Disclaimer: Measurement has been carried out with a single positional placement, as it is the only spot of which an acoustic seal is successfully obtained.
The internal design of an isodynamic headphone can be described as a vibrating system consisting a patternized conductive membrane on top of a magnetic system, with magnetic rods configured to push each other apart on a yoke. With a magnetic flux generated at the magnets, the membrane, which is situated perpendicular to the magnetic flux, is driven as current runs through. Carl A. Poldy, the mastermind behind the development of AKG K1000, describes the working principle of an isodynamic driver in detail on Loudspeaker and Headphone Handbook written by John Borwick:
"...The resultant magnetic field at the membrane is in the plane of the membrane and perpendicular to the current in the conductors...The pattern of the conducting layer, which resembles a printed circuit, is arranged so that the whole membrane is driven in phase. In order to increase the total force, each conductor is divided into a number of tracks which are mechanically in parallel but electrically in series..."As its conducting layer is enlarged to fortify the driving force, the surface area of the membrane increases as well, hence lowering the mechanical constraint of the driver itself. And rises the greatest technological advantage of an isodynamic driver:
"..The flat response down to the lowest frequencies is a natural result of the relatively low mechanical impedance of the membrane, and requires no special effort on the part of the acoustics engineer. Even the effect of leaks under the cushion has no dramatic influence on the response..."With its low acoustic impedance, an isodynamic driver is more lenient when it comes down to linearity under load than an electrodynamic driver. And according to Dr. Mendel Kleiner, the author of Acoustics and Audio Technology,
"The advantage of this design is that one does not need to worry about diaphragm resonances other than the fundamental resonance, such as rocking mode resonances. Since the film has low mass, any high frequency resonances will be damped by the radiation impedance."
Released in 2002, Fostex T50RP is an orthodynamic headphone too, but it is implemented with the manufacturer's proprietary RP (regulated phase) technology, employeeing "a copper foil etched polyimide film to provide resistance to high level input peaks of up to 3000mW combined with a neodynium magnet for high sensitivity and excellent transient handling", according to the manufacturer.
The RP technology, which is comprehensively described in 特開２００２－１４２２８９, essentially addresses two fundamental limitations of the conventional design.
1. Poor damping characteristic of the membrane
2. Deviation with the driving direction of the vibration system
Fostex solves the former issue by forming a conductive membrane on top of a vibration damping layer, and removes unnecessary oscillation of the vibration system. For the latter, by patternizing the conductors in zig-zag, the driving direction of the membrane is equally distributed, hence lowering the distortion along.
CON: There is a substantial amount of reflection & resonance in the bass due to the housing design, which is acoustically untreated & Poor quality stock earpads making acoustic seal almost impossible. An audible ringing is present at 10 kHz, but can be tamed by placing the driver to the rear side of the head, just like with Sony MDR-1R.
ON SECOND THOUGHT #2: A separate article dedicated to the modification of T50RP will be up soon.
C. A. Poldy, Chapt. 12 in Loudspeaker and headphone handbook, Borwick, J., ed., Oxford: Reed, 1994.
M. Kleiner, Acoustics and Audio Technology. J. Ross Publishing, 2012.