A SHORT HISTORY OF THE MICROPHONE - The microphone pervades our daily lives through the sound we hear on radio, television and recordings, paging in public spaces, and of course
in two-way communications via telephone. In this chapter we will touch upon some of the highlights of more than 125 years of microphone development, observing in particular how most of the first 50 of these years were without the benefits of electronic amplification. The requirements
of telephony, radio broadcast, general communications, and recording are also discussed, leading to some conjecture on future requirements.
As children, many of us were fascinated with strings stretched between the ends of a pair of tin cans or wax paper cups, with their ability to convey speech over a limited distance. This was a purely mechano-acoustical arrangement in which vibrations generated at one end were transmitted
along the string to actuate vibrations at the other end. In 1876, Alexander Graham Bell received US patent 174,465 on the scheme shown in Figure 1–1. Here, the mechanical string was, in a sense,
replaced by a wire that conducted electrical direct current, with audio signals generated and received via a moving armature transmitter and its associated receiver. Like the mechanical version, the system was reciprocal.
Transmission was possible in either direction; however, thepatent also illustrates the acoustical advantage of a horn to increase the driving pressure at the sending end and a complementary inverted horn to reinforce output pressure at the ear at the receiving end. Bell’s further experiments with the transmitting device resulted in the liquid transmitter, shown in Figure 1–2, which was demonstrated at the Philadelphia Centennial Exposition of 1876. Here, the variable contact principle
provided a more effective method of electrical signal modulation than that afforded by the moving armature.
The variable contact principle was extended by Berliner in a patent application in 1877 in which a steel ball was placed against a stretched metal diaphragm, as shown in Figure 1–3. Further work in this area was done by Blake (patents 250, 126 through 250, 129, issued in 1881), who used a platinum bead impressed against a hard carbon disc as the variable resistance element, as shown in Figure 1–4. The measured response of the Blake device spanned some 50 decibels over the frequency range from 380Hz to 2000Hz, and thus fell far short of the desired response. However, it provided a more efficient method of modulating telephone signals than earlier designs and became a standard in the Bell system for some years.
Another interim step in the development of loose contact modulation of direct current was developed in 1878 by Hughes and is shown in Figure 1–5. In this embodiment, very slight changes in the curvature of the thin wood plate diaphragm, caused by impinging sound waves, gave rise to a fairly large fluctuation in contact resistance between the carbon rod and the two mounting points. This microphone was used by Clement Ader (Scientific American, 1881) in his pioneering two-channel transmissionsvfrom the stage of the Paris Opera to a neighboring space. It wasvHughes, incidentally, who first used the term microphone, as applied to electroacoustical devices.
The ultimate solution to telephone transmitters came with the development of loose carbon granule elements as typified by Blake’s transmitter of 1888, shown in Figure 1–6. Along with the moving armature receiver, the loose carbon granule transmitter, or microphone, has dominated telephony up to the present. Quite a testimony to the inventiveness and resourcefulness of engineers working nearly 130 years ago.
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