International projectionist (Oct 1931-Sept 1933)

18 INTERNATIONAL PROJECTIONIST JulyAugust 1933 to 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, if the output of an amplifier is 6 watts, while the input is .06 watt, 6 a 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 to some generally accepted standard energy value, either implied or expressed. For comparison purposes, acoustic experts usually refer sound intensities to "minimum audibility." Minimum audibility," or "threshold of hearing," as it is sometimes called, 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 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, such as those coming from the ordinary loudspeaker. Another reference intensity generally known as "zero level" has, therefore, been generally adopted by communication and sound engineers. An idea of the intensity of sound at "zeio level" may be had if it is remembered that speech from a telephone receiver held tightly against the ear is about zero level when it is just too loud to be comfortable. Engineers have agreed that, expressed in electrical power, this "zero level" should be taken as .006 watt. Thus, compared to "zero level" of .006 watt, the 6-watt output of the amplifier mentioned under (1) above would correspond to a power ratio of 6 or a ratio of 1,000. From the .006 table it is seen that this ratio represents 30 decibels. Hence it is said that the output level of the amplifier is "30 decibels above zero level" or, simply, "30 decibels." Just as room temperature is commonly expressed as, say, "70 degrees" without specifying that it is "70 degrees al>ove zero," sound engineers commonly refer to the output of an amplifier as being, say, "30 decibels" without specifying that they mean "30 decibels above zero level." "Output Level" and "Gain" Care should be taken not to confuse the "output level" of the amplifier with the "gain" of the amplifier referred to under (1) above. Each is commonly expressed in decibels, although, as explained, the output level should, strictly speaking, be expressed as "decibels above zero level." Thus, while the out-put level of this amplifier is 30 decibels (above zero level), its gain is only 20 decibels, as figured above. This will be more readily understood when it is considered that the input (.06 watt) is already 10 decibels above zero level. This is figured from the power ratio .06 watt .06 of or or 1 0 which, from zero level .006 the table, corresponds to 10 decibels. If the input of 10 decibels above zero level is increased by the 20decibel gain of the amplifier, the output level will, of course, be 30 decibels above zero level. This is also in accordance with what we said earlier about the addition and subtraction of decibels. The table given below shows the approximate power ratios corresponding to from 1 to 30 decibels. There are two convenient relations to remember in dealing with decibels. The first of these is that ten decibels correspond exactly to a tenfold change in power. The second relation to be remembered is that in order to make a sound louder by 3 decibels it is necessary to approximately double its power. What this means can be more easily pictured when you consider that if the Niagara Falls power plant output were in the form of sound energy, it would be necessary to construct another power plant of the same capacity in order to increase the sound output by 3 decibels. In the same way for 6 decibels increase, the power would have to be about four times as great, for 9 decibels about eight times as great, and so on. )wer Ratios Decil 1.25 1 1.6 2 2.0 3 2.5 4 3.2 5 4.0 6 5.0 7 6.3 8 8. 9 10. 10 100. 20 1,000. 30 One cannot learn to drive an automobile by reading an instruction book. The same is true in studying the use of the decibel; it is necessary to actually apply the principles outlined to specific problems before they will be thoroughly understood. It is suggested, therefore, that the reader take advantage of any opportunities which may arise to make practical applications of the tables and explanations given. A little practice along these lines will convince him that the decibel is an extremely simple and useful unit. Interesting Acoustic Problems Involved in Unique Outdoor Theatre Installation COME very interesting acoustic prob^ lems were involved in the recent installation by RCA "Victor Co. of Photophone apparatus in the new DrivIn Theatre (open air) in Camden, N. J. The analysis of the problem involved included a study of the number and location of loudspeakers to give not only a uniform distribution or coverage of sound over the entire parking area but the proper illusion of sound as coming from the screen; also, a study of the amount of power or size of equipment required to give an adequate loudness of sound within the automobiles and to override the general noise level due to nearby auto traffic and wind. Pull low-frequency response for giving richness to music is doubly important in outdoor installations because the reverberation which usually builds up the low-frequency response of indoor installations is entirely absent outdoors. For this reason and because of its narrow distribution angle (33° arc) the directional baffle loudspeaker was chosen for the Drive-In Theatre. The intensity of sound from a loudspeaker is greatest along its axis and the intensity diminishes for points removed from the axis, that is, for increasing azimuths. In the horizontal plane a uniform distribution of sound is obtained from three loudspeakers, because in passing across the field of sound from one loudspeaker axis to the next, the increasing intensity of the one compensates for the decreasing intensity of the other. In the vertical plane, a fairly uniform distribution of sound is obtained because the increasing intensity due to decreasing the distance from the loudspeaker compensates for the diminishing intensity due to the increasing azimuths. The narrow distribution angle of the directional baffle loudspeaker is of particular benefit in outdoor installations in reducing the amount of power