![]() ![]() Whereas then the ambulance goes past you and moves away, the sound waves are spread across an increasing distance thus reducing the frequency you hear so the pitch seems lower. As the police car travels towards you the number of waves is compressed into a decreasing distance, this increase in the frequency of sound waves that you hear causes the pitch to seem higher. It is all relative as the siren's frequency doesn't change. This apparent change in pitch to the observer is due to sound waves effectively bunching together or spreading out. We've all heard how a siren changes as a police car rushes past, with a high pitch siren upon approach, shifting to a lower pitch as the vehicle speeds away. The Doppler Effect was first described (opens in new tab) by Austrian physicist Christian Doppler in 1842 and many of us experience the Doppler effect firsthand almost every day without even realizing it. ![]() The concept of redshift and blueshift is closely related to the Doppler effect - which is an apparent shift in soundwave frequency for observers depending on whether the source is approaching or moving away from them, according to the educational website The Physics Classroom (opens in new tab). Redshift, blueshift and the Doppler effect ![]()
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