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MANAGERS AND PROJECTIONISTS 985
decrease to .0015 inches. Thus the width of the band on the sound track is a measure of the frequency of the sound recorded. The difference in shade between the light and dark bands is a measure of the intensity of the sound.
"As most bodies vibrate or tend to vibrate in harmonic motion, the sound waves gradually increase and decrease. Therefore, the striations usually are not sharp but gradually fade from one to another.
"This simple example of recording a single fixed frequency is seldom met in practice. The fundamental frequencies of the human voice vary from 60 to 1300 cycles and the overtones are as high as 10,000. Two musical tones from different instruments of equal intensity and frequency, but of different quality, will differ in the manner in which one striation fades into another. In orchestral recording many frequencies varying over the entire audible range of 20 to 10,000 cycles should be recorded at the same instant. The resulting *sound track showing all of the varying frequencies is extremely complex and shows practically no uniformity or regularity.
"The picture, Fig. 384, is that of a strip cut from the center of a sound track and magnified to 30 times its original size. Plotted vertically under it are the approximate corresponding percentage transmissions of total light. As stated during the course of the text, this graph with a variable vertical scale could represent:
(1) The vibration of the diaphragm of the microphone.
(2) The microphone current.
(3) The Aeo light voltage.
(4) The intensity of the Aeo light.
(5) The exposure of the negative.
(6) The transmission of the print.