Man's physical universe

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SOUND PRODUCED BY VIBRATIONS IN MATTER 453

records can be greatly amplified by a special apparatus designed for

the purpose of giving curves similar to those produced by the phonodeik.

The phonograph not only records sound but also reproduces the

sound thus recorded, because the diaphragm is caused to vibrate as

the needle runs over the sound track. Thomas Edison's phonograph

was followed by a series of improved machines under such trade names

as the "graphophone," the "gramophone," the "electrola," and the

"victrola." Today a very useful application of the phonograph is the

dictaphone.

Phonograph records are first made in wax, dusted with graphite, and

electroplated to make the master disks. These master disks are used

to make faithful impressions on plastic materials of various compositions

in the hydraulic press. The loudness of a sound produced by

a record depends on the depth of the track, while the pitch depends

upon the relative frequency of the elevations and depressions of the

vertical cut record. If the records are played too slowly the pitch is

cut down. In the case of lateral-cut records, the loudness depends

upon the swings of the track away from the center line.

Modern instruments use electrical transcription which employs

the vacuum tube (as described in Unit VII). Some sound motion

pictures use a sound track produced on film by a device in which sound

energy is transformed so as to vary the intensity of light falling on the

film.

Light passing through this sound track produces variations in

the light received by the photoelectric cell, which changes the light

into electrical impulses. These variations in electrical current are

then amplified, as in the radio, by vacuum tubes, to a point where they

can operate electromagnets powerful enough to cause rather large

diaphragms to vibrate and produce the usual sound of the loud-speaker.

Overtones Are Vibrations of Greater Frequency than the Fundamental

Vibration.

The time required for one complete vibration is called a period, while

the number of complete vibrations (cycles) per second is called the

frequency of a vibrating system.

A string is capable of vibrating as a whole, thus producing the

fundamental tone, or it may also vibrate in segments, giving not only

the fundamental vibration but a series of higher tones. These higher

tones, produced by vibrations of greater frequency, are called overtones.

When the set of vibrations has frequencies in the ratios of the

natural numbers, 1 : 2; 3 : 4; etc., it is called a harmonic series. The

overtones of many vibrating bodies are not harmonic. For example,

the overtones in the xylophone are in the ratios 1 : 2.756 : 5.404, etc.

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