Moving iron speaker

A moving iron speaker consists of a ferrous-metal diaphragm or reed, a permanent magnet and a coil of insulated wire.

Moving iron horn loudspeakers developed from earphones after the first amplifying device which could drive a speaker, the triode vacuum tube, was perfected around 1913.

It is only practical to make small drivers with this technology, large diaphragms have too much mass, and hence inertia, for passable frequency response.

This has remained a popular type of transducer design, being used in: Poor bandwidth and modest output are limitations of most of these devices.

[citation needed] Modern headphones that use this technology incorporate damping to greatly improve sound quality.

Such devices can be used on valve (vacuum tube) amplifiers, but if used with transistors some precaution to prevent overcurrent at low frequency is often necessary, such as a series resistor or capacitor.

Alternatively the amp can be chosen to drive the speaker resistance, though this will result in worse impedance mismatch and thus output power far below the amplifier design spec.