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 pitch of the siren of an approaching ambulance changes as it approaches you. This is an example of which of the following?
sound radiation |
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Doppler effect |
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reflection |
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refraction |