How do headphone drivers work? - Planar, Electrostatic, Dynamic and BA explained!

In the previous parts of our Explain series, we've talked through how digital audio works, how DACs convert digital audio to analogue, why you need an amplifier, how they work, and what separates different classes of amplifier. But there's one very important thing that we've not talked about yet, and that is how you get sound from a headphone or speaker. So today, we're going to talk drivers. Not those drivers, these drivers. Drivers, or acoustic transducers, are the physical components that convert electrical energy to sound energy. They move or drive the air to vibrate, and vibrating air is sound. If you're into audio, you'll probably know that there's a lot of different types of drivers that show up in various different headphones or speakers, so what are the differences between them and how do they work?

Brought to you by Headphones. com. If you like educational content like this and want to help support it, then consider Headphones.com for your next audio purchase, and buy with confidence thanks to their 365-day return policy. Let's start with the most common type of driver, the dynamic driver. This is what you'll see in the vast majority of speakers and headphones. The part that actually pushes and moves air is called the diaphragm, which usually looks like a rigid cone or dome. Around this you'll find some form of flexible surround which allows the diaphragm to move back and forth. What you can't see is that attached to the diaphragm on the rear is a coil of wire, called the voice coil, and this voice coil sits inside or around a permanent magnet.

When nothing is fed to the driver, this coil of wire is not magnetic, and so the voice coil just sits still. It does not interact with the magnetic field from the permanent magnets at all. But when you feed an electrical current from an amplifier through this coil, one which then pushes or pulls against the magnetic field from the permanent magnet and causes the coil to move. Because the voice coil is attached to the diaphragm, the diaphragm is moved and now we have a moving surface driving movement of air. Put a sine wave electrical current through the coil and the driver moves back and forth in a sine wave fashion. Put some ice J. J. fish through the coil and the driver will produce a sound that quickly encourages all of your guests to leave.

Dynamic drivers have a lot of benefits. They can be constructed relatively easily and affordably, and they can be designed with extremely large, rigid diaphragms and high excursion limits, allowing them to move significant volumes of air at low frequencies and achieve great bass response, which is why you'll pretty much never see subwoofers using anything other than dynamic drivers. But this comes with the trade-off of the diaphragm and coil being heavier and therefore harder to move quickly to reproduce high frequencies. And this is why in speakers you'll usually see larger dynamic drivers handling low frequencies, since they can move more air and don't need to move fast, the high frequencies. When you look to spend more money on headphones in particular, you'll quite often see another type of driver show up, the planar magnetic driver, and these do show up in speakers and in ear monitors as well, so what's the deal with these?

Planar magnetic drivers don't use a rigid diaphragm like dynamic drivers do. Instead, they use a very thin tension sheet of material between a grid or array of magnets. This ultra-thin sheet is the diaphragm, and instead of a voice coil, it has metal electrical traces adhered to or etched into it. This sheet is not magnetic by itself, nor are the traces, but when you pass an electrical current through the traces, an electromagnetic field is induced and this again interacts with the magnetic field from the permanent magnets to push or pull the traces, and thus the diaphragm, towards or away from them. This movement of the diaphragm drives motion of air and therefore creates sound. Planar magnetic drivers have some potential advantages and disadvantages versus dynamic drivers.

Since the overall mass of a couple micron thin sheet of material and ultra thin traces material cone and relatively heavy coil of wire in a dynamic driver, they can have a lower mass for the moving parts and this can potentially allow for lower distortion and higher frequency capability. Whilst they need to be fairly close to the magnet structures and therefore can't usually move as much as some dynamic drivers will, their larger surface area compensates for this and they can still move a lot of air. This, combined with a small, completely sealed volume between the driver and ear, called the front volume, usually means they can achieve exceptionally flat, well-extended sub-bass performance. Also, be quite sensitive to changes in pressure and the quality of the seal of the front volume, in some cases losing most of their sub-bass output if not sealed properly.

Though this is dependent on the level of compliance or stiffness of the diaphragm. Having an array of magnets in between the driver and the listener's ear can obstruct the propagation of sound waves and cause some problems, and they're also just outright more difficult and expensive to manufacture, especially if you want to keep the tensioning of the diaphragm and the thickness of both the diaphragm and the traces on it extremely consistent; an area where one driver may be more flexible and move differently from another part, or to ensure that one driver behaves the same as the next driver. Consistency can be a bit of a challenge when making planar magnetic drivers. But there is one more type of driver that, at first glance, looks very similar to a planar magnetic driver but in practice works a very different way, and that is the electrostatic driver.

Electrostatic drivers work differently to other types of drivers because they don't use electromagnetic force at all to move anything. They use electrostatic force, hence the name. Experiment where you charge up a balloon by rubbing it on your clothes and then seeing how it can pull your hair up towards it? That is electrostatic force, no magnets involved, just a traction of a positively charged mass to a negatively charged mass, and electrostatic headphones basically take this to the extreme. Like a planar magnetic driver, an electrostatic driver has an ultra-thin sheet of tension diaphragm material, but instead of magnets, it's sandwiched between two metal grids or stators. The diaphragm itself is coated with a very thin film of resistive material which can In most electrostatic systems, a bias voltage of either 230 or 580 volts is applied to the diaphragm, but this can be as high as 18,000 volts in some more complex systems.

The bias voltage does not change in response to the audio signal, it's fixed, and it's just there to make sure that the diaphragm itself is holding a constant static charge, and will therefore be electrostatically repelled or attracted to nearby charged elements, i. e. the stators. And unlike a planar magnetic headphone, where the audio signal is passed through the traces on the diaphragm, in an electrostatic headphone, signal is applied to the stators. As the audio signal goes positive, a positive voltage is applied to one stator, and an equal but opposite negative voltage is applied to the other stator. Alike charges repel each other and opposite charges attract, so the highly positively charged diaphragm is electrostatically repelled from the positive stator and attracted to the negative stator.

As the audio signal changes and becomes negative, the signal applied to each stator changes and so the diaphragm is now attracted and repelled in the opposite direction. The diaphragm moves the air, and so you make sound. Electrostatic drivers have the benefit that the diaphragm material can be extraordinarily light compared to other types of driver. You don't need any metal voice coil or metal traces, just an ultra-thin resistive coating, allowing for extremely low mass membranes, meaning potentially higher frequency reproduction and potentially lower distortion. The stators can also be more acoustically transparent than the magnet array in a planar magnetic driver, however they have some significant disadvantages too. Proximity to the stator limits the maximum excursion of the driver, and so electrostatic headphones often aren't able to output low frequencies at high SPLs.

In fact, in speaker systems, you'll often see electrostatic drivers handling the higher frequency reproduction, and something like a dynamic driver handling the low frequency reproduction. Though there are also significant room acoustics reasons for this too, which is a little bit beyond the scope of this video. Electrostatic drivers also require dedicated electrostatic energizers and can't be run on normal amplifiers. You need something that can not only supply the DC bias voltage to the diaphragm, but that can also supply several hundred volts of audio signal to the stators. There can also be some risk to longevity. Planar drivers relying on electromagnetism might be risky to use when angle grinding, since small fragments of metal could get pulled in. The easy solution there is don't do that.

Electrostatic charge, however, attracts stuff that is all around us, dust and moisture, and high humidity in particular can be kryptonite to electrostatic drivers. If either of these build up then they can potentially change the resistivity of the you'll end up with either a channel imbalance or if it builds up in a particular spot and that particular area of the driver is now more or less electrostatically attracted and repelled to the stators, that bit can move more and get stuck to the stators. In fact, because the diaphragm can hold a static charge for quite some time, it's actually recommended that once you're done using electrostatic headphones, you unplug it and put your thumb over the connectors to discharge the diaphragm and prevent it from attracting dust and moisture whilst it's sat on your shelf.

Don't worry, we're covering those next. IEMs or in-ear monitors can and do use many of the same driver types found in full-size headphones and speakers, but one type of driver that shows up almost exclusively in IEMs is the balanced armature or BA driver. Balanced armatures look like small metal boxes, but inside they're quite clever little machines. They have a stationary metal coil and a pair of permanent magnets. Running through this coil and in between the magnets is a highly magnetically permeable U-shaped reed or armature. With a rod connected to a diaphragm, above in this instance, and all of this in a sealed metal enclosure. When no current is applied to the coil, the armature is in a neutral or balanced position.

When a voltage is applied to this coil, the armature is magnetized and then interacts with the field from the permanent magnets, being pulled or pushed in one direction. The coil and the magnets are both stationary, only the armature itself moves, and when it does so, it then pushes the diaphragm with the attached rod, moving the air and creating sound. BA drivers also have some unique benefits. Firstly, because this coil is stationary, light or having any real restriction on the number of coils or the thickness of the wire, giving you more flexibility with driver impedance and allowing you to have highly efficient drivers. This is why hearing aids almost exclusively use BA drivers, since they really need the batteries to last as long as possible.

They're also extremely small, making it easy to pack many of them into a tiny in-ear monitor. This also then allows you to have many different drivers handling different frequency ranges, or pair them with other driver types and achieve a desired product tuning driver to do exactly what you want. Their sealed design also provides much better acoustic isolation than a planar or dynamic driver, which can be important for many of the situations in which in-ear monitors in particular are used. The main downsides to BAs are typically that a single balanced armature usually can't reproduce the same range of frequencies you can get out of a dynamic driver, and so you'll often need multiple drivers to get a good tuning. They also typically have less desirable distortion characteristics than dynamic or planar drivers.

Been covered here, but let us know if you'd like to see more of this. But is there a best type of driver that you should be shopping for? No. Drivers all have pros and cons, most of which are situational and heavily affected by the design of the product as a whole. Never buy a product just because it has a particular type of driver in it. Always look at reviews, measurements, and feedback for the product that you want to buy, since there is no overall best type of driver. If there was, every product would be using it, and there are various different products with any given type of driver, that are terrible, and others which are fantastic.

In fact, even for some of the stereotypes that exist for certain driver types, maybe one's got a reputation for being more detailed than the others, but does that matter if it's got no base to it? Or maybe that reputation for detail was because it didn't have base in the first place. Instead, look for products that are tuned well, that have the kind of base response you want, and don't have problems elsewhere. That's what you should be focusing on. Don't shop for driver types and driver stories, shop for good products. And if you'd like to see us cover more types of drivers, like AMTs or maybe even plasma drivers, then let us know in the comments below and we might do a part two of this. But if you've got any questions you wanted to ask me about music, gear, driver types, anything else at all, then come and say hey on the Headphones.com Discord server or the Headphones.com forum and I and other wiggly air enthusiasts will endeavour to help. Until next time, I'm Golden Sound, you're watching The Headphone Show by Headphones.com, and I'll see you next time.