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Figure 2. Examples of transducer devices that were available c. 1925. a: Drawing showing the internal construction of an early moving coil speaker. Note the large “field coil” (left) that provides the static magnetic field required for actuation. From Rice, 1929. b: Photograph of a speaker built as shown in a. c: Patent drawing showing the internal construction of a horn loudspeaker driver. From Pridham and Jensen, 1923. d: A horn loudspeaker using the driver from Pridham and Jensen, 1923.
    Progress from the 1920s Through the 1950s
The 1920s and 1930s hosted the beginnings of electroa- coustics as known it today, and the vacuum tube initiated a growing electronics industry. This was both enabled and enhanced by the start of the broadcast radio industry and by the growing audio and movie recording industries. The avail- ability of high-input impedance preamplifiers enabled the use of both condenser microphones and dynamic microphones. These microphones have significant advantages of lower inter- nal noise and greater available bandwidth compared with the lower cost carbon microphones that continued to be used in
the telephone system. For loudspeakers, the availability of electronic power amplification enabled public address sys- tems in auditoria and audio presentation in movie theaters.
Significant developments in material science brought the availability of strong permanent magnets using ferrite ceram- ics and Alnico metal alloys. (Alnico is a family of metal alloys of iron with aluminum [Al], nickel [Ni], and cobalt [Co].)
Without these good magnets, earlier magnetic transducers generally needed field coils powered by direct current to gen- erate sufficient magnetic fields. The new magnetic materials
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