The coil is oriented coaxially inside the gap, a small circular volume (a hole, slot, or groove) in the magnetic structure within which it can move back and forth. The gap establishes a concentrated magnetic field between the two poles of a permanent magnet; the outside of the gap being one pole and the center post (a.k.a., the pole-piece) being the other. The center post and back-plate are sometimes a single piece called the yoke.

Very few manufacturers use electrically powered field coils as was common in the earliest designs. The size and type of magnet and details of the magnetic circuit differ, depending on design goals. For instance, the shape of the pole piece affects the magnetic interaction between the voice coil and the magnetic field, and is sometimes used to modify a driver's behavior. As well, a 'shorting ring' or cap is sometimes used near the magnetic gap to reduce adverse distortion effects of high current in the voice coil.

IPOD Speakers