Sound and Sound Waves

Think about the sound of the siren on a fire truck as it approaches and passes you. What happens to the pitch of the siren? The Doppler effect occurs when the source of a sound is moving. As the source of a sound approaches the listener, the sound waves get pushed together. The frequency and pitch will seem higher to the observer. After the source passes the observer, the sound waves get pulled farther apart. The frequency and pitch will seem lower to the observer.