Sound needs a medium

Can sound travel through nothing?

Put an electric bell inside a glass jar and switch it on: you hear it ring. Now pump the air out slowly. The sound grows fainter and fainter, although you can still see the hammer striking. Let the air back in, and the sound returns.

So sound needs something to travel through: a medium. It travels through air, water and solids, but not through a vacuum. On the Moon, which has no air, astronauts talk by radio.

What travels?

The particles of the medium do not travel with the sound. Each one moves a little to and fro and pushes on its neighbour. It is the disturbance that travels, the way a push passes along a line of people.

Compressions and rarefactions

As the vibration pushes forward, it crowds the particles together: a compression, where the pressure is high. As it pulls back, it spreads them apart: a rarefaction, where the pressure is low. A sound wave is a train of compressions and rarefactions.

The distance from one compression to the next is one wavelength, $\lambda$ (the Greek letter lambda).

A longitudinal wave

In sound, the particles vibrate along the direction the wave travels. Such a wave is longitudinal. In a transverse wave, like a wave on a rope, the particles move at right angles to the direction of travel.

You should find that the red particle swings back and forth about one place, while the crowded regions move steadily to the right. The pressure curve is highest wherever the particles are bunched up.

Watch out. A compression to the next rarefaction is only half a wavelength.

A worked example

A loudspeaker makes a sound whose compressions are $0.68\ \text{m}$ apart. How far is a compression from the next rarefaction, and how many compressions fit in a corridor $6.8\ \text{m}$ long?

1. Wavelength. Compressions are one wavelength apart: $\lambda = 0.68\ \text{m}$.
2. Half a wave. Compression to rarefaction: $\dfrac{0.68\ \text{m}}{2} = 0.34\ \text{m}$.
3. In the corridor. $\dfrac{6.8\ \text{m}}{0.68\ \text{m}} = 10$ compressions.

Practise with new numbers

Try it in Python

The program prints a row of particles, crowded and spread out like a sound wave.

Remember these

• Sound needs a medium; it cannot travel through a vacuum.
• A sound wave is a train of compressions (high pressure) and rarefactions (low pressure).
• Sound is longitudinal: the particles vibrate along the direction of travel.
• Compression to compression is one wavelength.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why can two astronauts on the Moon not talk directly, without a radio?

2. In a compression, is the pressure high or low?

3. Is a wave on a stretched rope longitudinal or transverse? And sound?

4. In a sound wave, a compression is $0.3\ \text{m}$ from the next rarefaction. What is the wavelength?