Worlds of David Darling > Children's
Encyclopedia of Science > Sounds Interesting > Chapter 1
The Science of Acoustics
a book in the eXperiment! series by David Darling
1. Good Vibrations
The world is full of sound – a dog's bark, the wail of a police siren,
the chatter of students in a classroom. Even when everything outside is
quiet, you can still hear your own breathing and the steady beating of your
| The sounds produced during a thunderstorm
are among the loudest in nature
Sound is one of the most important ways we have of sensing our surroundings
and communicating with other people. Sometimes it can be a nuisance, as
when an airplane or a car roars by. Then we call it noise. On the other
hand, many sounds, such as those of musical instruments, are extremely pleasant.
Sounds come in every variety imaginable. But what exactly is sound? How
is it made and how does it travel?
Sound is a form of energy. This fact becomes most clear when a loud sound
causes something nearby to rattle or move about. For example, if a television
set or a record player is turned up high, the sound from it may cause small
objects in the room to shake.
Sound energy is produced by anything that moves back and forth. A mosquito's
wings, for instance, beat back and forth rapidly. This causes similar back-and-forth
movements, or vibrations, in the surrounding air. When the air vibrations
reach our ears we hear the mosquito's familiar hum.
A "Sound Gun"
You will need:
- A cardboard tube
- Plastic wrap
- A nail
- A birthday-cake candle
- A dish
- Some sand
What to do:
Cover both ends of the tube with plastic wrap. Make sure that the
plastic is stretched tight. Use the nail to make a small hole in the
middle of the plastic at one end. Put some sand in the dish and stand
the candle upright in the sand. Light the candle. Hold the "sound
gun" so that the hole is about 1" from the candle flame. Tap the plastic
at the other end of the tube with your finger. What do you hear? What
happens to the flame? Try to explain your observations.
Warning: Always be careful when using matches. If in doubt ask an
adult for help.
You will need:
- A Slinky toy
- A broom handle or other long round pole
- Two chairs
- A long jump rope
What to do:
Thread the coils of the Slinky on to the broom handle. Rest the broom
handle across the backs of the two chairs, as shown. Hold the Slinky
by both ends and pull it apart, without stretching it too far. Jerk
one end slightly. Observe how the coils of the Slinky move. Jerk one
end hard, then more gently. Notice what happens in each case.
Hold one end of the jump rope and ask a friend to hold the other end.
Shake your end of the rope once. Shake it hard and then more gently.
What do you observe?
The Slinky and the jump rope behave like a length of air. When you
jerk or shake one end, you set up a vibration. The way the Slinky
and jump rope move is similar to the way air moves when an object
Pushes and Pulls in the Air
The air next to a vibrating object is alternately squeezed together and
pulled apart as the object moves back and forth. This causes the air farther
away to be squeezed together and pulled apart, too.
| A vibrating string pushes
and pulls on the air around it
You can picture how this works by thinking about a long line of people,
in which each person is holding on to the shoulders of his or her neighbor
in front. If the person at the very back suddenly pushes forward, a wave
of pushes travels down the line. If the same person pulls back, a wave of
pulls is set in motion. The bigger the starting push or pull, the bigger
the wave that spreads out. In the same way, an object makes a louder sound
if its vibrations are large than if they are small.
Sound and Matter
Sound waves do not only travel through air. In water, sound can be heard
over great distances. Whales, for example, seem to be able to communicate
with each other underwater even if they are tens of hundreds of miles apart.
Sound can only pass easily through hard materials such as metal, stone,
bone, wood, and glass. Soft substances, however, including fabric and rubber,
quickly absorb sound waves. They can be used to deaden the noise of a car
engine or prevent echoes in a concert hall.
The one thing that no sound can travel through is empty space. So, when
astronauts visited the moon they had to communicate using radios. Only by
touching helmets would they have been able to talk to ne another directly.
This would have allowed the vibrations of their voices to pass through the
material of their helmets.
Sometimes you will see movies in which spacecraft make noises when their
engines are fired or they are hit by the enemy. But, in truth, all such
action would be completely silent from outside.
Something to Travel Through
You will need:
|What do you hear when a friend
taps a wall some distance away?
|What do you hear through your
cup and string "telephone"?
- A stone
- A tuning fork
- Two plastic cups
- Fine string
- A tape measure
- Coarse string
- Fishing line or nylon thread
- A portable tape recorder
What to do:
Stand with your ear next to brick wall. Ask a friend to tap on the
wall some distance away. Can you hear anything? Can you hear the tapping
if your ear is not pressed to the wall? If so, does it sound the same?
Strike the tuning fork and hold it just in front of your chin. Listen.
Strike it again and press the rounded end of the tuning fork against
your chin. Listen once more. Is the sound louder, quieter, or about
the same? Try to explain your results by thinking about how the vibrations
of the tuning fork are carried to your ear.
The next time you are in a swimming pool or a bath, lie back so that
your ears are underwater. Can you hear anything? If so, how is that
Make a hole in the bottom of each plastic cup. Cut off a length of
about 30 feet of fine string. Push one end of the string through the
hole in one cup and the other end through the hole in the other cup.
Knot the string to stop it from coming out of the holes. Stand facing
a friend and pull the string tight. Hold your cup to your ear while
your friend talks slowly and clearly into the other cup. What do you
hear? What happens if you let the string go slack?
Taking it further:
Record a passage from a book on the tape recorder. Hold your cup to
your ear while your friend holds the other cup to the loud speaker
of a tape recorder. Again. keep the string tightly stretched. Repeat
the experiment using a 30-foot length of fishing line instead of fine
string. Does this affect the quality of what you hear? Can you explain
your results? Repeat the experiment using coarse string. Again, describe
When sound waves strike a hard, solid object, such as the side of a building,
they bounce back. That is, they are reflected. A softer surface, however,
soaks up most of the energy in the sound waves and reflects very little.
This explains why sounds in a bare room appear very loud, but in a room
with carpeting, curtains, and soft furnishings, sounds are more muffled.
|Locating fish with sonar
A sound that bounces back is called an ECHO. If you stand a long distance
away from a large wall or a steep mountain and shout, your calls will return
to you a short time later. The farther you are from the reflecting surface,
the longer is the gap between making the call and hearing the echo. Why
do you think this should be?
Ships and submarines use beams of sound to measure the depth of water, to
chart the seabed, and to find shoals of fish. The system is called SONAR,
which stands for sound navigation and ranging. A device under the hull sends
out regular pulses of sound. These bounce off the seabed or off shoals of
fish and return to the ship. After the echoes have been picked up, they
are turned into a picture on a screen.
Bouncing Sounds Around
You will need:
- Two long cardboard tubes
- A ticking watch
- A wall
What to do:
Ask a friend to hold one of the tubes at an angle to the wall with
the ticking watch at the other end of the tube. Hold the other tube
to your ear and aim it at the same spot on the wall as your friendís
tube is pointing. Can you hear anything? If not, alter the angle your
tube makes with the wall (still pointing at the same spot). Is there
anything special about the position at which you can hear the watch?
What does this tell you about what is happening to the sound waves
when they hit the wall?