Both technically and sonically, these open-backed headphones represent a breakthrough in portable monitoring.
Innovation in music technology is usually about incremental improvement rather than radical change. It's quite unusual to be confronted with a product that is genuinely the first of its kind, but the HEDDphone is just that.
Headphones for hi-fi and studio use have typically employed one of three transducer technologies. The vast majority are like dynamic microphones in reverse, and are based on the principle of electromagnetic induction. A coil of wire surrounds a permanent magnet: current passing through the coil causes it to move relative to the magnet, and a diaphragm attached to the coil translates this motion into sound.
Much less common are electrostatic headphones, which are directly related to the capacitor microphone. In this case, the diaphragm forms part of a variable capacitor, and is set into motion through being alternately attracted to and repelled by a charged backplate.
Finally, planar magnetic or magnetostatic headphones have enjoyed a revival in recent years. As in moving-coil headphones, the motive principle is induction, but the coil is replaced by wires embedded within a flat (planar) diaphragm, which is suspended within a magnetic field.
What all of these technologies have in common is that there is a trade-off between the surface area of the diaphragm and the quality of the sound that can be achieved. Making the diaphragm larger means less displacement is required in order to generate a given sound pressure level, and this generally means lower distortion. But there is a limit to how large a headphone diaphragm can be made whilst retaining structural integrity; and, of course, there's not much point in having a diaphragm much larger than the human ear.
Enter the HEDDphone, and a fourth driver technology that — in theory at least — eliminates this compromise. Like a planar magnetic driver, the Air Motion Transformer is driven by induction, and has wires embedded in the diaphragm...