All headphones use some type of Driver, sometimes called a “sound transducer” (mainly by people showing off!) to produce sound.
Essentially drivers convert the electrical signals sent through the cable, or the air if you’re using Wireless or Bluetooth, to the sound waves that your ears hear.
There are several different types of drivers in use today such as Dynamic Drivers, Balanced Armature (or BA) Drivers, Electrostatic Drivers and Planar Magnetic Drivers. In addition, there are a growing number of multi-driver headphones that use a combination of driver technologies to utilize the relative strengths of the various types of driver.
So, let’s get into the most common type of driver and probably what powers your headphones, the Dynamic Driver.
How Do Dynamic Driver Headphones Work?
So first up we have the most common type of headphone driver, the Dynamic Driver, also known as a moving coil driver.
This type of driver consists of a stationary magnet attached to the frame of the headphone to create a static magnetic field. There is also a diaphragm, basically, a thin membrane that will move to generate the sound waves, usually shaped as a cone and hence called the cone, although in most earphones it is pretty much flat, and a coil known as the voice coil.
The voice coil is typically made of copper, but more exotic materials are used in more expensive earphones, which is uncharged initially. Once the current from the input signal is passed through it, it gains a magnetic field. As the input current varies from positive to negative, the magnetic field in the coil switches from north to south.
The magnetic field of the voice coil interacts with the magnetic field of the permanent magnet and the voice coil is repelled and attracted by the permanent magnet as its magnetic field fluctuates between north and south
The voice coil acts as a solenoid, the technical term for something that converts electrical energy into force and movement. Since the diaphragm is attached to the voice coil, this movement causes it to move in sympathy, displacing air.
The airwaves that are created by the moving air are picked up by our ears as sound.
To increase the volume, the input current is simply increased, thereby causing more air to be moved and hence more sound to be produced. At some point, the diaphragm reaches the end of its range and cannot move any further. This is the point that we hear distortion.
Bass frequencies require more air to be moved, so require larger diaphragms, but this requires more energy and, since they have more momentum, they find it much harder to mimic the fast movements of higher frequencies.
Practical dynamic driver sizes for headphones are anywhere from tiny drivers, as little as 7 millimeters, that fit inside IEMs, to big 55 millimeter drivers capable of the pounding bass that you’ll find in over-ear headphones. In hi-fi speakers these become larger still.
Pros and Cons of Dynamic Drivers
One of the biggest limitations of these is that they are relatively large, but also require a back volume to let the air move. As a minimum, this is typically around the same size as the driver itself.
This can be done within the enclosure itself, or by creating a vent, essentially creating an infinite space. This means that for in-ear monitor (IEM) application they are limited as the vent will reduce their weatherproofing and requires the vent to be outside the ear canal.
The main benefits of these drivers are that they are inexpensive to produce, keeping manufacturing costs relatively low for entry-level products, although, if engineered correctly, they can deliver a wide enough frequency range with enough accuracy that even high-end audiophile grade products can be built using this technology.
Since they can be used in such a wide variety of equipment and their ability to reproduce sound at affordable cost has lead to their massive popularity
How Do Balanced Armature Driver Headphones Work?
While dynamic drivers are hugely popular for earbuds, headphones, and loudspeakers for in-ear monitors they are less popular as we mentioned above.
Phiaton MS 100 BA Balanced Armature Earphones
Balanced Armature (BA Drivers) excel. BA drivers are much smaller and they’re incredibly power efficient. In addition, they require little back volume, so the driver is all you have to find space for.
Since a BA driver does not need to displace air to produce sound, an IEM with BA driver will not need an air. This allows them to be created with much better isolation which in turns keeps out environmental noise and gives you a more detailed sound.
So while some in-ear monitors do use dynamic drivers, the characteristics of BA drivers make them ideal for IEMs and for many other applications such as hearing aids as they also use much less power to drive them.
The BA driver features a reed that is highly magnetically permeable that is pivoted and then perfectly centered or balanced between the poles of a permanent magnet. This balance is why they are called balanced armature drivers.
A coil surrounds the armature so that when an alternating current (AC) signal passes through the static coil, it creates a magnetic flux in the reed. This flux creates an imbalance in the reed and causes motion in the reed as it pivots. This motion is then transferred through a coupling rod to a lightweight and very stiff diaphragm.
If the diaphragm is free of resonances in the audio band it addresses, BA drivers will produce a clean, accurate sound but typically have a fairly narrow frequency response range. They also don’t really deliver head crushing bass and are very limited in performance at the extremes of the human hearing range.
However, since they are so small, they are often combined with other BA drivers in a passive crossover network. In this set-up, each driver is focused on a certain frequency, and a crossover ensures that only the relevant frequencies are passed to each driver.
This approach expands the overall frequency range of the earphones and commercially you can find IEMs with as many as 4 balanced armature drivers, but some custom made ones contain as many as 20.
Do note though that when shopping for headphones more drivers does not always mean better. The crossover is never perfect and the drivers may not be great quality and many put the crossover right where our hearing is at its best – between 2kHz and 5kHz!
So do your research on the quality of the drivers and electronics inside a pair of IEMs before you pay some ridiculous amount of money just because they have a ton BA drivers!
As a final note, there are quite a few IEMs that make use of both BA Drivers and Dynamic drivers. The BA Drivers take care of the higher frequencies and the dynamic driver takes care of the low. These are called multi-driver headphones
How Do Multi-Driver Headphones Work?
As mentioned above, both dynamic drivers and BA drivers have strengths and weaknesses. By using the larger size and response of the dynamic drivers to reproduce the bass, you can overcome one of the main issues with BA drivers as they struggle with lower frequencies.
By allying your dynamic driver with one or more BA driver, you can reduce the power consumption dramatically, while also getting the beautiful sound reproduction in the mids that BA drivers do so well.
The 1more Triple Driver and Quad Driver IEMs are great examples of this, using a dynamic bass driver with two or three BA drivers for the higher frequencies.
How Do Electrostatic Driver Headphones Work?
Electrostatic drivers are a little different to dynamic drivers as they use an electrically charged diaphragm rather than a charged coil. The aim with these is to reduce the stages between the signal and the output by directly moving the diaphragm with the signal, rather than moving a coil, that then moves the diaphragm.
These drivers consist of a thin diaphragm, typically made of coated polyethylene tetra phthalates film which is electrically charged. This is then suspended between two perforated metal plates that act as electrodes. These are called stators.
The diaphragm is kept the correct distance away from the stators with spacers. These not only keep the diaphragm from moving too far and help to tune out unwanted resonances that might color the sound.
The charge on the diaphragm remains constant, usually positive. The electrical signal of the sound is boosted up and applied to the stators which produce an electric field between them – one becomes positive, the other negative.
The negative stator draws the diaphragm towards it, while the other pushes it away. Since the musical signal is alternating, the stators switch in time with the signal between positive and negative. This causes the diaphragm to be repulsed and then attracted by each stator in turn.
The back and forth movement of the diaphragm forces air through the perforations or the curves on the stators. As the diaphragm moves along with the constantly changing electrical signal of the musical recording, the air also moves in time, which creates sound waves which your ears hear.
As you might expect, the lightweight diaphragm and direct coupling mean these electrostatic drivers are generally capable of producing a very wide range of frequencies. Their frequency response often extends well above a human’s audible limit of approximately 20 kilohertz.
The other advantage is that because the entire diaphragm is charged, the force of the movement is spread across the entire face, unlike dynamic drivers, where it is just the voice coil.
Added to the reduced weight of the diaphragm, means the reproduction with electrostatic drivers is less colored than other forms of drivers as phase distortion is also reduced.
However due to the improvements in dynamic drivers and the high cost of production, electrostatic drivers are only really present in extremely high-end audiophile grade headphones as you can see from the above video!
Since most people’s ears will not benefit from the increased range and they are more expensive and require stronger amplification the benefit for most people simply do not justify the extraordinary cost. However, the Electrostatic Driver’s close cousin is beginning to find favor in high-end headphones – Planar Magnetic Drivers
How Do Orthodynamic or Planar Magnetic Headphones Work?
As we have talked about, traditional headphones use a dynamic driver which basically is a cone-shaped diaphragm and voice coil with a magnet to drive them. This style has been around for a long time and they are very popular even with audiophiles.
The Audeze SINE Planar Magnetic Headphones – one of the first to be powered with an integrated Lightning Cable
So what’s the problem with them? The main issue is that a cone-shaped speaker reproduces audio very inconsistently across the frequency range. This means that the cone may respond well at 3 kHz but not so well at 8 kHz, so the audio that you’re listening to is not going to be as accurate as you might like.
On top of this, the entire speaker system has got a lot of moving parts, which means there are a lot of joints stuck together. Over time these will deteriorate so sound reproduction is going to get worse with age.
Instead of using a voice coil to drive a cone to create the soundwaves you hear, these planar magnetic drivers essentially turn the voice coil into the creator of the soundwaves by making the diaphragm the voice coil.
Electrostatic drivers have a permanently charged diaphragm whereas a planar magnetic driver you have two incredibly powerful magnets, separated by spacers and a diaphragm that conducts to create a magnetic flux, which has the input signal run through it to act as the voice coil.
This diaphragm is created from stuff that came out of the NASA space program and it is incredibly thin and lightweight, it is typically one-tenth the width of a human hair and has now been borrowed by audio technology manufacturers.
When there is no signal, the filament has no charge and so sits between the magnets. The diaphragm has a circuit “printed” onto it that be charged by an input signal – the shape of this circuit is used to tune the frequency response of the diaphragm depending on the arrangement of the magnets.
So when an electrical charge is applied to the filament it creates a magnetic flux. Depending on whether this charge is positive or negative it causes the diaphragm to be attracted and repulsed by the poles of the magnet. The old adage of opposites attract applies here, so a positively charged filament will be repulsed by the positive pole and attracted by the negative pole of the magnet.
An an audio signal fluctuates between positive and negative when this is applied to the filament, the magnetic flux changes along with the electrical signal. This causes the diaphragm to be pushed and pulled back and forth in time with the signal. The diaphragm’s movement then creates the soundwaves in the air, producing audio.
So in essence, it is a very simple concept, and very similar in essence to the cone of a dynamic driver, but the audio is extremely accurate and extremely sensitive so you get a much better reproduction of the original recording.
This happens because you have this very lightweight diaphragm is directly charged, so the powerful magnets have little trouble moving it, so it is able to respond to very slight or very fast changes – so the subtle sustain of an acoustic guitar is discernible and the attack of a snare drum gets more of a “crack” to its sound.
Since the diaphragm itself is also comparatively large, it can reproduce more frequencies that a cone ensuring that the bass the mids and the highs are all being well reproduced without the need for additional drivers and crossovers causing signal loss and interference.
Their biggest strength of Planar Magnetic headphones is in the lower frequencies as the diaphragm’s ability to move air is much better than dynamic drivers and particularly balanced armature drivers. If you are a fan of bass, you will love these – you will hear bass tones that are simply lost with other headphones.
However, due to the limitations of the real world, there are a few issues that come to light in the implementation. To make this design work, the magnets need to be fairly large, and the opposing force they exert on each other means they need to be braced to hold the separation.
This means there is a significant amount of trapped air held within the structure. This needs to be moved before the sound emanates and this “springiness” can cause a slowness of response and some resonances that will color the sound reproduction, especially with the treble.
Tuning/damping is key to getting the best out of these drivers, but it means that there will always be some additional compromise compared to Electrostatic drivers that do not have this issue.
While the technology has been around for a while, any headphones using this technology needed a lot of voltage to drive them (although a tiny current is required) In the past this meant you typically needed a good amplifier to drive them, making them useless for portable devices.
However with new technology shrinking the technology and requiring less power to operate so they can be powered via a lightning connector or similar, so they are becoming more mainstream and probably offer the best overall sound unless you want to spend $50k and not leave your house!!
The market is heavily dominated by dynamic drivers as they are pretty good even at the cheaper end of the market and can sound pretty darn good at the top end too.
Balanced armature drivers have distinct issues with frequency response, but their tiny size, price and ability to be used with several drivers have seen them gain in popularity recently and are now found in some very impressive sounding earphones.
At the top end you have electrostatic and magnetic planar drivers. Electrostatic undoubtedly sound incredible, but their price, weight, and need for high-end amplifiers make them little more than a rich-mans plaything. However recent advances have seen planar magnetic headphones increasingly common at the higher end of the market. Prices have tumbled from thousands of dollars to only a few hundred and becoming more practical with lightning connector power.
So while we will still see dynamic and balanced armature drivers rule the roost at the lower end of the market, I fully expect to see more planar magnetic headphones hitting the market in the next few years.