How big is the density change?

The idea

The amplitude of a sound wave is the maximum change in density in a compression (or rarefaction) compared with the average density.

Two waves

Two sound waves pass through the same air. In one, the air at the crests is squeezed a lot more than in the other.

Which of the two do you think has the larger amplitude?

• The one with the bigger density change
• The one with the smaller density change
• They have the same amplitude

Change the squeeze

Use the slider to change the maximum change in density. The numbers are only examples.

Measured from the average

The amplitude is the biggest change in density in a compression (or a rarefaction), measured from the average density. On the graph it is the height of a crest above the dashed line.

The amplitude of a sound wave is the maximum change in density in a compression (or a rarefaction) compared with the average density. A larger change in density corresponds to a larger amplitude.

Notes

The amplitude is the maximum change in density (in a compression or rarefaction) compared with the average density. Larger change, larger amplitude.

Check yourself

What is the amplitude of a sound wave?

What does a larger change in density correspond to?

In one picture the crest of wave P is 3 divisions above the average density and the crest of wave Q is 1 division above it (an example). Which has the larger amplitude?

The amplitude is a change in density compared with which value?

• The maximum change in density, compared with the average density — correct. Yes!
• The distance between two consecutive crests. That is the wavelength. Amplitude is the maximum change in density compared with the average density.
• The number of oscillations in one second. That is the frequency. Amplitude is the maximum change in density.
• A smaller amplitude. A larger change in density corresponds to a larger amplitude.
• A larger amplitude — correct. Yes!
• A longer wavelength. Wavelength is the distance between consecutive crests. A larger change in density means a larger amplitude.
• Wave Q. Wave Q has the smaller change in density, so the smaller amplitude.
• They have equal amplitudes. Their maximum changes in density are different, 3 and 1, so their amplitudes differ.
• Wave P — correct. Yes!
• The average density — correct. Yes!
• The maximum density of the wave. The amplitude is the change compared with the average density, not with the maximum density.
• Zero density. The change is compared with the average density of the medium.