Sound and Light
Introduction

Sound and light both travel as waves. The properties of these waves differ quite considerably. Sound waves travel a million times slower than light waves. They have wavelengths between 1 centimetre and 10 metres, and will easily diffract round corners. Light waves have much smaller wavelengths, and only diffract through very small holes. This difference is the reason why you can often hear things that you cannot see.
Travelling sound
The bell in Fig.1 below has been placed in a bell jar. Press the button to activate the bell and again to remove the air from the jar. Watch what happens as the pump sucks the air out of the bell jar.

Figure 1.   A bell inside a bell jar. The blue circles represent the sound waves being emitted.
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Can you still see the bell when there is no air in the bell jar?
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Does this mean that light can travel through a vacuum?
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Can you still hear the bell?
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Which of these statements is true?
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Making sound
When you bang a cymbal, it vibrates. The air molecules around the tuning fork also start to vibrate. The vibration gets passed through the air as a vibrating sound wave.
Figure 2.   A sound wave moving through air.
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What kind of wave is sound?
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This helps explain why sound cannot travel through a vacuum. If there are no molecules to vibrate, then there will be no sound. Sound can only travel through a material. It will travel through air, through walls (and other solid objects), and through water and other liquids. On the other hand, a light wave is not made of vibrating particles. It is a wave of changing electric and magnetic fields which can exist in a vacuum.

How sound behaves
Echoes are made when sound bounces off a wall or a distant mountain.
What do we call this?
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Sound does not bounce off all materials, however. Sometimes it is absorbed. The absorption of sound must be considered when concert halls are designed. Without this consideration, the clothes of the audience would absorb most of the sound, and some of the audience would not hear anything. Snow also absorbs sound. This is why snowy winter nights always seem so quiet.

What is the term for when a wave bends around an obstacle or a corner?
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Can you see around corners?
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Can you hear around corners?
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The human ear is most sensitive to sounds with wavelengths in the range 5 to 20 cm. The
wavelength
The wavelength of a wave is the distance from one peak to the next, or from one trough to the next.
wavelength
of light is a lot shorter than this.

By comparing light and
sound waves
Sound is carried by a wave and needs a material to travel through. Sound waves are longitudinal and are produced by vibrating sources such as musical instruments.
sound waves
, which type of wave diffracts more?
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Volume and amplitude
Very loud music can shake your body. This is because the air molecules in the sound wave are moving very fast and have a lot of
energy
Energy is the capacity to do work. The SI unit of energy is the joule (J).
energy
. When they hit your body you can actually feel it. Your ears are very sensitive to sound, and loud music has enough energy to damage your hearing severely.

To measure sound in a laboratory, a sensitive microphone is connected to a cathode ray oscilloscope (CRO). The microphone turns the longitudinal sound wave into a voltage wave which changes with time. The CRO displays this changing voltage as a
transverse wave
In a transverse wave, the material moves at right angles to the direction of motion of the wave. Ripples on a pond are an example of transverse waves.
transverse wave
. The
amplitude
In general, the amplitude of a quantity is a measure of its size. When talking about waves, the amplitude of a wave is the maximum height of a wave measured from its rest position.
amplitude
of the wave on the CRO is proportional to the amplitude of the sound wave. They both have the same
frequency
In general, the frequency of an event describes how often it occurs. When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.
frequency
.


Click on the figure below to interact with the model.

 Figure 3.  Volume and amplitude.



Turn on the circuit in Fig.3 above. The graph has the same form as the line you would see on a CRO. You can alter the amplitude of the wave by moving the slider in the circuit.

Investigate how the amplitude of the sound wave is related to its volume.

Increasing the amplitude of a sound wave produces a …
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A sound wave with a larger amplitude carries more …
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Summary


Sound waves need a material to travel through.

Sound waves are longitudinal and are produced by vibrating sources.

The volume of a sound corresponds to the amplitude of the sound wave.

Sound waves diffract around corners.
Exercises
1. Decide whether sound will travel through each of the objects below.
  • A balloon filled with hot air
    A balloon filled with helium
    A vacuum
    A wooden pencil
    A metal fork
    A glass of water
    A pair of headphones
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2. Decide whether you could hear a sound or not in each of the following situations.
  • The propeller of a boat underwater
    A drum being struck in air
    A radio playing in deep space, which is a vacuum
    An astronaut speaking in a spacecraft
    A tree falling in a forest on a planet with no atmosphere
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3. Decide whether each of the statements below are true or false.
  • Light can travel through a vacuum.
    Sound can travel through a vacuum.
    Light can travel through air.
    Sound can travel through air.
    Light can travel through wood.
    Sound can travel through wood.
    Light waves are longitudinal.
    Sound waves are longitudinal.
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4. When you hear a police siren, what moves from the siren to your ears so that you can hear the sound?
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5. In which direction do the air molecules between you and the siren move?
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6. For a source to make a sound, what must it do?
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Figure 4.  
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7. A microphone is connected to a CRO. The output signal when a sound is made is shown in Fig.4 above. The volume of the sound is increased. Which picture below shows the signal in the oscilloscope now?
Figure 5.  
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8. A vibrating cymbal is placed inside a bell jar as shown in Fig.5. Initially there is air inside the bell jar. What can you hear outside the bell jar?
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9. The air inside the bell jar is slowly sucked out. What can you hear outside the bell jar?
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10. A sensitive microphone is connected to a CRO and placed outside the bell jar. Which of the diagrams below shows the display of the CRO as the air is sucked out of the bell jar?
11. Finally all the air is sucked out of the bell jar. What can you hear outside the bell jar now?
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Well done!
Try again!