Final answer:
The change in observed sound frequency due to the movement of the source relative to an observer is called the Doppler effect. As the source approaches, the sound frequency increases (higher pitch), and as it moves away, the frequency decreases (lower pitch).
Step-by-step explanation:
The phenomenon you are asking about is known as the Doppler effect, which describes the change in frequency of a wave in relation to an observer that is moving relative to the source of the wave. When a sound-producing source, like a police siren or a train whistle, moves towards you, the sound waves are compressed, leading to a higher sound frequency and a higher pitch. Conversely, as the source moves away from you, the sound waves are stretched, resulting in a lower frequency and a lower pitch. This is because the relative motion affects the intervals at which the sound waves reach the observer.
When a vehicle, such as an ambulance, drives past you, you will hear a noticeable shift from a high pitch to a lower pitch as it passes. The closer the vehicle passes, the more abrupt is the shift in pitch. The rate at which the vehicle moves also greatly affects the degree of this shift. If the source, like a jet airplane, approaches at speeds close to or exceeding the speed of sound, even more fascinating phenomena connected to the Doppler effect can occur.