The Doppler effect occurs when the source or listener (or both) of sound waves is moving. If they're moving closer together, the listener perceives the sound with a higher pitch and, when they're moving apart, the listener perceives the sound with a lower pitch.
The Doppler effect occurs when the source or listener (or both) of sound waves is moving. If they're moving closer together, the listener perceives the sound with a __________ pitch and, when they're moving apart, the listener perceives the sound with a __________ pitch.