The voice coil is the only part of the loudspeaker that is directly connected to the amplifier.  The coil consists of a length of thin (typically #30 AWG) enameled wire that is wrapped in a few layers around the bobbin.  One end of the coil is attached to a positive (+) terminal and the other end is attached to a negative (–) terminal; both terminals are mounted to the speaker frame.  The bobbin is a hollow cylinder of thin metal (usually aluminum) that provides support for the voice coil and directs heat away from the wire.  Heat is a byproduct of the wire’s normal resistance to electrical current; a typical loudspeaker has a coil with three to eight ohms of resistance in the wire.  Thus, the power rating of the voice coil and bobbin assembly is a function of wire gauge, bobbin area and bobbin thickness.  The design is a compromise between the ability of the assembly to withstand heat and the requirement to minimize weight.  A voice coil with thicker wire and a heavier bobbin requires a large (i.e. expensive) magnet to produce an acceptable sound level from a loudspeaker.

The cylindrical shape of the voice coil causes the formation of a doughnut-shaped magnetic field around the bobbin when an amplifier signal is applied to the coil (Fig. 2). The direction and intensity of this field changes in response to the electrical polarity (positive or negative) and the power of the amplifier signal.

The loudspeaker magnet assembly consists of a ring-shaped magnet that is glued between two heavy steel disks (Fig. 3). The rear disk incorporates a cylindrical structure called a pole piece (usually the disk and pole piece are cast or machined as a single unit). The pole piece is centered within a circular opening in the front disk, which together form the air gap of the magnet assembly. The magnetic field circulates within the path that is formed by the disks and pole piece, and is concentrated within the air gap.

The voice coil assembly is suspended by the centering device in the air gap (Fig. 4). The inside diameter of the bobbin is large enough to encircle the pole piece, and the outside diameter (including the thickness of the voice coil windings) is smaller than the circular opening in the front disk. The assembly is placed about halfway along its length at the center of the air gap when the loudspeaker is at rest, i.e., when no amplifier signal is present. The assembly is thus positioned to move backwards and forwards within the air gap without rubbing against any part of the magnet assembly.

The voice coil magnetic field direction is alternately reversed and aligned by the amplifier signal to the unchanging direction of the magnetic field in the air gap (Figs. 3, 4). Thus, the voice coil assembly is attracted and then repelled by the field of the magnet, which forces both the assembly and the attached loudspeaker cone to move backward and forward. A higher power signal increases the intensity of the voice coil field, which increases the forward and backward travel distance or displacement of the assembly and cone in the air gap, with a corresponding rise in sound level.

Note: for clarity, the magnet assembly is shown in the illustrations as a single unit.