Movie Makers (Jan-Dec 1953)

Record Details:

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Diagrams by J. C. Vogel, ACL GINAI A 1 263 ORIGINAL SOUND WIRES AMPLIFIED MICROPHONE AMPLIFIER SPEAKER SOUND FIG. 5: Reproduction of sound, from mike thru speaker, is seen above. FRAME LbhfEj „ FRAME i£~ ! MEMBRANES FIG. 6: Sound waves, impinging on crystal elements of microphone, are transmuted by them into small electrical signals. mental tones. They are thus pure or fundamental in tone because they were generated by an electrical device called an oscillator. In actuality, very few of the sounds which we hear are fundamental tones. For example, when a key on the piano is struck, the tone we hear is a composite tone consisting of the fundamental with several harmonics or overtones superimposed on it. In Fig. 4 curve (A) represents the sound wave given off by a violin string vibrating as a whole. This is the fundamental sound wave. Curve (B) represents the sound wave of the string vibrating in halves. It shows what is called the second harmonic. Curve (C) represents the sound wave of the string vibrating in thirds. It shows what is called the third harmonic. Curve (D) is the composite sound wave which we would hear. It is obtained by adding or subtracting the vertical heights of waves (A) , (B) and (C) for each frequency as indicated at the position marked (X-X). The harmonics or overtones are weaker sound waves that accompany the basic or fundamental sound wave produced by any musical tone. They correspond to the little ripples on the surface of a big ocean wave. The process of adding, without limit, still finer wave forms to larger wave forms accounts for the ability of sound waves to transmit simultaneously each of the necessarily complex and overlapping frequencies of all sounds, from symphony orchestras to the pounding of ocean waves. In the complex of orchestral sound it is the harmonics or overtones which distinguish easily each particular instrument. THE RE-CREATION OF SOUND So much for the theory of sound. Let us now consider how the amateur movie maker uses sound to provide narrative, music and special effects for his pictures: to emphasize the action he sees on the screen; to set the tempo of the action and establish the mood of the pictures. To provide narrative and special effects which are not recorded on phonograph records he uses a microphone, -PERMANENT MAGNET CONE OR DIAPHRAGM FRAME FIG. 7: Amplified signals cause coil to move speaker cone, creating air waves "heard" by the ear as sound. PICK-UP RECORDING AMPLIFIER SPEAKER ^OUND0 FIG. 8: Reproduction of sound, from record to speaker, is seen here. c lamping pad cover protector s pring nose piece insulator x ^°usi n9^ //tor que wire JJIlil i nilf y =£ZP\ bearings leads crystal moisture proofed Stylus FIG. 9: Mechanical vibrations from record, picked up by needle, are borne by torque wire to crystal, where strain creates electric signal. an amplifier and a speaker in order to have them loud enough for an audience to hear. Fig. 5 shows how these are connected together. The original sound produced near the microphone sets up air waves which upon reaching the microphone are changed to electrical waves of the same form as the sound waves. In a crystal microphone, for instance, the electrical waves are transformed into voltages. These voltages, which are very small in magnitude, are connected to the amplifier. The amplifier builds up the small voltages to currents of sufficient strength to operate the speaker. These currents flowing through the speaker are converted to mechanical vibrations which compress and decompress the surrounding air to produce sound waves of the same wave form as the original. These sound waves reach your ears and you hear the narrative or sound effect, but with greater volume. ELEMENTS OF THE MICROPHONE Let us now consider separately each of the units in this sound system. There are many types of pickup microphones, but we will limit ourselves to the one most generally used, the crystal microphone. This microphone, as diagramed in Fig. 6, consists of two crystal elements assembled [Continued on page 270]