Projection engineering (Jan-Dec 1931)

Page 26 PROJECTION ENGINEERING What is the Decibel? By Gordon S. Mitchell ONE of the most used, yet least understood terms in sound recording and reproduction is "decibel." Previous to 1923 the engineers of the Bell Telephone system used the expression "mile of standard cable" to designate a definite level of sound transmission ■ — being the amount of sound heard in a telephone receiver after the actuating currents had passed through one mile of standard telephone cable. In 1923, the term "transmission unit" was adopted merely as a temporary term until the matter had been considered by all who had occasion to use a unit of sound transmission. For very obvious reasons, a unit of universal definition was desirable, and with this idea in mind, the International Advisory Committee on Long Distance Telephony in Europe was asked to devise such a unit. There were two units proposed — one based on the decimal logarithm, and one based on e2, of the Naperian system of logarithms. Certain European countries, already using the Naperian unit, were reluctant to change. Engineers of the Bell System, invited to sit in on the conferences, made the suggestion that the basic power ratios of the two systems be standardized as 101 and e2, and that decimal multiples designated by the usual signs be used with either base. Numbers of the two systems corresponding to a given power ratio would differ by fourteen per cent. Inasmuch as the conference had reached a deadlock, it was finally recommended that the telephone systems of Europe use one or the other of the two approved systems of units, designating the one used by the name given it by the committee. The unit based on the Naperian system was given the name Neper, from the inventor of that system of logarithms, and the unit based on the decimal system was given the name Bel, from Alexander Graham Bell of telephone fame. Within a short time the Bell system in the United States adopted a unit which they called the decibel — this unit being one tenth of a bel and named in accordance with standard decimal nomenclature. The abbreviation db was adopted at the same time. The mathematical definition of the decibel is given by the following equation; S db = 10 log — So where S is the loudness of the sound under consideration and So is the loudness of a sound of identical pitch which is barely audible in acoustically dead surroundings. The loudness as expressed in the above formula is measured in any convenient unit of energy per cubic centimeter. To arrive at a common understandable definition for the decibel let us consider a sound of any given pitch which is just audible. The loudness of sound reproduced in the usual theatre is about one million times this just S audible sound. The ratio — ■ becomes So 1,000,000 ■ . The logarithm of 1,000,000, 1 as given by the table of natural logs is 6, which makes the loudness level of the sound originally under consideration 6 x 10 or 60 decibels. To convert back from the decibel level as read from the TU meter, a reverse process is used. Let us consider a level of 40 db. Dividing 40 by 10, a quotient of 4 is obtained. From the table of antilogs, we find that the number whose log is 4 is 10,000 — which means that the sound which plays on a level of 40 db. has ten thousand times the energy content that a barely audible sound in the same surroundings would have. The decibel, as evolved from the first "mile of standard cable" of the Bell Company, through the "transmission unit" as proposed to the International Advisory Committee, has been generally adopted as the standard unit of energy content for sound waves. The advent of talking pictures with recording of all types and varieties of sound has brought the decibel into common use by all connected with this kind of work. While originally intended for telephone usage, the decibel has taken its place in sound recording and talking picture reproduction and has become a term of common usage in the studio and the theatre. TECHNICAL ASSISTANCE CONTRACT FOR THE SOVIET CINEMA INDUSTRY "THE Amkino Corporation, which ' represents the Soyuzkino (United Motion Picture Industry of the Soviet Union), announced recently the conclusion of a contract with Joe W. Coffman, president of Audio-Cinema, Inc., whereby the latter will become consultant in connection with the development of sound films by the Soyuzkino. Mr. Coffman left a short time ago for the Soviet Union. L. Monosson, president of the Amkino Corporation, stated that the present contract is the first providing for American technical assistance in the Soviet motion picture industry. "The possibilities of sound motion pictures are keenly realized in the Soviet Union," said Mr. Monosson. "The motion picture is regarded as one of the most important means of educating the people of the country. At the same time the cinema occupies a high place among the arts. In order to further general cultural and, particularly, artistic development in the country, the Soviet authorities are anxious to apply the most advanced technical methods of sound motion picture production. It is likely that American technique may play a role in the Soviet motion-picture industry commensurate with that which it plays in other fields of the economic life of the U. S. S. R." The Amkino Corporation sells and distributes in the United States films produced by Soviet motion-picture companies. It purchases for shipment to the U. S. S. R. studio and theatre equipment, raw film, and other motionpicture machinery and materials, and also American pictures. Copies of films purchased here are printed in the United States on American or Canadian raw stock. Many of the Soviet films imported into the United States have won a distinguished place for themselves for artistic merit in respect to direction, act ing and photography. Industrial and educational pictures of scientific value are also imported. A number of Soviet directors have been invited by American producing companies to direct pictures in this country. One of the bestknown of these, Sergei Eisenstein, is at present connected with the Paramount-Publix Corporation in Hollywood. Since its organization in 1926 the Amkino Corporation has released fortytwo feature pictures and a number of news reels. Of the feature films one each was released in 1926 and 1927, eight in 1928, twenty-one last year, and this year through July, eleven. ▲ BOOK REVIEW PHOTOCELLS, oy V. K. Zworykm and E. D. Wilson, John Wiley & Sons, New York, 210 pp., illustrated. This new book contains much useful information on the design and operation of photocells as used in various industries.