A pair of Stax SRD-6 Electrostatic headphones. A tiny pair of speakers stuck to your head.
You give power to the box, it charges the membrane to a high voltage (perhaps 450V DC) and then the internal transformer raises the speaker drive to a nice high voltage (perhaps 300V AC) as well. This is routed to perforated stator electrodes on both sides of the membrane and the differential electrostatic force moves the membrane in sync with the music. Very retro, needs a decent amp before it will work with an iPod I expect, gotta try them out still.
Check out this cool site on how to make your own.