The effect observed when a moving observer perceives changes in frequency is known as the Doppler effect. This phenomenon occurs when there is relative motion between a sound source and an observer, resulting in a change in the frequency of the sound waves. As the source approaches the observer, the frequency increases, causing a higher pitch. Conversely, as the source moves away, the frequency decreases, leading to a lower pitch. The Doppler effect is commonly observed in various contexts, including acoustic waves and light waves.