The wave moves, the air does not

The idea

A sound wave is a series of alternating compressions and rarefactions passed on through a medium by particle collisions; the particles only vibrate about their mean positions and never travel with the wave.

Sound crossing the room

Your friend calls out from the other side of the classroom. You hear her. Sound has travelled across the room.

Do you think the air particles that were near your friend travel across the room to your ear?

• No, each particle only moves to and fro about its own place
• Yes, they travel all the way to your ear
• Yes, but only the particles in the first row

Follow one particle

Turn on Mark one particle, then press Play. Watch the red particle and the compressions (C).

A disturbance, not a flow

The pushes are passed on by collisions. A compression moves forward because each particle bumps into the next. Each particle only goes to and fro about its own place.

A sound wave is a series of alternating compressions and rarefactions passed on through a medium by particle collisions. The particles only vibrate about their mean positions. They never travel with the wave.

Notes

A sound wave is a disturbance (alternating compressions and rarefactions) passed on by collisions. The particles only vibrate about their mean positions.

Check yourself

What is a sound wave?

A sound wave passes through the air. What does one air particle do?

How is a compression passed on through the air?

On a slinky, a mark is made on one turn. A disturbance travels along the slinky. What does the mark do?

• A series of alternating compressions and rarefactions moving through a medium — correct. Yes!
• A stream of air particles flowing from the source to the listener. The particles do not flow along. They only vibrate about their mean positions while the disturbance moves.
• A single compression with nothing after it. A sound wave is a series of alternating compressions and rarefactions, not a single region.
• It travels with the wave to the listener. The particles of the medium do not travel with the wave. They just vibrate about their mean positions.
• It vibrates about its mean position — correct. Yes!
• It stops moving until the wave has passed. As the wave passes, the particle moves to and fro about its mean position.
• The air moves forward like wind. The wave moves without the actual flow of the particles of the medium.
• The source throws the compression forward like a ball. A compression moves forward because the compressed particles collide with their neighbours, and so on.
• The compressed particles collide with the particles ahead of them — correct. Yes!
• It oscillates about its position of rest — correct. Yes!
• It travels along the slinky to the other end. A mark on a turn does not travel along the slinky. It only oscillates about its position of rest.
• It stays still. Each turn oscillates about its own position while the disturbance travels from one end to the other.