Movie Makers (Jan-Dec 1953)

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262 SOME ELEMENTS OF SOUND A movie making engineer discusses, in their simplest terms, the fundamentals of sound JOHN R. HEPELE, ACL WHAT is sound? Insofar as the amateur movie maker is concerned, it is a vibration cf the air which enters the ear and is transmitted to the brain, causing a sensation which humans call sound. Graphically speaking, the vibration travels through the air in very much the same manner that waves, initiated by a stone dropped into a pool of water, travel on the surface of the water, expanding at a certain rate in all directions from the source. In Fig. 1 a bell is the source of sound. When the clapper strikes the body of the bell, the bell is caused to vibrate. This vibration is passed on to the surrounding air which is alternately compressed and decompressed. These alternate compressions and decompressions of the air fall on the ear and we hear the ringing of the bell. The source might have been a violin or a trumpet, in which case you would hear a musical tone. Or, as a person speaks to you, his vocal cords are the sound source and you hear his voice. SOUNDS HAVE AMPLITUDE Loud sounds are like big waves on the water. In Fig. 2 the upper curve represents a fundamental sound of large amplitude, or a loud sound. Weak sounds are like small waves on the water. The lower curve represents the same sound, but with small amplitude or a weak sound. The amplitude of the wave, whether it be a water wave or a sound wave, is equal to the vertical distance between the crest and the trough of the wave. SOUNDS HAVE FREQUENCY Waves of low-pitched sounds are long and rolling, as shown in the solid curve of Fig. 3. There are wide spaces between the crests. High-pitched sounds produce waves in rapid succession with short spaces between the crests, as shown in the dotted curve. The number of complete waves passing a given point in one second is referred to as the frequency of the sound. In the illustration the lowpitched sound completes one wave or cycle in one second, whereas the high-pitched sound completes four waves or cycles in the same time. Audible sound frequencies cover a range from about 16 cycles per second to about 20,000 cycles per second, the range varying considerably with the hearing ability of individual ears. Orchestral music generally is a mixture of sound waves from about 30 cycles per second to about 18,000 per second, depending upon which instruments are used to produce the sounds. The frequency range of male speech is from 100 cycles per second to about 8,500 cycles per second and of female speech from about 160 cycles per second to about 10,000 cycles per second. SOUNDS HAVE OVERTONES Let us now suppose that we are listening successively to two sound waves of differing frequencies — one vibrating at about 1000 cycles per second, the other at about 2300 cycles per second. Our ears tell us immediately that the first souad is lower in pitch than the second. But they also tell us that both, of these frequencies are pure or funda ONE. SECOND LOW PITCH HIGH PITCH FiS. 3: Frequency, the number of complete waves passing a given point in 1 second, is a measure of sound's pitch. FIG. 1: Vibrations from the struck bell are carried by the oir to the ear to create in the brain a sensation of sound. CREST LOUD SOUND CRE.ST TROUGH TROUGH WEAK SOUND FIG. 2: Amplitude, which designates the relative loudness of any sound, is measured from wave's trough to its crest. FIG. 4: Overtones B and C, called second and third harmonies, combine with pure tone A to create the composite sound wave D.