International projectionist (Jan-Dec 1935)

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AND ITS USES w w A w E JL PROBABLY no technical term in sound motion picture work is so frequently used and so little understood by the non-technical man as is the unit by which sound intensity is measured — the decibel. To the technical men, accustomed to electrical units and mathematics, the term presents little difficulty, but to others it is apt to be confusing, because of their unfamiliarity with it. The word decibel (abbreviated "db") was formed by combining "deci," meaning one-tenth, with "bel," the fundamental unit named in honor of Dr. Alexander Bell, inventor of the telephone. It meets the need for a unit by means of which one amount of energy either in the form of electricity or sound, can be compared with another. In making such a comparison we could say, for instance, that one energy is twice as great, ten times, a thousand times, or a hundred million times as great as another. Such large numbers are obviously inconvenient to use. For instance, the energy of the loudest sound the human ear can tolerate is greater than the energy of the faintest sound the ear can detect by several million times. A more simple system of working with these large numbers is therefore desirable. The decibel furnishes such a system. Response of the Ear Before discussing the matter further, it will be helpful to consider briefly how the ear responds to sounds of different intensities. The reason for the peculiar ability of the ear to handle wide ranges of sound energy is that the impression of intensity is, fortunately, not directly proportional to the amount of sound energy reaching the ear. What is meant by this can perhaps best be illustrated by considering two glass water tumblers of about the same capacity, one the conventional cylindrical tumbler, the other shaped like a funnel or an inverted cone. If we fill these tumblers with water a spoonful at a time, the level of the water in the cylindrical one will rise by the same amount each time a spoonful is added. The level of the water in the funnel-shaped tumbler, however, will rise rapidly at first, but as it becomes more nearly filled the increase in level resulting from the addition of a spoonful can scarcely be noticed. The human ear responds to sound in much the same way as the water level in the funnel-shaped tumbler does to water. As the sound energy is increased in equal amounts, the added sensation of intensity (loud From I.P., July, 1933 ness) resulting from these increases becomes less and less. If, however, each time the sound energy is changed it is increased by the same percentage of its previous value, the result will be equal increases in the sensation, that is, it will appear to get louder in equal steps. This is indeed a wise provision of nature since it makes the ear sensitive to weak sounds and at the same time protects it from the loud sounds. It is important that this principle be constantly kept in mind, as it will be of considerable aid in obtaining a clear understanding of the subject. Figuring Power Ratios Without going into.the mathematics of the subject, let us examine the numbers in Table 1. It is very evident that there is a definite relation between these figures. Those in Column B obviously represent the number of times the number 10 must be multiplied by itself to give the larger figures shown in Column A. The figures in Column C are ten times the corresponding figures in Column B. Since there is this definite relation between Column B or C and Column A, evidently we can make unnecessary the handling of the large numbers shown in Column A if we use instead the figures in Column B or C. If now we consider Column A, B and C not as simple figures but as power ratios, bels and decibels, respectively, the relationship involved is apparent. By "power ratio" we mean the number of times the larger of the two powers being compared is greater than the smaller. Thus, if we are comparing a power of 2 watts with one of 20 watts, the power 20 ratio is — ■ == 10. 2 Aside from avoiding the handling of large numbers, there is another advantage, perhaps even more important, in using bels or decibels as in Columns B or C instead of power ratios as in Column A. In combining two power ratios, we must multiply or divide them, whereas the corresponding bels or decibels need only be added or subtracted. For example, if one amplifier in TABLE 1 Column A Column B Column C (Ratios) 1 (Bels) 0 (Decibels) 0 10 1 10 100 2 20 1,000 10,000 100,000 1.000,000 3 4 5 6 30 40 50 60 By D. C. Mc GALLIARD creases the sound energy 100 times and a second amplifier takes the output from the first one and increases it 10,000 times, the total increase has been 100 x 10,000, or 1,000,000 times. From the table we see that power ratios of 100, 10,000, and 1,000,000 correspond, respectively, to 20, 40, and 60 decibels. It is evident therefore that the total increase could have been figured much easier by simply adding the decibels corresponding to 100 and 10,000 (20 + 40 = 60) and then referring to Column A for the answer. Unfortunately, in actual practice power ratios do not often work out to round figures such as 100 or 10,000 as used in the example given; they are much more likely to be uneven figures, such as 96 or 9,585. The advantages of using decibels are usually much greater than would appear from the example given, and by their use we are enabled to greatly simplify many calculations which would otherwise be very tedious. There are two principal uses of the decibel : 1. To compare one sound intensity with another. For instance, if the energy of one sound is one hundred times as great as another, we say that the first sound is 2 bels, or 20 decibels, greater than the second. Thus, it the output of an amplifier is 6 watts, while the input is .06 watt, a 6 power ratio of , or 100, we say .06 that the amplification, or "gain" of the amplifier, is 20 decibels. 2. To measure the absolute value of sound energy by comparing it with some generally accepted standard energy value, either implied or expressed. For purposes of comparison, acoustic experts usually refer sound intensities to "minimum audibility" (or "threshold of hearing," as it is sometimes called) which may be defined as the weakest sound which can be heard under absolutely quiet conditions. The power of such a weak sound is unbelievably small, being of the order of only a ten thousand millionth of a microwatt (a microwatt is a millionth of a watt), — another indication of the sensitivity of the ear. Thus, when the acoustic engineer refers to a sound as having an intensity of 50 decibels, the statement is actually incomplete, it should be said that the intensity is "50 db. above minimum audibility," or "50 db. above threshold." "Minimum audibility" is much too small to be used as a reference intensity for relatively loud sounds, [11]