Final answer:
The Doppler effect explains how the pitch of a sound changes as a source moves relative to an observer. Sound from a fire truck's siren is perceived as higher pitch when approaching a stationary listener and lower pitch when moving away.
Explanation:
The Doppler effect in sound waves describes how the pitch of a sound changes when a source of sound is in motion relative to an observer. In the case of a fire truck's siren moving toward a stationary hearer, the quality of the sound is perceived as higher pitched than its actual frequency. As the fire truck moves away from the stationary hearer, the perceived quality of the sound becomes lower pitched than the actual frequency.
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