4.2.2 - The Loudspeaker
In the early days of radio, in order to listen to the audio frequencies the sets generally used the well- known earphones originally derived from the telephone. However, earphones were not a reliable devices for the proper reproduction of the sound waves and soon the first loudspeakers appeared on the market.
It is interesting to notice the loudspeaker had hardly changed since 1925. However, over the years the loudspeaker driving units may be broadly classified into the following two types:
Moving iron and moving coil.
In the moving iron type, the attraction between the pole pieces of a permanent magnet and a magnetic diaphragm, rod, or reed is made to vary in accordance with the variations of the signal current flowing through coils of wire. Moving iron type driving units may be further subdivided into iron diaphragm and balanced armature types. In the moving coil type, the mechanical forces and motion are developed by the interaction of the varying magnetic field produced by the flow of the signal current in a conductor, and that set up by a strong magnet provided which can be either a permanent magnet or an electro-magnet.
Horn speaker circa 1926
Schematic of balanced armature loudspeaker
Primitive type of 2000 Ohm earphone made in Germany circa 1925.

Bakelite framed loudspeaker using balanced armature driving unit model 2007 made by Philips.

Loudspeaker using balanced armature
Loudspeaker using balanced armature driving unit made by Sferavox, EUA 1926
First type of dynamic loudspeaker made in the USA by Stewart Warner
Primitive type of loudspeaker made in the USA by Crosley


Due to the lack of high flux density magnetic material, in early thirties it is launched in the market the electrodynamic loudspeaker as known as auto-dynamic loudspeaker also. Basically in this type of acoustical transducer the voice coil is energized by an electromagnet powered by direct current supply.