So, you’re looking at headphones and somewhere in the specifications or description the type of driver employed is mentioned. It might be a dynamic driver, or planar magnetic, or something completely different.
What do those terms mean? Is one better than the other? What are their strengths and weaknesses?
Well, let’s go through them. Just remember, in this article I’m talking about the motors inside the headphones (and earphones) that produce the sound, not the overall design of the headphones proper. I dig deeply into that subject here.
First, a little background
Sound is transmitted through the air in the form of pressure waves. These are formed by the sound-producing item – whether cello or loudspeaker or headphone driver – vibrating in the air, pushing then pulling to create those waves. Every form of headphone driver has to do just that. It vibrates a surface, usually called a diaphragm. That’s almost a constant of headphone design. (I’m using words like “almost” and “exceptions” because, well, there are always exceptions it seems. For example, Air Motion Transformer drivers don’t have a flat diaphragm at all! See below.)
There are particular challenges for headphone driver design. For one thing, with most designs the one driver has to handle the entire frequency range. There are exceptions (of course), as we’ll see. And obviously there are space limitations, especially with in-ear earphones.
Most importantly, headphones and earphones require a kind of tuning judgement beyond that in loudspeakers. With loudspeakers the aim is, broadly speaking, a flat frequency response. With headphones, and even more so with earphones, a perfectly flat frequency response delivered into your ear canals would sound terrible. Our heads and ears and the air between you and loudspeakers (or other sources of sound) shape the sound significantly. Actually, heavily.
Don’t believe me? Well here are the frequency response measurements of four quite different headphones taken by Tyll Hertsens at what was then InnerFidelity, and which is now hosted by Stereophile:
Those headphones are all highly regarded models, priced at around a thousand dollars each and all are well reviewed. They include open and closed back, dynamic, planar magnetic and electrostatic driver models. I’ve used all four of these headphones extensively, and have enjoyed them all.
But what you can’t say is that the measured frequency response approaches anything like ruler flat. From one or two kilohertz and up, the responses of all four bounce around all over the place, by up to 30 decibels in some cases. (The bass end of the responses, though, are quite representative of what I hear.) However they do exhibit a general trend, and this general trend appears to be present in most headphones generally regarded as good.
Okay, with that background let’s get into the types of motors that make those diaphragms vibrate. If indeed they do.
Also known as moving coil drivers, these are by far the most common drivers in headphones, and indeed in loudspeakers in general. Effectively, headphone dynamic drivers are simply small loudspeakers. In a loudspeaker, the diaphragm is connected to a cylindrical former, around which a coil has been wound. Often in headphones the former is dispensed with, and the coil stands on its own. The coil and former, or just the coil as appropriate, are placed in a carefully shaped space inside a permanent magnet. When a signal is applied to the coil, it creates a magnetic field which interacts with the magnetic field of the permanent magnet, making the coil and thus the diaphragm move.
Here is an exploded view of a dynamic driver from Focal.
From left to right the parts are the front housing, then the surround/suspension, then the diaphragm (since this is the Focal Utopia, the diaphragm is made from beryllium), then the coil, the magnet and the rear housing. See the deep groove in the magnet in which the coil is placed.
Simple and effective, and because they have been around so long – they really got going in the 1950s – dynamic driver designs have been highly refined.
As with all headphone drivers, careful damping is applied during manufacture to avoid audible resonances and shape the output for a pleasing sound.
Some of the other considerations involve trade-offs in the design of the coil. The sensitivity of the headphones is directly related to the number of coils: the more windings the stronger the force. But the more windings, the greater the weight of the moving parts, which can impact high frequency performance. The weight of the coil can be reduced by using thinner wire, but that increases impedance. So it should be no real surprise that headphones come in a wide range of impedances and sensitivities.
(Why “moving coil”? The earliest loudspeakers and headphones were “moving iron” designs, in which the coil was stationary and wound around a permanent magnet. It modified the magnetic field. A metal diaphragm or reed responded to this change. And thus you had sound. As we’ll see, this general system lives on today with balanced armature drivers.)
Planar magnetic drivers
Dynamic drivers apply their force to the diaphragm in a specific location, so the diaphragm needs to be stiff to avoid distortion. That adds weight. Planar magnetic drivers using the same electromagnetic principles as dynamic drivers, but instead of the coil being would on a former, the “coil” is a flat, printed trace covering the entire surface of an extremely thin diaphragm, often made of Mylar. Permanent magnets are placed on both sides of this. The passage of a signal through the printed trace creates a field which interacts with the magnetic field and thus moves the diaphragm.
Here is an exploded view of a planar magnetic headphone driver from Audeze.
From left to right you can see Audeze’s “Fazor” plate (this is Audeze tech to smooth the wavefront, employed in some models), a stator to hold the magnets, the magnets themselves, the diaphragm with the printed conductive trace, the other set of magnets and a stator, and then a grille to protect the various parts.
A lot of high-end headphones now embrace the planar magnetic design, but that doesn’t mean that they are necessarily better than dynamic. Even though it makes sense, the level of development of dynamic drivers can’t be easily surpassed.
However, one characteristic they have which can be quite useful is that they tend to have a smooth impedance curve across the frequency spectrum. The coils in dynamic headphones are inductors and in combination with the natural capacitance of various components within headphones, they form a resonant circuit. That often leads to an impedance curve that varies wildly by frequency. And that unpredictably affects the signal they receive from some headphone outputs.
Planar magnetic? Not so much. The trace is frequently a kind of snake pattern over the surface of the diaphragm, but it might also be a kind of spiral, or even (in a very early model from Yamaha) several such spirals. None of those has a strong inductance characteristic. And that means they can often be used nicely with less-than-ideal sources.
Both of those use primarily magnetic interactions. Electrostatic drivers work very differently. As with magnetic poles, like electrical charges repel, opposite ones attract. Electrostatic drivers work by placing a very thin diaphragm – usually PET plastic and only a few microns thick – between two perforated metal plates. The plates are called stators. A strong charge is applied to the diaphragm – in modern Stax headphones it’s 580 volts – while the signal is boosted to between 300 and 600 volts and applied to the stators, one pole to one side, one to the other. This changes the charge on each stator, tracking the signal, and these in turn attract and repel the charged diaphragm.
The advantages are similar to planar magnetic in the sense that you have a very light diaphragm which is being driven very evenly across its whole surface. The main downside is that the voltages involved are very high, so a special driver box is required. Those high voltages kind of sound dangerous, given that they’re only centimetres from your head. But in reality, the drivers are incapable of delivering anything but the slightest amount of current. If anyone has ever been hurt by electrostatic headphones, reports of it are too hard for me to find.
The most famous producer of electrostatic headphones is the Japanese firm Stax. Here is one of its diagrams illustrating how the signal changes the direction of the forces on the diaphragm.
Balanced armature drivers
I mentioned “moving iron” driver designs before. They were perhaps the very earliest type of loudspeaker driver, but they went out of fashion with the introduction of dynamic or moving coil drivers. Yet they live on today in the form of balanced armature drivers. In these designs, the coil is not moving. That makes design easier in some ways because you don’t have to worry too much about the weight of the coil, which means you can use more turns of thicker wire, increasing sensitivity while keeping impedance down.
They work by having the coil wrapped around one end of a metal “reed”. The signal provides variable magnetisation to the reed that matches the input signal. The other end of the reed is placed between two permanent magnets. The magnetic fields interact, causing the reed to vibrate in keeping with the signal. The end of the reed pokes out beyond the magnets and a mechanical link transfers the vibration to a diaphragm.
This rough schematic from Wikipedia shows roughly how it works:
As shown, there is no signal going through the coil, so the reed or armature is “balanced”, not tilted towards either the north or south poles. When the AC signal is running one way, the reed will be magnetised and tilt away from the like pole, pushing or pulling the diaphragm as it moves. As the AC signal starts running the other way, the reed will move back in the opposite direction.
What this diagram does not show is how incredibly tiny these balanced armature drivers are. The components are arranged much more efficiently and packed in small rectangular boxes, with a port on the end to allow the generated sound to come out. They are so small that some earbuds incorporate multiple drivers. It’s not uncommon for there to be two treble, two midrange and two or more bass drivers in each earbud. Their relative weakness is in the bass, so there are also hybrid models which use a dynamic driver for bass. Or, alternatively, sometimes more bass balanced armatures are added to increase their level.
Air Motion Transformer headphones are highly unusual. I think there is only one model on the market, from HEDD (Heinz Electrodynamic Design). AMT drivers use a thin folded membrane of PET plastic with a metal trace etched on its surface. Think of the bellows on the sides of an accordion.
A magnetic field is provided by permanent magnets. Despite a lot of reading, I haven’t been able to determine their precise placement or how they translate the signal into movement. What everyone focuses on is the form of the movement. Rather than just pumping in and out like the diaphragms on all other types of headphone or earphone, the folds in the material squeeze air out. This apparently provides a mechanical advantage, accelerating air to four or five times the velocity provided by a regular diaphragm.
That makes for excellent high frequency performance. In loudspeaker applications, AMT drivers are usually employed for upper midrange and treble, but don’t do well in the bass. The HEDD HEDDphone is designed to be full range.
It does seem that the design is fairly low in sensitivity, requiring around ten decibels more power that most over ear open back headphones.
Are there others? Of course. For example there’s the crystal earpiece, which uses the piezoelectric effect of some crystals to produce sound. And no doubt there are plenty of others. Engineers are inventive and there are lots of ways to vibrate air. However, few have made it into products, or those which have don’t seem to be around any more (such as the Stax Electret driver headphones), or the technology simply might not be amenable to high fidelity applications.