microphones use the same working principle as the ribbon loudspeaker, only in reverse.
To learn about ribbon microphones, check out my in-depth article titled The Complete Guide To Ribbon Microphones (With Mic Examples).
Magnets In Moving-Iron Loudspeaker Drivers
The moving-iron loudspeaker was the first effective electromagnetic loudspeaker design. Today, the design is pretty well relegated to balanced armature headphone drivers, but this driver type is still worth mentioning.
For information on balanced armature headphones, check out my article The Complete Guide To Balanced Armature IEMs/Earphones.
Let's have a look at the balanced armature headphone driver design to explain the magnets of a moving-iron speaker driver:
In the driver above, the magnets are placed above and below a conductive armature that is physically balanced in a system. The magnetic poles are to be opposite to the top and bottom of the balanced iron/armature and will produce a concentrated magnetic field at the arm.
The audio signal is passed through a stationary coil which causes a varying magnetic field in the coil. This field is then extended to the armature, which causes it to move between the two magnets.
As the armature moves, the mechanically coupled diaphragm moves and produces sound directly proportionate to the audio signal.
Magnets In Magnetostrictive Loudspeaker Drivers
The magnetostrictive speaker design is much less known than the above-mentioned driver designs. However, it uses magnets and, therefore, should be mentioned in this article.
Let's have a look at a simplified cross-sectional diagram of the magnetostrictive speaker driver:
As the name would suggest, the magnetostrictive driver work on the principle of magnetostriction.
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape during the process of magnetization.
The sound of magnetostriction can be heard as the dreaded 60-cycle hum (or 50-cycle hum depending on where you live) that you’ll hear from power mains transformers.
So as the audio signal is passed through the stationary coil, a varying magnetic field is extended to the magnetostriction core.
The magnetostriction core is made of many thin magnetic plates (magnets) stacked together.
As the plates experience a varying magnetic field, they change shape ever-so-slightly in shape and move a diaphragm that propagates sound waves. When the coil is not experiencing an audio signal, the core and its magnets return to their original shape.
What Kind Of Magnets Are Used In Loudspeakers?
Most high-quality loudspeakers will use rare earth Neodymium magnets.
Neodymium magnets are the strongest kind of permanent magnet available. They were invented in the 1980s and have since become standard in high-quality electromagnetic audio transducers (headphones, microphones and, of course, loudspeakers).
Neodymium magnets are made from an alloy called NIB, which combines neodymium, iron and boron (Nd2Fe14B). They produce very strong magnetic fields and weigh considerably less than other magnets.
To further improve upon their performance, Neodymium magnets are coated with nickel or resilient plastic to improve durability and resistance to corrosion or rust.
Of course, not all electromagnetic loudspeakers have top-of-the-line Neodymium magnets. Other headphone magnet materials that also perform well include:
- Alnico (composed primarily of iron, aluminum, nickel and cobalt, hence the name).
- Ceramic (an alloy of iron oxide and strontium carbonate).
Do All Loudspeakers Need Magnets?
Not all loudspeakers work on electromagnetic principles. There are two other common working principles utilized by non-electromagnetic loudspeaker driver designs:
Neither of these loudspeaker types requires magnets to transduce energy and turn audio into sound.
How does a speaker work? A speaker works primarily as a transducer that converts electrical energy (audio signals) into mechanical wave energy (sound waves). It does so with a driver element (transducer) and an enclosure. The audio signal affects the driver and causes it to vibrate a diaphragm in order to produce sound that mimics the audio signal.
Does the size of the speaker magnet matter? The size of the magnet in a loudspeaker is not as important as the power of the magnetic field produced by the magnet. Generally speaking, a larger voice coil and heavier magnet can handle more power. Still, the magnet size will likely play a minor role in determining the speaker frequency response and sensitivity.
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