Why does the frequency of a siren change as it moves past you?

The frequency of a siren changes as it moves past you due to the Doppler effect.

The Doppler effect is a phenomenon in physics that describes how the frequency of a wave changes based on the observer's viewpoint. It is named after the Austrian physicist Christian Doppler, who proposed this idea in the 19th century. When a source of sound, such as a siren, moves towards an observer, the waves are compressed, leading to a higher frequency or pitch. Conversely, when the source moves away from the observer, the waves are stretched, resulting in a lower frequency or pitch.

Imagine you are standing on the side of a road and an ambulance with its siren on is approaching you. As it gets closer, the sound waves from the siren are compressed because the ambulance is moving towards you. This compression of waves results in a higher frequency, which your ears perceive as a higher pitch. This is why the siren seems to get louder and higher in pitch as the ambulance approaches.

Once the ambulance passes you and starts moving away, the opposite happens. The sound waves from the siren are now being stretched because the ambulance is moving away from you. This stretching of waves results in a lower frequency, which your ears perceive as a lower pitch. This is why the siren seems to get quieter and lower in pitch as the ambulance moves away.

This change in frequency due to the relative motion of the source and the observer is known as the Doppler effect. It is not only applicable to sound waves but also to other types of waves like light and radio waves. For instance, astronomers use the Doppler effect to determine whether a star or galaxy is moving towards or away from us by analysing the change in frequency of the light it emits.

In summary, the change in frequency of a siren as it moves past you is a practical demonstration of the Doppler effect. It's a fundamental concept in wave physics, with wide-ranging applications from medical imaging to astronomy.