As radio grew, during the 1920s, from a laboratory curiosity to a new means of home entertainment, it became apparent that some means of reproduction other than the usual single-person headphones had to be found. Radio designers first took a single headphone and tried improving it, with a stronger coil and larger diaphragm, and then feeding the output to a horn similar to those found on acoustic phonographs.

This led to a sort of loudspeaker that enabled more than one listener to enjoy a radio program. There were many such speakers on the radio market for a few years (such as RCA’s UZ-1325), but their tinny sound and lack of fidelity soon led designers to consider a vibrating paper cone as the reproduction device.

Typical cone-based speakers of the time consisted of a paper cone connected at the apex to a metal rod. This rod, in turn, was connected to a driver known as a speaker motor. The speaker motor was an assembly made up of a horseshoe-shaped permanent magnet with two pole pieces at each end of the magnet, and a small gap between those pieces. Coils of fine wire were wound around each pole piece, and audio voltage from the radio receiver would energize the coil, thus producing a varying magnetic field. The driving rod was mounted on its other end to a small metal vane suspended in the gap between the pole pieces, and the magnetic variations caused the vane to vibrate in accordance with the audio voltage. Those audio vibrations were transmitted via the metal rod to the cone and converted into sound.

These speakers were capable of good reproduction; some of the later models, if correctly matched as to impedance, had a decent sound – but nothing approaching even medium fidelity. They were frequently installed in ornate metal housings and were often part of the total aesthetic design of a radio receiver.

The electrodynamic speaker was developed in the late 1920s as a method of providing better sound reproduction than had previously been possible, while saving a bit on the total cost of the radio receiver. Permanent magnets were not inexpensive in those days, and tended to lose their magnetism as time went on.

The problem, then, was the magnet. Radio engineers realized that, if they used the principle of the electromagnet, they could produce a very powerful magnetic field. Replacing the filter inductor usually found in the radio’s power supply with just such an electromagnet could do this. Not only would this produce a better magnet and eliminate the more expensive filter inductor, but also the improved filtering action would result in cleaner power for the radio. The electromagnet in such speakers became known as the field coil.

It was about this time that someone hit upon the idea of replacing the metal vane and rod assembly with a moving coil. Rather than the coil remaining stationary around the pole pieces, engineers replaced the entire assembly with a coil of very fine wire wound around a paper tube, which was then attached to the speaker cone. This voice coil, as it came to be known, was positioned in the air gap between the pole piece of the electromagnet and the U-shaped frame that surrounded it. Audio voltage from the receiver caused the voice coil to vibrate, and those vibrations were transmitted to the cone, resulting in a more reasonable reproduction of the original audio program. The design was good enough that today, during restoration, all that’s frequently necessary is replacement of the old cone to result in a speaker that works well.

In later years, as good permanent magnets became less costly, and as better receiver designs dispensed with the need for stringent power supply filtering, the field coil disappeared from speakers. This design, known as the permanent-magnet speaker, is largely unchanged today, aside from better magnets and cone materials.


SOURCES

Ghirardi, Alfred A., Modern Radio Servicing. New York: Radio & Technical Publishing Co., 1935.
Kendall, Lewis F, Jr., and Koehler, Robert Philip, Radio Simplified. Philadelphia: The John C. Winston Company, 1925.
Rutland, David, Behind The Front Panel: The Design & Development of 1920s Radios. Philomath, Oregon: Wren Publishers, 1994.

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