A wire coil that, when charged with electrical current, produces a magnetic field. Field coils were utilized in early 20th-century loudspeakers, prior to their replacement by permanent magnets. Remarkably, many audiophiles consider field coil loudspeakers to be sonically superior to models using Alnico or other magnets.
Speakers are the primary source of distortion in the playback chain. Field coil-based drivers, designed properly, drastically reduce these distortion levels. With this technology, the driver is controlled much more accurately. Drivers vibrate at hundreds and even thousands of times per second. Permanent magnets actually lose strength slightly with each vibration. This causes a loss of low-level information and a blurring of the signal. The more complex the music becomes, the more of a problem this loss of control becomes. The permanent magnet essentially modulates the signal.
Field coil drivers, with their own power supplies, do not exhibit these irregularities in strength and so have much less distortion than their permanent magnet counterparts. However, they are significantly more expensive to build into loudspeakers, and that economic fact spelled their doom. A few contemporary manufacturers now build field coil speakers for audiophile sound systems.