Doppler effect
The pitch you hear changes when source and listener move toward or away from each other.
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Analogy
An ambulance siren gets higher as it approaches and lower as it leaves.
Example
f' = f v / (v - vs)f' = 700 x 343 / 313f' = 767 Hz
In practice
A 700 Hz siren at 30 m/s coming toward you sounds like about 767 Hz.
Questions people ask
Does the Doppler effect work for light?
Yes. Light from a moving source is shifted in frequency. Moving away shifts it toward red, and approaching shifts it toward blue. Astronomers use the redshift of distant galaxies to show that the universe is expanding. The formula differs from sound because light needs no medium.
Why does pitch change with the Doppler effect but loudness does not directly?
Pitch depends on how many wave crests reach your ear each second. A moving source squeezes or stretches the crests, changing that rate. Loudness depends mainly on intensity, which depends on distance, so a siren gets louder as it nears for a separate reason.