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How Planar Magnetic Headphones Work

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) have never gathered a wide following. On the other hand both planar magnetic speakers and headphones have gathered cults of rabid fans … with good reason, as it turns out.


Terminology!
The 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 crossbreed 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.


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. My guess is that the high-end planar magnetic headphone makers (Audez’e and HiFiMAN) need to move the drivers a little forward and angle them back towards the ear to make improvements here.


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 and spacious.


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.


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.


(As a side note: I’ve asked for some help over in the Head-Fi Ortho thread from members who would be willing to ship me their modified headphones for measurement. I’ve been very pleased by the response, and you should start seeing measurements of these DIY modified planar magnetic headphones within a few weeks.)


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.


The Sound of Planar Magnetic Headphones
There is no one particular sound to these headphones, of course, but there are a few common characteristics worth mentioning about the new cans from HiFiMAN and Audez’e (pronounced like odyssey). Headphones from both these makers have about the best bass response I’ve heard. It’s tight, linear, and beautifully textured. Tubas and plucked stand-up bass notes are fully recognizable and characteristic. One-note bass is a thing of the past with these cans; just extraordinary.


Treble response seems to be an issue with planar magnetic headphones likely due to an ongoing need to keep working on the damping issues. The HiFiMAN products tend toward the tizzy side, but I’m very happy to report that they seem to have been making worthwhile adjustments with the release of the various models (HE-4, HE-5, HE-5LE, and HE-6), and with the introduction of their soon-to-be-released can (the HE-500 that I’ve had the pleasure of listening to for the last 2 weeks) I think you’ll find that they’re very close to spot on.


The Audez’e LCD-2 has difficulties in the other direction in that many find them a bit too relaxed in the highs. I find this to be true as well, but need to remark that I tend to like a warm sounding can, and with extended listening I adjusted very well to the presentation of the LCD-2. They make a GREAT headphone for hip-hop and rock where bass slam is strongly desired, and the relaxed top-end permits lengthy listening to the often-harsh recordings of the genres.


Imaging is likewise very good on these headphones, but it seems to me the problems in the treble may be holding them back from the imaging that may one day be realized with headphones of this type. I encourage the makers to continue their quest for good treble, and therefore imaging, as I’m certain their efforts will be rewarded.


Summary
I’m simply amazed that two small companies can enter the high-end headphone world with a long ignored technology and produce what amounts to world-class sound from their efforts. Obviously, planar magnetic headphones are here to stay. I hope the big corporate makers are watching, and, more important, listening to these new headphones. I heartily applaud the efforts of Audez’e and HiFiMAN on the work they’ve done, and will heartily recommend both makers products in upcoming reviews.


Well done!


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