What makes a dictaphone or a phonograph repeat your words?
What makes the wind howl when it blows through the branches of
trees?
Why can you hear an approaching train better if you put your
ear to the rail?
If you were to land on the moon tonight, and had with you a tank
containing a supply of air which you could breathe (for there is no
air to speak of on the moon), you might _try_ to speak. But you would
find that you had lost your voice completely. You could not say a
word. You would open and close your mouth and not a sound would come.
Then you might try to make a noise by clapping your hands; but that
would not work. You could not make a sound. "Am I deaf and dumb?" you
might wonder.
The whole trouble would lie in the fact that the moon has practically
no air. And sound is usually a kind of motion of the air,--hundreds of
quick, sharp puffs in a second, so close together that we cannot feel
them with anything less sensitive than the tiny nerves in our ears.
If you can once realize the fact that sound is a series of quick,
sharp puffs of air, or to use a more scientific term, _vibrations_ of
air, it will be easy for you to understand most of the laws of sound.
A phonograph seems almost miraculous. Yet it works on an exceedingly
simple principle. When you talk, the breath passing out of your throat
makes the vocal cords vibrate. These and your tongue and lips make the
air in front of you vibrate.
When you talk into a dictaphone horn, the vibrating air causes the
needle at the small end of the horn to vibrate so that it traces a
wavy line in the soft wax of the cylinder as the cylinder turns. Then
when you run the needle over the line again it follows the identical
track made when you talked into the horn, and it vibrates back and
forth just as at first; this makes the air in the horn vibrate exactly
as when you talked into the horn, and you have the same sound.
All this goes back to the fundamental principle that sound is
vibrations of air; different kinds of sounds are simply different
kinds of vibrations. The next experiments will prove this.
EXPERIMENT 54. Turn the rotator rapidly, holding the corner of
a piece of stiff paper against the holes in the disk. As
you turn faster, does the sound become higher or lower? Keep
turning at a steady rate and move your paper from the inner
row of holes to the outer row and back again. Which row has
the most holes in it? Which makes the highest sound? Hold your
paper against the teeth at the edge of the disk. Is the pitch
higher or lower than before? Blow through a blowpipe against
the different rows of holes while the disk is being whirled.
As the holes make the air vibrate do you get any sound?
This experiment shows that by making the air vibrate you get a sound.
The next experiment will show that when you have sound you are getting
vibrations.
EXPERIMENT 55. Tap a tuning fork against the desk, then
hold the prongs lightly against your lips. Can you feel them
vibrate? Tap it again, and hold the fork close to your ear.
Can you hear the sound?
[Illustration: FIG. 96. An interesting experiment in sound.]
The experiment which follows will show that we usually must have air
to do the vibrating to carry the sound.
EXPERIMENT 56. Make a pad of not less than a dozen thicknesses
of soft cloth so that you can stand an alarm clock on it on
the plate of the air pump. The pad is to keep the vibrations
of the alarm from making the plate vibrate. A still better way
would be to set a tripod on the plate of the air pump and to
suspend the alarm clock from the tripod by a rubber band. Set
the alarm so that it will ring in 3 or 4 minutes, put it under
the bell jar, and pump out the air. Before the alarm goes off,
be sure that the air is almost completely pumped out of the
jar. Can you hear the bell ring? Distinguish between a dull
trilling sound caused by the jarring of the air pump when the
alarm is on, and the actual _ringing_ sound of the bell.
[Illustration: FIG. 97. When the air is pumped out of the jar, you
cannot hear the bell ring.]
The experiment just completed shows how we know there would be no
sound on the moon, since there is practically no air around it. The
next experiment will show you more about the way in which phonographs
work.
EXPERIMENT 57. Put a blank cylinder on the dictaphone, adjust
the recording (cutting) needle and diaphragm at the end of the
tube, start the motor, and talk into the dictaphone. Shut
off the motor, remove the cutting needle, and put on the
reproducing needle (the cutting needle, being sharp, would
spoil the cylinder). Start the reproducing needle where the
recording needle started, turn on the motor, and listen to
your own voice.
Notice that in the dictaphone the air waves of your voice are all
concentrated into a small space as they go down the tube. At the end
of the tube is a diaphragm, a flat disk which is elastic and vibrates
back and forth very easily. The air waves from your voice would not
vibrate the needle itself enough to make any record; but they vibrate
the diaphragm, and the needle, being fastened rigidly to it, vibrates
with it.
[Illustration: FIG. 98. Making a phonograph record on an old-fashioned
phonograph.]
In the same way, when the reproducing needle vibrates as it goes over
the track made by the cutting needle, it would make air vibrations too
slight for you to hear if it were not fastened to the diaphragm. When
the diaphragm vibrates with the needle, it makes a much larger surface
of air vibrate than the needle alone could. Then the tube, like an ear
trumpet, throws all the air vibrations in one direction, so that you
hear the sound easily.
EXPERIMENT 58. Put a clean white sheet of paper around the
recording drum, pasting the two ends together to hold it in
place. Put a small piece of gum camphor on a dish just under
the paper, light it, and turn the drum so that all parts will
be evenly smoked. Be sure to turn it rapidly enough to keep
the paper from being burned.
Melt a piece of glass over a burner and draw it out into a
thread. Break off about 8 inches of this glass thread and tie
it firmly with cotton thread to the edge of one prong of a
tuning fork. Clamp the top of the tuning fork firmly above
the smoked drum, adjusting it so that the point of the glass
thread rests on the smoked paper. Turn the handle slightly to
see if the glass is making a mark. If it is not, adjust it so
that it will. Now let some one turn the cylinder quickly and
steadily. While it is turning, tap the tuning fork on the
prong which has _not_ the glass thread fastened to it. The
glass point should trace a white, wavy line through the smoke
on the paper. If it does not, keep on trying, adjusting the
apparatus until the point makes a wavy line.
[Illustration: FIG. 99. A modern dictaphone.]
Making a record in this way is, on a large scale, almost exactly like
the making of a phonograph record. The smoked paper on which a tracing
can easily be made as it turns is like the soft wax cylinder. The
glass needle is like the recording needle of a phonograph. The chief
difference is that you have struck the tuning fork to make it and
the needle vibrate, instead of making it vibrate by air waves set in
motion by your talking. It is because these vibrations of the tuning
fork are more powerful and larger than are those of the recording
needle of a phonograph that you can see the record on the recording
drum, while you cannot see it clearly on the phonograph cylinder.
[Illustration: FIG. 100. How the apparatus is set up.]
In all ordinary circumstances, sound is the vibration of _air_. But
in swimming we can hear with our ears under water, and fishes hear
without any air. So, to be accurate, we should say that sound is
vibrations of any kind of matter. And the vibrations travel better in
most other kinds of matter than they do in air. Vibrations move rather
slowly in air, compared with the speed at which they travel in other
substances. It takes sound about 5 seconds to go a mile in air; in
other words, it would go 12 miles while an express train went one.
But it travels faster in water and still faster in anything hard like
steel. That is why you can hear the noise of an approaching train
better if you put your ear to the rail.
[Illustration: FIG. 101. When the tuning fork vibrates, the glass
needle makes a wavy line on the smoked paper on the drum.]
WHY WE SEE STEAM RISE BEFORE WE HEAR A WHISTLE BLOW. But even through
steel, sound does not travel with anything like the speed of light.
In the time that it takes sound to go a mile, light goes hundreds of
thousands of miles, easily coming all the way from the moon to the
earth. That is why we see the steam rise from the whistle of a train
or a boat before we hear the sound. The sound and the light start
together; but the light that shows us the steam is in our eyes almost
at the instant when the steam leaves the whistle; the sound lags
behind, and we hear it later.
_APPLICATION 42._ Explain why a bell rung in a vacuum makes
no noise; why the clicking of two stones under water sounds
louder if your head is under water, than the clicking of the
two stones in the air sounds if your head is in the air; why
you hear a buzzing sound when a bee or a fly comes near you;
how a phonograph can reproduce sounds.
INFERENCE EXERCISE
Explain the following:
251. The paint on woodwork blisters when hot.
252. You can screw a nut on a bolt very much tighter with a
wrench than with your fingers.
253. When a pipe is being repaired in the basement of a house,
you can hear a scraping noise in the faucets upstairs.
254. Sunsets are unusually red after volcanic eruptions.
255. Thunder shakes a house.
256. Shooting stars are really stones flying through space.
When they come near the earth, it pulls them swiftly down
through the air. Explain why they glow.
257. At night it is difficult to see out through a closed
window of a room in which a lamp is lighted.
258. When there is a breeze you cannot see clear reflections
in a lake.
259. Rubbing with coarse sandpaper makes rough wood smooth.
260. A bow is bent backward to make the arrow go forward.