Magnetic materials and magnetic fields are all around us but it may come as a surprise to you that the headphones you’re wearing have magnets, too!
Why & How Do Headphones Use Magnets? Headphones act as transducers of energy, converting electrical energy (audio signal) into mechanical wave energy (sound waves). They often do so via electromagnetic induction which requires a conductor (that carries the signal) and a magnetic field that is supplied by magnets.
That’s a quick and easy response. In this article, we’ll discuss magnets and their role in headphones with much more detail to ensure you understand why and how headphones use magnets!
Related article: Do Microphones Need Magnetism To Work Properly?
The Role Of Magnets In Headphones
Magnets play a huge role in most headphones. This is because most headphones require a magnetic field for the driver to function properly.
The headphone driver is the key transducer component that turns the electrical audio signals into sound waves for our enjoyment (or displeasure). The simple fact is that most headphone drivers rely on magnets to effectively convert energy.
The role of magnets is to produce a permanent magnetic field that will attract and repulse the electrically charged diaphragm or the conductor that is controlling the diaphragm.
So to answer the question shortly, magnets are required if the diaphragm of a dynamic headphone is to move and produce any sound.
Let us dive further into the role magnets play in headphones.
Headphones As Transducers & Electromagnetic Induction
As we’ve discussed in the intro paragraphs, headphones are transducers of energy. This means they convert one form of energy into another form of energy.
More specifically, headphones convert electrical energy (audio signals) into mechanical wave energy (sound waves). Many headphones do so via electromagnetic induction.
Electromagnetic induction states that a voltage will be produced across an electrical conductor in a changing magnetic field. Conversely, it means that a voltage across a conductor (that comes with current passing through a conductor) will cause a magnetic field around that conductor.
So if we can pass alternating current (audio signal) through a conductive wire or coil we can produce a magnetic field around it. If this coil is placed in proximity to a magnet, the two magnetic fields will cause the coil and magnet to repel or attract one another.
When it comes to headphone drivers, having permanent magnets and some sort of conductive diaphragm design would allow us to turn audio signals into sound waves. This brings us to headphone driver design.
For more info on headphones and their role as transducers, check out my article How Do Speakers & Headphones Work As Transducers?
Moving-Coil Headphone Driver Design
By far the most common headphone driver design is the dynamic moving-coil. Let’s have a look at how the moving-coil driver works to better understand why and how magnets are used in headphones.
The moving-coil dynamic headphone driver has three key components:
- Conductive “moving” coil
- Magnetic structure (magnets + pole pieces)
Here is a simplified cross-sectional diagram to get a better picture of the moving-coil drive before we get into how it works:
First off, because this article is about magnets, let’s talk about this oddly shaped magnet and its poles.
In order to produce the most efficient and effective magnetic field for coil movement, the magnetic structure must have its north pole to the interior of the cylindrical coil and the south pole to the exterior of the cylindrical coil.
The coil itself must have just enough room to move back and forth within a cutaway. Too much distance between the coil and the magnet will reduce the effectiveness of the magnetic “pushing and pulling” of the coil and diaphragm.
So how do we achieve such a magnetic structure? Well, it takes one strong central ring-shaped magnet and a few pole pieces to extend the poles of the central magnet. 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 north pole underneath.
A thicker ring-shaped pole piece is used to extend this south pole and provide 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.
Now back to how the driver acts as a transducer.
When an audio signal is sent to the headphones, an alternating current (the audio signal) passes through the coil and causes the coil to produce a magnetic field. Because the coil is suspended in such a strong magnetic field already, this current causes the coil to move.
The audio signal causes forward and backward motion due to its alternating nature. This results in the coil moving in accordance with the audio signal.
Now for the fun part. The conductive coil is attached to a diaphragm. Not only does the diaphragm hold the suspended coil in place but it moves along with the coil when an audio signal is applied.
The diaphragm pushes and pulls air and produces sound waves as it does so. Since it moves along with the coil and the coil moves relative to the audio signal, the sound waves represent the audio signal!
In this design, which makes up the vast majority of headphone drivers today, the magnets are absolutely essential if the driver is to produce sound.
The Sennheiser HD280 Pro (link to check the price on Amazon) is an excellent example of a pair or moving-coil dynamic headphones:
Note that there are two other important types of dynamic headphone drivers that work with magnets and transducer energy via electromagnetic induction.They are known as planar magnetic and balanced armature and are described in greater detail in My New Microphone’s article titled What Are Dynamic Headphones And How Do They Work?
As an aside, loudspeakers and studio monitors also utilize the same moving-coil dynamic design, only on a bigger scale. Dynamic microphones, which we talk about a lot in this blog, also share this moving-coil design, though the wiring and conversion are in reverse.
To learn more about moving-coil dynamic microphones and speakers, check out the following My New Microphone articles:
• What Is A Dynamic Microphone? (Detailed Definition + Examples)
• The Complete Guide To Moving-Coil Dynamic Microphones
• How To Turn A Loudspeaker Into A Microphone In 2 Easy Steps
• Do Microphones Need Loudspeakers Or Headphones To Work?
• How To Plug A Microphone Into A Speaker
What Kind Of Magnets Are Used In Headphones?
High-quality dynamic headphones nearly all utilize 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 audio transducers (speakers, microphones and, of course, headphones).
Neodymium magnets are made from an alloy called NIB which combines neodymium, iron and boron. 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 headphones have top-of-the-line Neodymium magnets. Other headphone magnet materials that also perform well include:
- Samarium Cobalt
Do All Headphones Need Magnets?
So far we’ve discussed moving-coil dynamic headphones in great detail and have touched on planar magnetic and balanced armature headphone drivers. All three of these types require magnets.
However, not all headphone transducers need magnets to function. Electrostatic headphones function on electrostatic principles rather than electromagnetic induction and do not include magnets in their design.
Rather than utilizing magnets, the electrostatic headphone driver has a positively biased (fixed charged) diaphragm that is sandwiched between two perforated metal stators, separated by spars. Note the biasing voltage required of the diaphragm makes electrostatic headphones active, meaning they require power to function properly.
The audio signal, then, is applied to the two metal stators (also known as plates). These stators/plates act as a sort of capacitor.
When the audio signal is positive, it applies a positive charge on the front plate and an equal but negative charge on the rear plate. This causes the positively charged diaphragm to move to the back. Conversely, when the audio signal is negative, the front plate becomes negatively charged and pulls the diaphragm while the rear plate becomes positive and pushes the diaphragm.
The Stax SR-007A MK2 (link to check the price on Amazon) is a great pair of electrostatic headphones.
Stax is featured in My New Microphone’s Top 13 Best Headphone Brands In The World.
That simple explanation of how electrostatic headphone drivers work is just to show that they do not require magnets.
To learn more about electrostatic headphones, check out my article The Complete Guide To Electrostatic Headphones (With Examples).
What are electrostatic headphones? Electrostatic headphones are active headphones that convert audio signals into sound waves via a positively biased diaphragm between two perforated stator plates. The audio signal voltage is applied across the plates and the difference in charge between the plates causes the charged diaphragm to move and produce sound.
How do headphone jacks work? A headphone jack outputs (and sometimes inputs) audio with multiple different connections. The jack accepts a plug that can have 2 to 5 poles (known as tip, ring(s) and sleeve) with each pole carrying its own signal. The typical TRS (tip-ring-sleeve) jack is wired as follows:
- Tip = left channel audio
- Ring = right channel audio
- Sleeve = common ground/shield and audio return path
Headphone jacks also come in a variety of sizes (diameters), including:
- 2.5mm (3/32″)
- 3.5mm (1/8″)
- 6.35mm (1/4″)
To learn more about headphone jack sizes, check out My New Microphone’s article titled Differences Between 2.5mm, 3.5mm & 6.35mm Headphone Jacks.