Most speakers today are permanent magnet which means they have a fixed magnetic field created by a magnet. With a fixed magnet speaker the voice coil is suspended in a gap between the poles of the magnet. The voltage that is applied to the voice coil causes it to move in and out.
A field coil speaker uses two coils; the voice coil and a field coil. Instead of a magnet, DC is applied to the field coil creating a magnetic field. This takes the place of the permanent magnet in creating the fixed magnetic field. Field coil-based drivers claim to drastically reduce distortion levels, and are able to control the driver much more accurately. Drivers in speakers vibrate up to thousands of times per second. Some claim that permanent magnets actually lose strength slightly with each vibration, causing a loss of low-level information and a blurring of the signal. Field coil drivers, with their own power supplies should not lose strength and so have much less distortion than their permanent magnet counterparts.
Like electrostatic speakers, field coil speakers have to be plugged-in to work. The sound gets more relaxed as you lower the voltage, and the highs and lows will be more extended and detailed as you raise the voltage. What you hear is just pure music beautifully reproduced.
Hope this helps. Happy Listening.