How Planar Magnetic Headphone Drivers Work

Ed Note: This article was originally published on May 11, 2011and has been edited and re-written with additional information for the "Headphone 101" series.

Like dynamic headphone drivers, planar magnetic headphone drivers use the electromagnetic interaction of a conductor with an audio signal on it immersed in the magnetic field of permanent magnets. But in this case, the conductors are not in a voice coil, but rather attached to the surface of a thin film diaphragm.

Planar-magnetic headphones have experienced a resurgence in popularity since about 2009 with offerings from companies like HiFiMANand Audeze initially, and later Mr. Speakers, OPPO digital, Abyss, Fostex, and others. But planar magnetic drivers in both headphones and speakers have been around for a long time.

Many will be aware of Magnepan and their speakers, for which they've coined and registered the name "Magneplanar" referring to their planar magnetic operating principle. Most won't know, however, that Yamaha similarly branded their planar magnetic headphones as "Orthodynamic" headphones, with a U.S. introduction in 1976. Just like with planar magnetic speakers, Yamaha's Orthodynamic headphones (and those by Fostex, MB Quart, and others) never gathered a wide following, but both have gathered cults of rabid fans...with good reason, as it turns out.

Terminology!
Headphone101_PlanarMagneticDrivers_Diagram_planarmagneticprincipleThe correct term to refer to this type of headphone is planar magnetic. Unfortunately, all of Headphonedom calls them Orthodynamic, a Yamaha marketing term. Well, I call a facial tissue a Kleenex, so no big deal I suppose, but it's not technically correct. Likewise Magneplanar (not often used) is a registered mark of Magnepan.

The other less used common term is Isodynamic, which means, "having equal force," and refers to the zones of evenly distributed magnetic force in the driver within which the electrical conductors are immersed. Isodynamic magnetic systems exist in numerous types of devices. For example, isodynamic separators can sort streams of powders of mixed materials having differing magnetic permeability.

So, in the world of headphones, Orthodynamic, isodynamic, and planar magnetic all mean the same thing. I'll probably use them somewhat interchangeably here to be sure people Googling for the terms will find the info...but planar magnetic is the correct term.

The Planar Magnetic Operating Principle
You can think of planar magnetic drivers as a sort of cross-breed between dynamic and electrostatic drivers. Like a standard dynamic headphone, planar magnetic headphones use the magnetic field around a conductor that has electrical current flowing through it to drive the diaphragm. Like an electrostatic driver, the diaphragm of a planar magnetic speaker is a thin sheet of flexible transparent film, but unlike an electrostat, the film has very thin, flat electrical conductors (wires ... but very flat ones) in it.

An array of magnets is placed in front of and behind the diaphragm such that the conductors are immersed in a very even field of magnetic flux (isodynamic magnetic field). When current is passed through the conductors, the magnetic field created by the current flow interacts with the isodynamic field created by permanent magnets, causing the conductors, and therefore the diaphragm, to move. The importance of the isodynamic field is to ensure that the relationship of current flow to force exerted on the diaphragm is constant regardless of the position of the conductor in its excursions through the field. The quality of the isodynamic field partly determines the linearity, and therefore contributes to the harmonic distortion content of the reproduced sound.

Other Magnetic Circuit Topologies
Headphone101_PlanarMagneticDrivers_Diagram_OtherDriverCircuits

Engineers are pretty good at finding a variety of solutions to a probem, so there are also numerous other configurations of magnetic circuits in planar-magnetic headphones.

The first diagram on the page shows how some of the older HiFiMAN headphones are configured; the diagram immediately above shows two alternatives that are used in the Audeze LCD-2 and LCD-3 headphones. In addition a number of planar-magnetic headphones use single-sided magnetic structures in order to reduce reflections around the diaphragm and reduce the weight of the headphones. Single-sided designs are used in the JPS Labs Abyss AB-1266 and HiFiMAN HE-560 and HE-400.

Headphone101_PlanarMagneticDrivers_photo_driver

Advantages of Planar Magnetic Drivers There are so many advantages to this type of driver in a headphone that I'm surprised it hasn't caught on more. Let's work our way down the list.

Planar Sound Wavefront --- In my opinion, this may be the most important characteristic advantage of Orthodynamic headphones. Standard dynamic drivers are fairly small and essentially operate as a point source of sound radiating a spherical section wavefront. When a spherical wavefront hits your ears it reflects on the outer ear in a geometrically different way than a planar wavefront. This causes the focusing of sound entering your ear to behave somewhat differently than it would normally. It is surmised that this disturbance of the reflective characteristics of your ear may inhibit normal localization of sound, and therefore disturb the audio image heard.

Headphones like the Stax electrostatics, AKG K1000 earspeakers, and Sennheiser HD 800 with it's large ring-radiator, are known for their excellent audio imaging likely due to the more planar wavefront they present the ear. I find the imaging (such as it is on headphones) to be quite good on the latest crop of planar magnetic headphones, though possibly not quite as good as those mentioned above.

It seems to me generally that getting the sound out of the large magnetic structure is problematic and causes some additional features in the time domain response of the headphones (additional second peak on the 300Hz square wave, for example). Good examples of attempts to mitigate this problem include the Audeze Fazor and single-sided magnetic structure designs like that found in the Abyss AB-1266.

Low Distortion --- Unlike dynamic drivers that are driven from the point at which the voice coil is attached (usually near the center), planar magnetic drivers are forced to move over their entire surface. This means they don't suffer from modal break-up found on traditional drivers when the cone surface starts wobbling in undesirable ways at higher frequencies.

Large and Powerful Diaphragm --- Getting powerful, tight bass response is difficult for most dynamic headphones as the driver surface area is relatively small and would have to make large excursions to move the volume of air that good bass response requires. The force used in electrostatic drivers (the static force the makes your socks cling together out of the drier) is relatively weak compared to the electro-magnetic force in planar magnetic drivers. Electrostatic drivers have trouble delivering the horsepower needed for big bass notes. The large surface area of the planar magnetic driver coupled with the powerful drive of the electromagnetic force permits large amounts of air to be moved with authority. My experience with planar magnetic cans is that they offer the best bass response of any type of headphone.

Responsiveness --- The diaphragm in an Orthodynamic headphone is very light, and the electromagnetic force is very strong, so the ability for the signal to accelerate the diaphragm is very, very good. Like electrostatic speakers, planar magnetic headphones tend to sound very coherent.

Easy on the Amplifier --- Unlike the coiled winding of a dynamic driver, which creates inductive peaks in the impedance characteristics of the headphone, current planar magnetic headphones use a serpentine pattern for their voice coil, which makes their impedance characteristics almost purely resistive. Though they sometimes need quite a bit of voltage to drive them, they are not difficult loads to drive at all.

Headphone101_PlanarMagneticDrivers_photo_desk

The Disadvantages of Planar Magnetic Headphones

Damping --- With electostatic drivers, the charge carrying stators on either side of the diaphragm can be very thin and sonically transparent. Planar magnetic driver diaphragms are surrounded on either side by relatively large structures. The magnets are fairly large, and the opposing force they exert on each other is significant, so sturdy metal structures hold the magnets in place. There is a significant amount of trapped air in volumes of various sizes that must be moved before sound is radiated out of the driver. The springiness and resonances that may exist in this trapped air volume can cause problems. Quite a bit of the design effort with planar magnetic headphones seems to be spent on getting just the right damping. The vintage Orthodynamic headphone scene is filled with various damping modifications done by enthusiasts.

Weight --- As really cool as neodymium magnets are, they're still heavy. The weight and size of a planar magnetic headphone driver makes these headphones potentially uncomfortable.

Summary
Planar-magnetic headphones use a thin diaphragm with electrically conductive traces on it immersed in a magnetic field. Some claim that because of the large, thin diaphragm, sound wave approach your ears with a flatter wavefront more similar to the arrival of real sound than dynamic driver headphones.

Planar-magnetic headphones tend to have powerful bass punch, and tend to be somewhat immune to driver break-up. Though they sometimes need a relatively high voltage to drive them, planar-magnetic cans usually have a very flat impedance response and will generally deliver consistant tonal characteristics on a variety of amplifiers.

On the other hand, planar-magnetic headphones tend to be heavy due to their bulky magnetic structure, and sonic performance can be difficult to achieve due to interference from the large magnets.

Resources
A very well written introduction to the technology from planar magnetic speaker maker Wisdom Audio.
A great source for information on rare headphones including the Yamaha Orthodynamic line is Wikiphonia.
Home pages for planar magnetic headphone makers HiFiMAN, Audeze, Mr. Speakers, OPPO digital, Abyss, and Fostex.

COMMENTS
bronson's picture

Which planar out of those you mention in your article has the hardest hitting bass?

Cool article, thanks.

TMRaven's picture

I've had extensive use with all of the Audeze lineup, and the Hifiman HE-400 and HE-560, plus the Oppo-PM1 and the Mad Dogs. Out of all of those headphones, the HE-400 and LCD-XC hit the hardest.

Btw Tyll, you need to change 400 with 400i in your article.

balkanguy's picture

The answer is NONE of the planars have the best bass. Just because the writer made the ERROR of claiming "My experience with planar magnetic cans is that they offer the best bass response of any type of headphone. " does not mean U should believe that absurd nonsense. The best bass BY FAR is from in-ear headphones. Even the $8 Koss KEB15i will wipe the floor with N E other type of non-in-ear phone. B sure 2 use tips from something like the Auvio 'for android', or 'element' or 'pearl bud 2.0' though, as the stock Koss tips R UNUSABLE & DESTROY the bass output because they CANNOT seal because they R 2 stiff. The high performance of this (current sound quality leader of all headphones regardless of price) is entirely by ACCIDENT, so take advantage & buy yourself some already & stop wasting your time chasing phantoms =)) They need a redesign however into a small enclosure that can B easily draped over the ear & pointed upward into the ear canal like a proper 'pro' phone is all. & that stupid 'inline mic' garbage needs 2 go.

bronson's picture

I think I will get the opportunity to check your $8 Koss earbuds at Christmas time, as Im sure they can be found inside Christmas crackers alongside key rings and toy soilders.

sszorin's picture

Can you please use English when posting a comment ?

balkanguy's picture

The 'fluke' Auvio Pearl bud ORIGINAL design had better slam than the Koss, & there was also a horrible MaxHell design with better bass (& I am talking clarity AND volume, not 'loud mud'), but the MaxHell had very uneven response. As 4 the Auvio, only SOME R better than the Koss, because that line is all over the map (no QC whatsoever).

Out of every 20 units of 'Pearl Bud' (original version) U will usually find 1 extra special in sound, with more of a 'tuunder' vibe 2 the deep lows ~ more 'through the floor' thing. The others all have GR8 bass, just now & then U luck out on a special set =) The Koss R fine tho'. Go try them! They don't go quite as loud as the Pearl Buds but that's something 2 maybe fix later. They go 'loud enough' 4 most people certainly, me included. Also there is a lot of talk about 'break in'. I've done testing with this, & basically what U R hearing is the things demagnetizing, as every time U work against a mag with voice coil energy it is TRYING 2 demag the mags. The lower efficiency also results in less damping & more driver 'swing' in some areas, giving a more mid-centric sound, with reduced bass & highs (more 'classical' feel). The clarity is also REDUCED due 2 this, tho' U may find it more 'pleasant'.

These differences occur only after a LOT of break in, like several days full blast. Most people saying they hear 'break in' R full of krap & not doing A/B comparrison. A few hours @ regular levels will not do jack squat on N E device whatsoever. Always buy @ least 3 of N E thing U plan 2 test = 1 2 use as 'control' & another 2 'test', & the xtra 4 being sure if 1 is faulty U have a 'spare' that is OK =D

Really gotta' love all these SUCKERS paying top dollar 4 planars & fancy amps & stuff. I used 2 sell planar speakers among other things, & 1 thing 4 sure they LACK slam. What they have is COMPRESSION that people confuse 4 'slam' & 'dynamics'. They actually drastically COMPRESS the dynamics, because the driver cannot move as far as easy. They can respond FAST on the rise time sure, but they can't keep going as easy. It's like routing a 'dynamic expander' (a device in studio setups that removes noise by quieting lower volume sections and jacking loud ones' cobined with a 'limiter' 2 emulate that 'compression' problem. They have 2 different problems occuring simultaneously, seemingly in conflict like they would cancel out & give U transparency, but no. They 'expand' the lower volume sections, & 'compress' the louder ones. Planars, in terms of dynamic contrasts, R a fuking MESS. Dynamics R much more linear.

bronson's picture

Thanks for your comment.

I've got the LCD-X which EQ's bass quite nicely.

I'd guess the closed back would hit harder mainly due to the closed back ear cups, but I wonder about the open back AB-1266 as Ive read that it has what some consider the hardest hitting bass out of other open back planars?

xnor's picture

I'm not so sure about that.
At low frequencies both dynamic and planar magnetic headphones work by pressurizing a sealed chamber - there is no wavefront.

At higher frequencies, we get traveling sound waves. The pinna is not a perfect funnel, so it really doesn't seem to matter whether you have a wavefront that supposedly hits the whole area of your ears or just a part of your pinna. The waves have to travel through the narrow ear canal eventually.

The main difference is frequency response at the ear drum. Dynamic headphones, at least in some of your measurements, seem to show fairly smooth FR >10 kHz while LCD2, 2r2, 3 etc. don't really.

Tyll Hertsens's picture
That comment in the article is related to the nature of dome drivers effectively act as a point source radiating a spherical section wavefront. Planars give a planar wavefront. Yes, we're talking about fairly high frequencies. The ear will focus a spherical sections wavefront into the ear differently than a planar wavefront. The contention is that we'll hear these two wavefront geometries differently, and is why imaging is better in electrostatics and HD 800. But as I said, PM headphones have other problems with sound getting through the magnets unchanged.

Yeah, the problem is trying to simplify things for an abstracted easy view...while the realities are quite complex. And I'm still learning.

xnor's picture

It's only a "point source" with "spherical wavefront" given a flat surface from which a small dome radiates sound into a much larger room.
That's not the case with headphones. In headphones we have relatively large drivers, radiating from within a special housing into relatively small enclosed volumes.

TMRaven's picture

While open planars are exceptional when it comes to distortionless bass extension down low and maintaining flatness in the entire bass and lower midrange region, I do agree that as a whole they never hit as hard as dynamics. Especially the Audeze stuff, is way overrated when it comes to slam factor. They're more atmospheric and limp sounding than hard-hitting, but they do extend really low really well.

Three Toes of Fury's picture

When it comes to the internet, chat forums such as this, and trolls (individuals which solely post negative and attacking comments) there is two rules:

1) You do not talk about fight club
2) Dont ever reply to trolls..its what they want and only serves to fan the fire.

Just this once Im going to break my rule #2...

@balkan guy....this is a great place to discuss, comment, review, suggest, and even offer criticism..provided it is constructive criticism. Your recent posts are typically very attacking in tone. Equally it appears you are a shill for Koss?

Now....here's the kicker...i actually tried the Koss buds you keep pushing. And you know what...they arent bad sounding for cheap-o buds. By no means are they the holy grail of cheap headphones that you tout, but they sound better than they cost which is always a plus.

Its a shame that you feel the need to attack and slander reviewers and site visitors to convey such suggestions. If you really do care about headphones and want to share your suggestions with fellow fans, perhaps try to do so in a less-attacking fashion? Offer constructive feedback and thoughts and you'll reach more folks.

Peace .n. Living in Stero

3TOF.

tony's picture

Planers are quite wonderful but dynamic driver technology can deliver an improved performance .
I've always loved the MG2s from Magnapan , a little Conrad-Johnson MV-45a tube amp is a beautiful combination system .
However , I sold ProAc Tablettes ( shoebox sized ) that equaled the beautiful ambience of the MG2s .
Nice as they are , Planers are limited by all that size and complexity . Focal , KEF , Genelec and probably many other outfits ( Sennheiser ) make dynamic drivers that go beyond the narrow performance envelope of Planers .
Still , Planers have found a happy buying public in the Headphone world of active hobby types . On the street , out and about , you mostly see the white wires going to people's ears which reminds me that I'm not normal when it comes to music reproduction . I do eat at Taco Bell ( like everyone else ) .

Tony in Michigan

balkanguy's picture

U got the guy with the colorful avatar avoiding trying out the Koss KEB15i, as if it is beneath him 2 even listen 2 them because they R not expensive. Sound quality is IRRELEVANT 2 him, like the typical Gib$on guitar buyer who refuses 2 acknowledge U can get the same SOUND from nearly N E Asian all-mahogany knockoff (because the tone comes mainly from the wood species used). Then U got this other IDIOT CLOWN who claims 2 have tried the Koss KEB15i, then obviously didn't even use the right tips, or does not believe what he is saying, trashing them because they R again... inexpensive. Did U use the tips from the Auvio 'for android' or 'element' or 'pearl bud 2.0'? No, U didn't. U can also use the tips from the TDK EB750, or many models of Maxell (mainly their large tips can B nice), such as those from the "MIX-WBK Item #: 196135". Does your deadbrain troll-tard azz mention the fact U did not use these exact sort of tips which R REQUIRED 2 release their awesome sound? No, I would bet my life U used the tips that came with them, which I already told U idiots R UNUSABLE because they R 2 stiff & thus fuk up the bass by not sealing properly.

So, all U nitwit 'cork sniffer' cows can go on throwing your money DOWN THE TOILET... hell it's YOUR $! Or U can save TONS of cash while simultaneously IMPROVING sound quality, by using the Koss KEB15i with the tips I mention, & N E person INSANE enough 2 think I am a shill 4 Koss obviously doesn't understand basic English, beause I do nothing but slam their HORRIBLE form factor & tip design! WAKE THE FUK UP =)) What needs 2 happen is that driver & filter arrangement, put into a smaller 'up-pointing' over-the-ear enclosure with tips like I described, & no 'inline mic' garbage.

Lastly, U fools, only 1 having even claimed 2 have tried them, STILL don't suggest N E thing that sounds BETTER! Y? Because NOTHING does! =) That's the truth & U know it! NOw go try them with the tips I suggested, then get on your soap box & tell the world what sounds better. If U find something that does, I will most certainly swithc 2 cheer leading 4 those INSTANTLY! I only care about PERFORMANCE! E-Mail me at 'zhotstuff at hotmail dot com' with something that sounds BETTER, or just STFU, cows.

balkanguy's picture

U can SELL your krap on Fleabay, simultaneously turning a PROFIT while improving the sound quality of your setup by going with the Koss I mentioned =D I am here trying 2 HELP U guys, AND myself, because perhaps one of U has stumbled onto something that is EVEN BETTER in sound quality! YAY! TELL ME WHAT IT IS!! (if it exists, which it probably does not.) I am in 'market research' phase. I've tried HUNDREDS of phones from ALL price categories, & they R all worse, except the odd (unpredictable from 1 unit 2 the next) Auvio Pearl bud originals. They ALL sound different (NO quality control WHATSOEVER). 1 of them if fuking MAGIC =D The rest I save & resell R good yes, but 1 is similar 2 the Koss I mentioned, but more ballzy & deep & dynamic. It's not a huge difference, but nice. I might have some1 reverse-engineer that set 2 find out how much extra glue they spilled on the diaphram or whatever it was 2 make them sound so nice ~;-) That is... if it turns out there is nothing out there better than the Koss KEB15i, & thank god that model of Koss exists so I can send U guys out 2 a reliable benchmark 4 ranking purposes.

So what sounds BETTER? Use the tips I suggested, & post your replies or E-Mail me. I care about SOUND QUALITY, not price or name. It could cost $100, 30 cents off Alibaba, or a THOUSAND dollars. Price has NOTHING 2 do with sound quality in the headphone market ~ ZERO!

castleofargh's picture

so what happened? someone hit you with a planar you when you were young and now you're traumatized?
what do you expect to do insulting everybody for no reason?
planar design usually aren't the best thing for loud low freqs because of the designs used. but it doesn't mean that the quality of the bass is bad. it can be accurate while not super loud.

balkanguy's picture

I will try N E phone regardless of price, & if it is better than the Koss I mentioned I will keep it as reference. However, if it is worse I will just return them, so it's not 'sticking it 2 me' with some kind of 'sucker punch'. It takes me only a few SECONDS 2 know what is good. I've heard so much stuff I have a few key sections of songs that tell me everything about their sound quality. I go through about 10 'review' phones a month - a hobby, N E thing I have not tried B 4, regardless of brand, model, or price. They all go back 2 the shops (usually =)) because they all pretty much suk compared 2 the Koss or Pearl Bud faves I mention (the Pearl buds R mostly junk ~ no QC so have 2 select out the baddies from the 1.0. The 2.0 verison is unusably tizzy).

Tyll Hertsens's picture
I've decided to ban balkanguy, he's prolific and meaningless, brings nothing of worth to InnerFidelity.

See ya.

Seth195208's picture

..and it is TERRIBLE! I can't believe I bought that piece of crap! Balkansguy needs to get on lithium. Bipolar is very treatable. What a joke.

sszorin's picture

He was kind of entertaining.

thelostMIDrange's picture

guitars. I can say I've not heard past or current planer phones that accurately portray what I hear whereas some inears and a lot of dynamics get a lot closer to that roundness and pluck that a traditional electric bass guitar has in the real world.....that being said, in 2014, there are other forms of 'bass' that are electronic and for those forms I'd say planars rule because of their linearity and control. But for rock n roll bass a sr60 grado or $30 in ear can compete and in some ways surpass a %1500 planar in terms of realness in a certain quality of bass guitar dynamics and realism...yes it's hard to get a dynamic distortionless and extended and so it's a tossup as to which is 'better' and really comes down to what a listener cares for in his music, but I'll wager 9 out of 10 musicians will agree that a good dynamic is more real than a pricey planar/ and likely will always be the case due to basic physics and limitations in the technology itself.

Three Toes of Fury's picture

Great stuff and helps explain some newbie questions i'd have like "why are those Audeze or Abyss cans so huge??".

One thing i cant figure out thou...wouldnt the magnetic array that is in front of the diaphram totally mess with the sound?? In the case of speakers and dynamic headphones, the last thing you'd want to do is place a bunch of posts between the sound source and the listener. Yet the magnet array of these seems to do just that. I would think those would cause all sorts of havoc with the sound waves coming off the diaphram.

Secondly: do all Planar Magnetic headphones have high Ohm ratings? Im still trying to work that variable out but i assume that all planar require decent amps to drive them and therefore would be high ohm rated?

Peace .n. SCIENCE!

3ToF

TMRaven's picture

Planar magnetics usually have lower resistance, typically from 16-50Ω. Planars used to be notoriously hard to drive due to low sensitivity, but each new planar coming out is getting more and more sensitive, so it's not a big deal anymore for the most part.

Rom's picture
Thujone's picture

Like TMRaven stated in his first post, the single ended designs include the HE-560 and HE-400i, instead of the HE-400. Also the HE-4 is single ended (and not discontinued yet).

I really like this article, Tyll. It was a great resource to me when I got into this hobby and I continue to use it to show buddies entering the hobby. Lots of good info and sweet diagrams!

Jazz Casual's picture

Nice work Tyll. The only planar magnetic headphone that I've liked as much as my dynamics is the Abyss. It is an unwieldy beast however.

skris88's picture

Re IEMs, I have never heard a good one. I simply cannot get any bass unless I push them in as hard as I can. That won't do for extended listening of course. I suspect my ear canal is deeper than average. So I avoid all IEM and similar earphones.

To say some IEMs are excellent value and excellent quality is useless for me and, perhaps, many others with my 'disability'.

For my main listening I have a pair of Magneplaner MG12 with 2 x 8" powered sub-woofers crossed over electronically at 40Hz. These were set up and referenced against a pair of Senheisser HD-650 headphones.

For bass accuracy and bass slam, I have not heard a better room-based system. The top end can be more refined but to me, it's "all about the bass".

I took my HD-650 into a HiHFiMAN dealer and compared them against their HE-400i

When it came to bass punch I initially felt the HD-650 did better. But I suspect I have gotten used to some looseness that the 650s have. The 400i are simply tighter and dryer. They win.

A pair of lovingly used 650s going cheap. Anyone? :-)