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THE DECIBEL IN THE MOTION PICTURE INDUSTRY*
V. C. HALL**
Summary. — The development of the term "decibel" is outlined, and its convenience in measuring the characteristics of electrical circuits discussed. The relation of the decibel to photographic density is pointed out and illustrated by calculations of the effect of ground noise reduction devices. Finally, the values of acoustic power of common sources of sound are given as the levels in decibels based on various reference points in use in the motion picture industry.
The use of the term "decibel" has increased rapidly since its introduction into the motion picture industry because it is a convenient method of handling quantities which might otherwise lead to cumbersome expressions. Many publications have been written concerning the decibel and its historical development and its application to the problems under consideration. These papers have been freely drawn from for examples and data for this summary. No new material is presented but it is hoped that a review of some of the ways in which the decibel enters into sound motion pictures may prove of value to those who are not familiar with this unit.
Development of the Term. — The decibel (db.) is the name which was chosen for the transmission unit (TU), in terms of which a great deal of telephone and talking motion picture apparatus is calibrated. Their values are identical, and the name itself was suggested several years before its final adoption. Units of similar type had been universal in telegraphy and telephony for many years, and came into being from the fact that an electric current representing a certain amount of power loses a certain fraction of that power for every mile, let us say, of transmission line over which it travels. The unit of electric power, representing the rate of doing electric work, is the familiar watt, in terms of which most electrical equipment is specified. If, then, a power of ten watts is started out over a telegraph line, it would drop to 0.9 of this in perhaps two miles. This nine watts would drop to 0.9 of 9, or 0.81,
* Presented at the Spring, 1931, Meeting at Hollywood, Calif. ** Research Laboratories, Eastman Kodak Co., Rochester, N. Y. 292