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.
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.