When you put a sea shell to your ear, how is it that you hear
a roar in the shell?
Why can you sometimes hear an echo and sometimes not?
If it were not for the fact that sound travels rather slowly, we
should have no echoes, for the sound would get back to us practically
at the instant we made it. An echo is merely a sound, a series of air
vibrations, bounced back to us by something at a distance. It takes
time for the vibration which we start to reach the wall or cliff that
bounces it back, and it takes as much more time for the returning
vibration to reach our ears. So you have plenty of time to finish your
shout before the sound bounces back again. The next experiment shows
pretty well how the waves, or vibrations, of sound are reflected; only
in the experiment we use waves of water because they go more slowly
and we can watch them.
EXPERIMENT 59. Fill the long laboratory sink (or the bathtub
at home) half full of water and start a wave from one end.
Watch it move along the side of the sink. Notice what happens
when it reaches the other end.
Air waves do the same thing; when they strike against a flat surface,
they bounce back like a rubber ball. If you are far enough away from
a flat wall or cliff, and shout, the sound (the air vibrations you
start) is reflected back to you and you hear the echo. But if you are
close to the walls, as in an empty room, the sound _reverberates_; it
bounces back and forth from one wall to the other so rapidly that no
distinct echo is heard, and there is merely a confusion of sound.
[Illustration: FIG. 102. When the wave reaches the end of the sink, it
is reflected back. Sound waves are reflected in the same way.]
When you drop a pebble in water, the ripples spread in all directions.
In the same way, when you make a sound in the open air, the air waves
spread in all directions. But when you shout through a megaphone the
air waves are all concentrated in one direction and consequently they
are much stronger in that direction. However, while the megaphone
intensifies sound, the echoing from the sides of the megaphone makes
the sound lose some of its distinctness.
WHY IT IS HARD TO UNDERSTAND A SPEAKER IN AN EMPTY HALL. A speaker can
be heard much more easily in a room full of people than in an empty
hall. The sound does not reflect well from the soft clothes of the
audience and the uneven surfaces of their bodies, just as a rubber
ball does not bounce well in sand. So the sound does not reverberate
as in an empty hall.
_APPLICATION 43._ Explain why a carpeted room is quieter than
one with a bare floor; why you shout through your hands when
you want to be heard at a distance.
INFERENCE EXERCISE
Explain the following:
261. It is harder to walk when you shuffle your feet.
262. The air over a lamp chimney, or over a register in a
furnace-heated house, is moving upward rapidly.
263. The shooting of a gun sounds much louder within a room
than it does outdoors.
264. A drum makes a loud, clear sound when the tightened head
is struck.
265. The pull of the moon causes the ocean tides.
266. Sand is sometimes put in the bottom of vases to keep them
from falling over.
267. It is difficult to understand clearly the words of one
who is speaking in an almost empty hall.
268. The ridges in a washboard help to clean the clothes that
are rubbed over them.
269. One kind of mechanical toy has a heavy lead wheel inside.
When you start this to whirling, the toy runs for a long time.
270. If you raise your finger slightly after touching the
surface of water, the water comes up with your finger.