The Science of Sound
Sound is a form of energy.
Try this test. Put your index and middle finger on your neck. Say the word "Aah" as loud as
you can. Then try it as soft as you can. You not only hear a sound, but you can feel a
movement inside your throat. When you say, "Aah", your vocal cords vibrate. That means
they move quickly back and forth. As your vocal cords vibrate they produce sound.
Sound waves
are compressional waves.
We demonstrated this wave
with a slinky. We stretched it
out between two students and
one student grabbed several
coils on the slinky, then let go.
You could observe the group
of coils go across to the other
student then come back to the
starting student. Did you
observe how some of the coils
stayed together? This is what
sound waves do.
A sound wave moves through matter, such as air, just like a wave
moves through a slinky.
We have drawn three pictures of a tuning fork to help you visualize
how air molecules might look around a tuning fork.
tuning fork #1
Look at tuning fork #1. When the tuning fork is at rest, the fork is
surrounded by molecules in the air.
As a tuning fork's prongs move apart because of a vibration, the
molecules ahead of it are crowded together. (See tuning fork #2). They
look like they are being pushed together. They bump each other.
tuning fork #2
As a tuning fork's prongs come back together, (see tuning fork #3) it
leaves a region that has fewer molecules than usual.
The region of a sound wave in which the molecules are crowded
together is a compression. The region of a sound wave in which
particles are spread apart is a rarefaction.
tuning fork #3
As a tuning fork vibrates, it causes molecules in the air to move. The
molecules bump into other molecules nearby, causing them to move.
This process continues from molecule to molecule. The result is a
series of compressions and rarefactions that make up sound waves.
When you spoke the word “Aah”, it is not the air you breathed that
traveled to someone else so they could hear it; it is the sound waves
that traveled. The particles of air simply moved back and forth.
Here are a couple of questions to think about. Write your own
response and then get ready to discuss with a partner.
Use the following diagram of a wave:
T
U
V
1. Which best demonstrates compression?
A. T, V and X
B. T, U and V
C. Only U
D. Only X
Describe why you chose this answer.
W
X
Y
2. There is an old question that goes like this: “If a tree falls in the forest and no one is
around to hear it, does it make a sound?” Ms. Stolz, Ms. Carelock, and Ms. Cole say “OF
COURSE!!!” Explain why they would say this. (It’s not just because they are geniuses!)
3. “Sound travels as mechanical waves, which need a medium.” We’ve been saying this
for weeks. Now consider the relationship between the movement of molecules and
production of sound waves in order to explain what this statement means.
How do you characterize sounds?
If you had to describe a specific sound to a friend, what words would you use? If you can't think of any words,
close your eyes and listen to the people around you talk. Are you able to tell who is talking without opening
your eyes? How? What are the differences between the sounds that different people make?
Perhaps you thought of describing a sound with the words loud or soft; high-pitched or low-pitched. These
words describe, or characterize, how we perceive sounds. Scientists, on the other hand, describe sounds with
characteristics that can be measured using instruments. We can relate characteristics that scientists measure to
the words we use to describe the sounds we hear. When we talk about loud or soft, scientists talk about the
amplitude of the sound. When we talk about the pitch of a sound, scientists use the word frequency.
Perceived Characteristic
Physical Characteristic
Loudness
Amplitude
Pitch
Frequency
Our ears are not sensitive to some of the characteristics that scientists use to describe sound. One important
characteristic that we cannot hear is the wavelength of the sound. However, you can see the wavelength of a
water wave. It is the distance from the top of one wave to the top of the next wave. You cannot see the
wavelength of a sound wave, but scientists can measure it using special instruments. Because sound travels in a
wave, we can relate the words that scientists use to describe sound to a picture of a wave.
This is probably what you think of when you imagine a wave. It is important to remember, though, that in a
sound wave, the particles are moving back and forth as if they were being pushed and pulled. They are not
moving up and down as this picture shows. However, we can use a picture like this to show the change in
pressure as a sound wave moves through. In the picture below you can see what the particles are doing - when
they are being squashed together (compressed under high pressure) - the wave goes up, and when they are being
pulled apart (expanded under low pressure) - the wave goes down.
The unit used to measure frequency is named Hertz, which is defined to be the number of cycles in one second. (This
unit is named after Heinrich Hertz, a famous 19th century physicist.) If you increase the frequency of sound (there are
more cycles in a second), you get a higher pitched sound. When you decrease the frequency, you get a lower pitched
sound. Amplitude, however, refers to the size of the sound wave. As the amplitude of the sound wave increases, the
intensity of the sound increases. Sounds with higher intensities are perceived to be louder. Relative sound intensities are
often given in units named decibels (dB).
One last question for you to think & write about before sharing with your partner….
4. Why would increasing the frequency of the wave create a higher pitched sound? (Hint, think about what was
happening to the medium during your Tuning Fork exploration you did in class).