sentence: Frequency seems to decrease when someone moves away from a sound source.
sentences_before: When a sound source is moving towards someone, the frequency seems to increase. The same thing happens when someone moves toward the sound source.
sentences_after: It also seems to decrease when the sound source moves away from someone. This is the Doppler effect.
concept_name: frequency
headings: Sound Sound The Doppler Effect
categories: Basic English 850 words