Mark Garlick/Science Photo Library/Getty Images Plus When the car moves away, the sound waves get stretched out, creating a sound that is lower in pitch. We hear these shorter waves as a higher pitch. Sound waves from a moving police car get compressed as the car moves toward the listener. The bell’s pitch will rise higher and higher until the moment it passes by. But if you’re standing at a train crossing when the train approaches at full speed, you’ll hear something very different. If the train starts moving very slowly, you won’t notice much difference in the bell’s sound. In this case, the bell’s pitch doesn’t seem to change. Meanwhile, you are standing on the platform. To picture how this works, imagine that a train is clanging its bell while it waits at a station. This change in apparent wavelength due to the source or observer moving is the Doppler effect. Moving away from the source will make the waves appear stretched out. Moving toward the wave source will make its waves appear smooshed. The same effect is seen when an observer moves toward or away from a wave source that is standing still. Waves behind the source get stretched out. Waves in front of the source get smooshed. But when a wave source is moving, its speed affects those wavelengths. The wavelengths of those waves are the same in all directions. When a source of waves isn’t moving, its waves expand outward in a regular, circular pattern. This feature of a wave is not impacted by the Doppler effect.) Explainer: Understanding waves and wavelengths (The part of a wave which causes loudness is its amplitude, or how tall the wave is. For sound waves, the wavelength relates to pitch. That is, how far it is from the top of one wave to the top of the next. This is due to the Doppler effect, which describes how waves - such as sound waves - change when their source is moving relative to an observer.Īll waves can be described by their length. You’ll hear the pitch rise as it gets closer to you, and then fall as it passes. Next time you hear a train whistling its approach, or an ambulance driving by with its siren blaring, listen closely.
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