Doppler effect

Definition

Doppler effect in plain English

You've heard this without knowing its name: a siren sounds higher-pitched as the ambulance races toward you, then suddenly drops lower the instant it passes and speeds away. Sound travels as waves, and pitch is just how squished-together those waves are — squished (high pitch) or spread out (low pitch). When a source moves toward you it 'catches up' to its own waves and crams them together, so you hear a higher pitch; moving away, it stretches them out, so you hear a lower pitch. The formula f' = f × (v ± v_o)/(v mp v_s) just calculates the pitch you actually hear (f') from the pitch the source makes (f), where v is the speed of sound and v_o, v_s are how fast the observer and source move.

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