The Moving-Iron Speaker

Author: N.H. Crowhurst

The principle of moving-iron speaker operation.

If you suspend a magnet or a compass needle over a wire connected to a battery, the needle will swing to one side. If you reverse the current in the wire, the magnet (or needle) will swing in the opposite direction.

The force given by the current in this setup is feeble - we could never produce appreciable sound output by passing the fluctuating sound currents through the wire (after attaching a diaphragm to the magnet). We can increase the force by using a stronger magnet, coiling the wire into many turns, and shaping the magnet so as to concentrate the effect of the coil. A strong magnet, however, must be heavy, and it would have difficulty moving at the speed of sound vibrations. For this reason, a large coil is used to provide the magnetism, but only a small piece of magnetic material (iron) is used to move the speaker cone.

In the early days of radio, thousands, if not millions, of these loudspeakers were made. Because of their construction, however, they could not respond very well to the wide range of frequencies used in sound, from the lowest tone given by an organ or other musical instruments - about 32 vibrations per second - to the highest overtone or harmonic required to give the correct "character" to sound. The iron or steel armature, moved by the currents, was too stiff to move freely enough at low frequencies and too heavy to move as rapidly as needed at high frequencies, hence the extreme frequencies did not get reproduced satisfactorily.

The moving-iron speaker.