Sound waves

6.03

Explain sound waves:

When a loudspeaker cone vibrates, it moves forwards and backward very fast. This squashes the air in front. As a result, a series of compressions (‘squashes’) and rarefactions (‘stretches’) travel out through the air. These are sound waves. They reach your ears

What are sound waves caused by?

Vibrations. Any vibrating object can be a source of sound waves.

Are sound waves transverse or longitudinal waves?

Longitudinal.

Why are sound waves longitudinal?

The air oscillates backwards and forwards as the compressions and rarefactions pass through it. When a compressions passes, the air pressure rises. When a rarefaction passes, the pressure falls. The distance from one compressions to the next is the wavelength.

Can sound waves travel through a vacuum (completely empty space)?

No they cannot.

Why can’t sound waves travel through a vacuum?

Sound waves need a material to travel through. This material is called a medium. Without it, there is nothing to pass on any oscillations.

Most sound waves we hear have travelled through air. But can sound waves also travel through solids, liquids and gasses?

Yes they can. When swimming underwater, and walls, and windows, doors, and ceilings can all transmit (pass on) sound.

What is diffraction.

You can hear someone through an open window even if you cannot see them. That is because the sound waves spread through gaps, or bend round obstacles, of similar size to their wavelength. Most sound waves have wavelengths between a few centimetres and a few metres, so they are diffracted by everyday objects.

What is interference of sound waves?

When identical sets of sound waves overlap, they may reinforce each other or cancel each other out, depending on whether they are in phase (‘in step’) or out of phase.

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If two loudspeakers are giving our steady, identical sets of sound waves as above, and you walk along in front of them, you can find loud positions where the waves reinforce and quiet positions where they cancel.

Constructive interference:

Waves reinforce.

Destructive interference:

Waves cancel.

Displaying sounds with microphone and oscilloscope:

Sound waves can be displayed graphically using a microphone and an oscilloscope as on the right. When sound waves enter the microphone, they make a crystal or metal plate inside it vibrate. The vibrations are changes into electrical oscillations, and the oscilloscope uses these to make a spot oscillate up and down on the screen. It moves the spot steadily sideways at the same time, producing a wave shape called a waveform. The waveform is really a graph showing how the air pressure at the microphone varies with time. It is not a picture of the sound waves themselves: sound waves are not transverse (up-and-down) - they are longitudinal.