You may have come across a number of headphones that are advertised as having "planar magnetic" drivers. But the question is, what does this mean? Is planar magnetic better? and if so, in what ways?
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A planar magnetic driver or transducer (the device that converts the electrical signal to sound waves) functions differently from dynamic drivers in that it uses a flat diaphragm rather than a typical cone or dome shaped membrane that you might find in common loudspeakers. The planar diaphragm also has a conductor trace on that allows it to react to the magnetic field produced by magnets on one or both sides of the diaphragm.
In the earlier days of planar magnetic headphones, designs were limited to double-sided magnetic arrays, such as in the HiFiMAN HE-500, HE-6, or the Audeze LCD-2. This made them quite heavy (over 500g). Over the years, designs have been improved and refined, both when it comes to how thin the diaphragm is (the thinner the better) and to the rest of the structure, allowing for single-sided magnetic arrays and other methods of making the headphones lighter. Modern planar magnetic headphones have benefited from these innovations in ways that also yield improvements to sound quality over their predecessors, however there are certain characteristics of some of those older models that enthusiasts still prefer.
Examples of planar magnetic headphones:
Dynamic drivers by contrast use a more traditional design with a cone or dome shaped diaphragm connected to a moving voicecoil. The voicecoil is moved by a charged magnet structure behind it. Dynamic drivers also vary in terms of how they're designed. Higher-end dynamic driver headphones from Focal for example use what they call a "formless" voicecoil, meaning the moving piece is much lighter and able to react faster to the signal it's being given. In this case, there's no center column that the voicecoil fits around, reducing overall mass.
So the most recognizable difference is the flat diaphragm of planar magnetic transducers, but the other key difference is the conductor trace for planar magnetic headphones is fixed on the diaphragm, while dynamic drivers have a conductor voicecoil behind the diaphragm. Both driver types function in a pistonic motion to produce sound waves, however the planar advantage of having the conductors directly on the flat surface of the diaphragm allow it to react more evenly within the magnetic field.
In the past, planar magnetic headphones were what you had to get in order to have good bass response - they were the only ones that had full bass extension down to 20hz, while dynamic drivers would commonly roll off below 50hz. These days, however, some enthusiasts actually prefer the bass response of dynamic driver headphones. The reason for this is that while you do get inherently better bass extension with planar magnetic headphones, many modern planars don't have quite as much punch or slam as dynamic driver counterparts.
There are of course a number of exceptions to this, like the high-end Audeze LCD-4 or HiFiMAN Susvara (and several others). However the trend for many newer planar headphones is that often what they gain in terms of efficiency and comfort compared to their older counterparts, they lose in dynamic impact (the 'punch' or slam quality). At the moment, it's unclear to me what causes this difference, it may be due to the prevalence of single-sided arrays with less magnetic force being exerted on the diaphragm, or it could be due to physical properties of the diaphragm itself.
As technology advances, we're getting thinner and thinner material for the diaphragm, and this allows planars to become more efficient, and also use lighter magnets (so that they're lighter overall). But this trade off also may be responsible for less slam and punch.
I recently had a conversation with Audeze CEO Sankar about this phenomenon. He indicated that there are several different factors that play a role in this.
Conversely, dynamic driver headphones have improved as well to be able to produce frequencies all the way down to 20hz - even the open-back ones (previously it was possible but only with closed-back designs). Even still, in most cases, planar magnetic headphones often have a more linear bass extension than dynamic driver headphones, even if dynamic driver headphones now have the ability to extend all the way down to 20hz. This means the advantage is still there to some extent, resulting usually in a more 'clean' sounding bass response - even if the headphone in question may not punch as hard as high excursion dynamic driver headphones, like the ones from Focal for example.
Other advantages include better image structure and clarity. This is especially noticeable at the extreme high end, like in the Audeze LCD-4 and HiFiMAN Susvara, however the effect is also recognizable at more entry-level price tags as well. This leads to the ability to isolate individual instrument lines in the mix better with planar magnetic headphones. When it comes to detail retrieval, it's not quite as straightforward however. There are a number of dynamic driver headphones like the Focal Utopia, that make use of unique materials like Beryllium or other design innovations to achieve incredible detail retrieval as well. Generally this is imparted by the stiffness and rigidity at the diaphragm's center - especially for treble frequencies - remember that dynamic driver diaphragms are not uniform the way planar magnetic diaphragms are.
So in summary, headphones with planar magnetic transducers often sound tighter, more controlled, and have better extension capabilities with less distortion for bass than dynamic drivers. There's a unique 'plucked' quality to the way tones come across, and in general, planar magnetic headphones are better at instrument separation and distinction. Modern high end dynamic driver headphones by contrast often have a bit better 'punch' or 'slam', even if they don't have as good image clarity and distinction capabilities (although often still quite good). Planar magnetic transducers also impart a certain type of sound or 'timbre' that makes them immediately recognizable as planars. In some cases they can sound a bit less familiar than what we might be used to with typical dynamic driver transducers, whether in headphones or speakers.
Lastly, another common distinction identified by listeners is that dynamic driver headphones often do a better job of even and consistent image distribution across the center of the stage. What I mean by this is that rather than a pan from left to right having an immediate crossover in the middle, there's a more gradual and consistent pan across the stage. There are enough exceptions and counterexamples here where this likely isn't an inherent limitation to planar magnetic driver headphones, but rather that currently there are a number of planar magnetic headphones that don't do this as well as dynamic driver headphones. It's important to remember that planar tech hasn't been nearly as common as dynamic drivers, so it's likely just a matter of figuring out what works best for image distribution - and there are a number of them that do this very well, like the HiFiMAN Ananda and Arya for example.
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This brings me to another interesting point of distinction. Many listeners who prefer dynamic driver headphones will report that they have a more 'natural' sound to them. There are even those who claim that when it comes to the ultra high end, planar magnetic headphones like the LCD-4 and Susvara are too good at image separation, clarity, detail, representation of fast transients - to a degree that detracts from the 'realism' of the music being listened to. Or in other words, there's a natural decay to the way instruments sound, and this is perhaps better represented in dynamic driver headphones.
There are two ways to think about this distinction in my opinion. The first is to conclude that dynamic driver headphones do indeed sound more 'natural and realistic'. The second is to remember that we're also much more used to hearing music through equipment with dynamic drivers, in both speakers and headphones. This familiarity may be the reason for the sense of realism. I think the answer to the question of which driver type sounds more realistic may instead come down to the question of whether 'realistic' entails "speakers in a room", or the way we naturally hear sound in the world. If it's the latter, then perhaps planar magnetic transducers do actually perform better.
To answer the initial question of "is planar magnetic better?", I think it really comes down to what you value most. Do you value image structure, clarity and distinction? Or do you value punch and slam? Do you want that 'plucked' character of planar magnetic sound? Of course, there are enough exceptions where you don't have to make any concessions one way or another - and not just at the highest price bracket. The HiFiMAN Sundara, for example, is still a somewhat punchy sounding planar magnetic headphone that comes in at only $350. It may not be as detailed or refined as some of the higher end options, but it's a great all-rounder that anyone considering getting into planar magnetic headphones should probably start with.
Conversely, there are enough dynamic driver headphones out there these days that also give you excellent bass extension, detail retrieval, imaging etc., that many of the planar advantages aren't as salient. With all of that said, my preference leans slightly towards the planar magnetic sound when done well, particularly for those advantages, even if I don't currently own any. What does this mean? Well for me, at a given price bracket, if there's a choice between a dynamic driver headphone and a planar magnetic headphone, I often find myself preferring the planar. But you may have different priorities, and so you have to answer that question for yourself.