What is an apparent change in frequency of a sound source that moves relative to an observer?

The Doppler Effect. It's a change in frequency cause by the motion of the sound source, the motion of the listener, or both. As a source of sound approaches, observers hear a higher frequency. When the sound source moves away, observers hear a lower frequency. This effect was discovered by an Austrian scientist named Christian Doppler. Example: An ambulance siren. As the ambulance approaches a stationary observer, the frequency seems to increase. As the ambulance moves farther away, the loudness of the siren seems to decrease.