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.

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.

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.

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.

Echoes are made when sound bounces off a wall or a distant mountain.

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.



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.

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.




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.

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.