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FUNDAMENTALS OF SOUND
The probability that projectionists will shortly extend their activities to include the servicing of sound picture equipment has generated renewed interest in the fundamentals of sound recording and reproduction. Each month in this department will appear material which will serve this interest.
This month's installment is presented through the courtesy of RCA Photophone, Inc., and is copyrighted by them. Reproduction of any part or all of the following without permission is forbidden.
THERE are various definitions of sound. Therefore, at the start it is well to have an understanding as to the definition of the word as used in this and in the succeeding information presented. Sound can be considered as a series of vibrations of the air of such fequency, or pitch, that it is audible to the human ear. Sound is sometimes defined as an audio sensation in the ear. Such a definition requires a person to be located in the vibrating air before sound can exist, and is therefore inconvenient in a discussion of sound reproduction.
Production of Sound. — Sound is produced when air is set into vibration by any means whatsoever, provided that the frequency of vibration is such that it is audible, but sound is usually produced by some vibrating object which is in contact with the air. If we take a string, such as used on a banjo or similar instrument, stretch it taut between two fairly solid supports a few feet apart, and pluck it, sound is produced which dies down in a fairly short time. When the string is plucked it springs back into position, but due to its weight and speed, it goes beyond its normal position, oscillates back and forth through its normal position, and gradually comes to rest. As the string moves forward it pushes air before it and compresses it; also, air rushes in to fill the space left behind the moving string. In this way the air is set into vibration. Since air is an elastic medium, the disturbed portion transmits its motion to the surrounding air so that the disturbance is propagated in all directions from the source of disturbance.
SOURCE OF WAVES
If the string is connected in some way to a diaphragm, such as the stretched drumhead of a banjo, the motion is transmitted to the drum. The drum, having a large area exposed to the air, sets a greater volume of air in motion, and a much louder sound is produced.
If a light piston several inches in diameter, surrounded by a suitable baffle board several feet across, is set in rapid oscillating motion (vibration), by some external means, sound is produced. The air in front of the piston is compressed when it is driven forward, and the surrounding air expands to fill up the space left by the retreating piston when it is
drawn back. Thus we have a series of compressions and rarefactions (expansions), of the air as the piston is driven back and forth. Due to the elasticity of air, these areas of compression and rarefaction do not remain stationary but move outward in all directions. (See Figure 1).
Propagation of Sound. — If we could measure the atmospheric pressure at many points along a line in the direction in which the sound is moving, we would find that the pressure along the line at any one instant varied in a manner similar to that shown by the wavy line of Figure 1 ; or. if we set up a pressure gauge at one point and could catch its
variations, we would find that pressure at regular intervals and in equal amounts above and below the average atmospheric pressure. Of course, we could not actually see the variations of the gauge because of the high rate at which they occur.
We can see wave motion in water, however, which is very similar to sound waves, with the exception that water waves travel on a plane surface, while sound waves travel in all directions. We are all fairly well familiar with what happens when a pebble is dropped into a still pool. Starting at the point where the pebble is dropped, waves travel outward in concentric circles, becoming lower and lower as they get farther from the starting point, until they are so small as not to be perceptable, or until they strike some obstructing object. If the pond is small, it will be noticed that the waves which strike the shore will be reflected back. If the waves strike a shore that is parallel to the waves, they will be reflected back in expanding circles. If the waves strike the shore at an angle they will be reflected at an equal angle. (See Figure 2.)
If the waves strike a concave (hollow), shore line, the reflected waves will tend to converge (focus), to a point. (See Figure 3.) The solid lines show the direction of the original waves, and the dotted lines show the direction and focusing of the reflected waves. Focusing of waves results in their reinforcement, which may cause them to build up to considerable proportion at one point.
If you can picture the same kind of wave motion in air, with the exception that the air waves expand as concentric spheres instead of circles, you will have a fairly good picture of a sound wave as it travels through the air. Sound waves are reflected in a manner similar to water waves, causing echo and reverberation. If the sound waves focus at a point, loud and dead spots are produced. These terms will be explained in more detail subsequently.
Wave motion has certain definite characteristics; these characteristics determine the loudness, frequency (or pitch ) , and tone of the sound.
Loudness. — Loudness (or amplitude), is determined by the amount of difference in pressure between the maximum compression and the maximum rarefaction. This corresponds in water waves to the vertical height of the crest above
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WAVES FROM THE SOURCE
FIGURE 2 Reflection of waves from a plane surface
FIGURE 3 Reflection of waves from a curved surface