Squeeze and stretch the air

The idea

A source moving forward compresses the medium into a compression, a region of higher than average density; moving back creates a rarefaction, a region of lower than average density; the two form alternately.

A piston in a tube

A long tube is filled with air. A piston at one end moves forward, then back, again and again.

What do you think happens to the air close to the piston?

• It is squeezed when the piston moves forward and stretched when it moves back
• Its density does not change
• It is squeezed both when the piston moves forward and when it moves back

Move the piston

Press Next step to see the air.

Higher and lower than average

When the piston moves forward the particles are crowded, so the density is higher than average. When it moves back, they are spread out, so the density is lower than average. The two regions follow each other.

A source moving forward makes a compression, a region of higher than average density. Moving back makes a rarefaction, a region of lower than average density. The two form alternately.

Notes

Forward: a compression (higher than average density). Back: a rarefaction (lower than average density). They form alternately.

Check yourself

What is a region of air with higher than average density called?

What does the piston make when it moves backwards?

In what order do compressions and rarefactions form as the piston keeps moving forward and back?

When the piston is not oscillating, how is the air in the tube?

• A rarefaction. A rarefaction is a region of lower than average density.
• A vacuum. A vacuum is a space with no matter. A region of higher density is a compression.
• A compression — correct. Yes!
• A rarefaction — correct. Yes!
• A compression. A compression is made when the piston moves forward.
• A vacuum. A vacuum is a space with no matter. Moving back leaves a region with lower than average density, a rarefaction.
• Compressions first, then rarefactions. They form alternately, because each forward move makes a compression and each backward move makes a rarefaction.
• Alternately, one after the other — correct. Yes!
• Compressions form but rarefactions do not. Each backward move of the piston makes a rarefaction, so rarefactions form too.
• It has a higher density near the piston. With the piston at rest the density is the same throughout, the average density. Higher density appears when the piston pushes forward.
• It has a lower density near the piston. With the piston at rest the density is the same throughout. Lower density appears when the piston moves back.
• It has a uniform density, called the average density — correct. Yes!