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).
3. 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 watch its variations,
we would find that pressure varied 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. You 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 perceptible, 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 a 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 are explained in more detail in Chapter XII.
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