International projectionist (Oct 1931-Sept 1933)

28 [NTERNATIONAL PROJECTIONIST February 1933 SERVICING SOUND EQUIPMENT Complete knowledge of projection and the necessary equipment is the projectionist's best insurance against unemployment. TROUBLE SHOOTING COMPLETE SOUND SYSTEMS AMPLIFIERS BATTERIES PH0T04ELECTRIC CELLS VACUUM TUBES PICK-UPS HORNS SPEAKERS UNITS FADERS GENERATORS MOTORS NON-SYNCHRONOUS EQUIPMENT, ETC. WHERE TO LOOK FOR TROUBLE HOW TO LOCATE TROUBLE WHY THE CAUSE OF TROUBLE Complete information on the Construction, Operation and Care of all sound systems. Sound Fundamentals in Review (Continued from page 24) WcfURES * '^'!?H00rERS MANUAL ICE7j !!?8"SHINGCa Sound Pictures and Trouble Shooters Manual by Cameron & Rider 1100 pages 500 illustrations Cameron Publishing Co. Woodmont, Conn. Here is my $7.50 send me by return a copy of SOUND PICTURES TROUBLE SHOOTERS MANUAL. NAME ... ADDRESS the trough of the wave. (See Figures 4 and 1.) Frequency. — Any one of a series of variations, starting at one condition and returning once to the same condition is called a "cycle." If we should fix our attention at some point on the surface of water in which waves exist, we would notice that at one particular point the water will rise and fall at regular intervals. At the time at which the wave is at its maximum height the water begins to drop, and continues until a trough is formed, when it rises again to its maximum height. Therefore, if we notice all the variations of height which one point on the surface of the water goes through in the formation of a wave, we will have witnessed a "cycle" of wave motion. The number of cycles a wave goes through in a definite interval of time is called "frequency." Therefore, the number of times the water rises, or falls, at any point in one minute would be called the frequency of the waves per minute, and we would express the frequency as a certain number of cycles per minute. In sound, the number of waves per minute is large, and it is more convenient to speak of the frequency of sound waves as the number of waves per second, or, more commonly, as the number of cycles per second. Thus, a sound which is produced by 256 waves a second is called a sound of a frequency of 256 cycles. When speaking of sound, "cycles" always mean "cyles per second." Considered from the standpoint of traveling waves, frequency is determined by the number of complete waves passing a certain point in one second, and this, of course, is equal to the number of vibrations per second generated at the source. In the same way, when a racer goes once around the race track and returns to the starting point, he completes a "lap," which, in this case, is just another name for a "cycle." Music seldom utilizes the full keyboard of the piano, the extremely high notes and extremely low notes being seldom used. Therefore a reproducing device which reproduces all frequencies from 50 to 4,000 cycles would be satisfactory in reproducing musical notes. However, there is another factor which enters into the consideration. This factor has to do with tone. Tone Quality. — The terms "quality" or "tone" of sound are used particularly with reference to music. A pure note of a given pitch always sounds the same and the frequency of this note is termed its "fundamental" or "pitch frequency"; but we are all familiar with the fact that notes of the same pitch from two different kinds of instruments do not give the same sound impression. This difference is due to the presence of overtones, sometimes called harmonics. Let us consider again the case of a taut string which is plucked to set it into vibration. If the string is plucked at its exact center, it will vibrate as a whole and give a very nearly pure note; but if it is plucked at some other point, say one-third of the length from one end, it will vibrate as three parts as well as a whole, and a change of tone will be noticed. If the string is plucked indiscriminately, various tones will be heard, all of the same pitch. Hollow cavities built into the bodies of the various musical instruments give them their characteristic tones, because the air chambers, called resonance chambers, strengthen overtones of certain frequencies and give a very pronounced tone to the instruments. Other instruments have built into them means of suppressing certain overtones, which help to give them their characteristic sounds. Character of Overtones The frequency of an overtone is always some multiple of the pitch frequency; that is, the second overtone has twice the frequency of the pitch note, and the third overtone, three times the frequency, etc. Overtones of twenty times the frequency of the pitch note are present in the sounds of some musical instruments, but overtones of this order are important only when the pitch note is low, because the frequency of the twentieth overtone of even a moderately high note would be beyond the ability of the human ear to detect. Overtones give character and brilliance to music, and their presence in reproduced sound is necessary if naturalness is to be attained. A reproducing device which reproduces frequencies from 50 cycles to 6,000 cycles will cover very well all the notes and overtones necessary for naturalness and distinctiveness. In singing, the range of notes covered is from 80 to 1,200 cycles, but this range cannot be covered by one person's voice. The frequency of 1,200 cycles does not represent the highest frequency used in ringing, because overtones of several times the frequency of the note are always present in the human voice. The presence of the overtone gives the pleasing quality to songs. This quality of the singing voice is called "timbre." The timbre of the voice transmits the emotions of joy, sadness, etc., from the performer to the audience, and therefore is very important in the enjoyment of vocal music. Wave-length.— Frequency in wave motion is related to wave-length. The \.y FIGURE 4 Properties of wave motion