The moving iron speaker was the earliest type of electric loudspeaker. They are still used today in some miniature speakers where small size and low cost are more important than best sound quality. A moving iron speaker consists of a ferrous-metal diaphragm or reed, a permanent magnet and a coil of insulated wire. The coil wraps round the permanent magnet to form a solenoid. When an audio signal is applied to the coil, the strength of the magnetic field varies, and the springy diaphragm or reed moves in response to the varying force on it. The original Bell telephone receiver was of this form. Large units had a paper cone attached to a ferrous metal reed.
There are several types of moving iron speaker. Old undamped moving iron speakers have a characteristic sound, with probably the worst technical specs of any known type of speaker usable for speech. Modern damped moving iron mechanisms can provide respectable sound quality, and are used in headphones.
Moving iron speakers were standard equipment on most pre-war radio sets (1910s to 1930s). (The earliest morse-only radio receivers used a sounding board and solenoid.) The better moving coil speakers were available from 1925, but like most new and better technologies they were relatively expensive.
Read more about Moving Iron Speaker: Design, Impedance, Defects, Uses
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—Chinese proverb.
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—Ralph Waldo Emerson (1803–1882)