Slide 1
A passing sound changes pitch
A siren approaching a listener is usually heard at a higher frequency than the same siren moving away. This is the Doppler effect.
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The source does not need to change its own vibration rate. The listener receives wave crests at different intervals because the source and observer positions change while the disturbance travels.
Slide 2
The approaching source compresses spacing
As a sound source moves forward, each new wave crest begins closer to the preceding crest in front of it. The wavelength ahead becomes shorter.
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Behind the source, successive crests begin farther apart. In still air the wave speed is set mainly by the medium, so altered spacing changes the frequency received at a fixed location.
Slide 3
Observer motion also changes arrivals
A listener moving toward incoming sound encounters crests more frequently. Motion away reduces the rate of encounters.
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For sound, source and observer motion are measured relative to the air. Wind can therefore alter propagation relative to the ground, making the medium part of a complete Doppler calculation.
Slide 4
Pitch and loudness are different
Doppler shifting changes frequency. A siren becoming louder as it approaches mainly reflects a separate change in received intensity.
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A source may get louder without changing pitch, or change pitch without a large intensity change. Separating these observations helps identify which parts of a recording come from distance and which from motion.
Slide 5
Use the component along the line
The shift depends on motion toward or away along the source–listener direction. Sideways motion can produce a changing geometry as the source passes.
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The largest approach or recession shifts occur when motion is nearly along that line. Near the closest point of a straight pass, the radial component changes sign, producing the familiar transition in pitch.
Sources
- Sound Waves and the Physics of Music
The Physics Classroom
- Anatomy of an Electromagnetic Wave
NASA