The main attraction people have to the Stax 'phones is their sound, which is quite different from that of dynamic headphones. Their diaphragm is vibrated through the modulation of an electrostatic field, rather than the electromagnetic drive principle used in dynamic headphones (nearly all headphones on the market are dynamics).
A transducer in dynamic headphone is really just a miniature loudspeaker. The signal goes through a coil attached to the diaphragm, which is placed within the field of a permanent magnet. The signal creates alternating electromagnetic fields. The resulting force moves the voice coil, along with the attached diaphragm.
An electrostatic headphone has no coil on its diaphragm. Instead, the diaphragm is a very thin sheet coated with graphite. It is surrounded by a pair of metal grids (stators) and charged to a high voltage. The audio signal is applied to the stators, which correspondingly attract and repel the diaphragm.
Electrostatic transducers tend to create a very quick, transparent sound, which appeals to many listeners. The moving mass is very low, which accounts for the speed. The rectangular form probably results from the shape of what is inside. Obviously they were not designed for looks. But they are light in weight, and comfortable. Many years ago I owned a pair of Stax SR-34's, which were not actually real electrostats (electerets, rather). They were not up to the standard of the real electrostats, which I could not afford.
You can find further writings about STAX 'phones in the reviews section. Have fun!
