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CHAPTER II
ELECTRICAL REPRODUCTION OF SPEECH
The art of telephony in its present form has for its problem so to
relate two diaphragms and an electrical system that one diaphragm will
respond to all the fundamental and harmonic vibrations beating upon it
and cause the other to vibrate in exact consonance, producing just
such vibrations, which beat upon an ear.
The art does not do all this today; it falls short of it in every
phase. Many of the harmonics are lost in one or another stage of the
process; new harmonics are inserted by the operations of the system
itself and much of the volume originally available fails to reappear.
The art, however, has been able to change commercial and social
affairs in a profound degree.
Conversion from Sound Waves to Vibration of Diaphragm. However
produced, by the voice or otherwise, sounds to be transmitted by
telephone consist of vibrations of the air. These vibrations, upon
reaching a diaphragm, cause it to move. The greatest amplitude of
motion of a diaphragm is, or is wished to be, at its center, and its
edge ordinarily is fixed. The diaphragm thus serves as a translating
device, changing the energy carried by the molecules of the air into
localized oscillations of the matter of the diaphragm. The waves of
sound in the air advance; the vibrations of the molecules are
localized. The agency of the air as a medium for sound transmission
should be understood to be one in which its general volume has no need
to move from place to place. What occurs is that the vibrations of the
sound-producer cause alternate condensations and rarefactions of the
air. Each molecule of the air concerned merely oscillates through a
small amplitude, producing, by joint action, shells of waves, each
traveling outward from the sound-producing center like rapidly growing
coverings of a ball.
Conversion from Vibration to Voice Currents. Fig. 1 illustrates a
simple machine adapted to translate motion of a diaphragm into an
alternating electrical current. The device is merely one form of
magneto telephone chosen to illustrate the point of immediate
conversion. _1_ is a diaphragm adapted to vibrate in response to the
sounds reaching it. _2_ is a permanent magnet and _3_ is its armature.