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SCIENCE
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PHYSICS
13 Sound
PHYSICS
3 Why is it that a bell
in a sealed bell-jar:
a can be heard
when the jar is
full of air?
b cannot be heard
when a vacuum
is created in the
jar?
Figure 13.6 These whales
communicate by soundwaves
Sound-waves can travel in a gas, a liquid or a solid, because they all contain
particles. When an object vibrates, it makes the particles next to it in the
gas, liquid or solid vibrate too. For example, when an object vibrates in air, it
pushes on the air particles around it.
Sound travels very well through a liquid. It moves faster and further than it
does in a gas. The humpback whale emits a series of sounds, called songs,
which travel thousands of kilometres through the ocean. It uses its songs to
communicate with other whales.
As the vibrating object moves towards the air particles it squashes them
together. The particles themselves are not compressed, they just come
closer together.
When sound travels through a solid, it moves even faster than through a
liquid because of the close interaction of the particles. However, the sound
does not travel as far. A snake detects vibrations in the ground with its lower
jaw-bone. The bone transmits the vibrations to the snake’s ears and allows the
snake to detect the footsteps of its prey.
As the object moves away from the air particles next to it, it gives the
particles more space, so they spread out. This movement of air particles
from a vibrating object can be modelled by using a slinky, as shown in
figure 13.5.
Reflection of sound
When light strikes a shiny surface, it is reflected. You may even see an
image of yourself reflected in a surface. You can find out what happens
when sound strikes a surface through the following simple experiment.
Are sound-waves reflected?
You will need:
this textbook and a ruler.
Hypothesis
Light-waves can be reflected so sound-waves should be reflected too.
One end is held firmly by the hand on the right and the other end is pushed
and pulled by the hand on the left.
Prediction
If a sound is reflected from a surface, you should be able to hear it.
to-and-fro vibration of the turns as the push-pull wave passes
Investigation
1 Close your book and hold it out about 20 cm in front of you, below the level of your mouth.
2 Keep the book below your head and say ‘ahhh’ for as long as you can.
3 Listen to the sound, then raise the book so that it is about 20 cm in front of your mouth.
wave direction
Figure 13.5
When using the slinky, which coils represent the air particles a) being close
together, b) spreading out?
2 What are the strengths and limitations of using the slinky as a model to
show how air particles pass a sound wave through the air?
CHALLENGE YOURSELF
Plan a modified investigation with the book, voice and ears to include a mobile
phone with a decibel meter app. If your teacher approves your plan, try it.
How will it confirm or contradict the evidence of the first investigation?
Figure 13.8 Sound cannot be heard through a vacuum
Sound-waves cannot pass through a vacuum because it does not contain
any particles. Figure 13.8 shows an experiment that demonstrates this. As
air is drawn out of the bell-jar with a pump, the sound of the bell becomes
quieter. When a vacuum is established in the bell-jar, the bell cannot be
heard, although the hammer can still be seen striking it.
Sound-waves are generated and travel in liquids and solids in the same way as
they do in gases. The particles in liquids and solids are held close together by
forces of attraction. In a liquid, however, the particles are further apart than in
a solid and can move around one another.
Figure 13.4 Producing sound by vibration
13 Sound
Scientists like to check their discoveries by performing different
investigations. The simple investigation using the ears to detect the
reflection of sound can be developed into a second investigation using
a sound detection meter which measures the energy in a sound-wave. A
cell phone can be converted into a sound meter by downloading a decibel
meter app. A decibel is a measure of sound energy and its symbol is dB.
Analysis and evaluation
Compare the sounds you hear when the book is below your mouth and in front of your mouth.
1
Conclusion
Compare your evaluation with your prediction and draw a conclusion.
Figure 13.7 This snake is
listening for vibrations in
the ground
4
Return to the contents page
5
6
Modelling sound reflection
You will need:
a slinky held firmly at one end by a friend. A second friend with a cell
phone camera (optional).
1 Stretch out the slinky between you and your friend.
2 Ask your friend to hold their end firmly, then push and pull the end you
are holding so that you set up waves as shown in Figure 13.5.
3 Look for waves reaching the end held firmly and setting up waves
back towards you.
4 If you find evidence that the model shows the reflection of sound, ask
another friend to film it.
Sometimes scientists make a model before they try an experiment. Here is
an example.
Modelling surfaces and sound reflection
The movement of sound energy can be modelled by using a tennis ball
(to represent an air particle). The surfaces can be provided by using a
metal tray and a cushion.
You will need:
a tennis ball, a metal tray and a cushion.
1 Ask a friend to stand about two metres away and hold up the tray.
2 Throw the ball at the tray and observe what happens to it.
3 Repeat steps 1 and 2 with the cushion instead of the tray.
What conclusion do you draw from using this model?
7
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