The Moving-Coil Speaker

Author: N.H. Crowhurst

The moving-coil speaker was designed to overcome these defects. The magnet can now be as big and heavy as we choose, because it does not have to move. The coil is small and light, and is suspended by a light flexible material that will allow free movement at all frequencies. This makes a much better loudspeaker.

Parts of a dynamic loudspeaker

But why was the coil made round, or cylindrical? The requirements for getting maximum force to drive the cone (or diaphragm) are a strong magnetic field, and as much length of wire as possible in the field. The narrow cylindrical air gap permits a strong field that is easy to create. The attraction between the poles is uniform at all points, hence the center pole does not try to get out of position due to magnetic pull. By winding a thin, flat, cylindrical coil to work in this gap, the coil is kept light in weight, with maximum strength in the direction needed for drive.