The basket or frame must be designed for rigidity to avoid deformation, which will change the magnetic conditions in the magnet gap, and could even cause the voice coil to rub against the walls of the magnetic gap. Baskets are typically cast or stamped metal, although molded plastic baskets are becoming common, especially for inexpensive drivers. The frame also plays a considerable role in conducting heat away from the coil.
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.
