If a source of solid is moving, a stationary listener will hear a sound of different frequency. This is called?
Answer Details
The phenomenon you're referring to is known as the Doppler effect. Simply put, the Doppler effect is a change in the frequency of a wave (such as sound or light) as the source of the wave moves relative to an observer.
For example, if a car is approaching you, the sound of its horn will seem to have a higher frequency (or pitch) than if it were stationary or moving away from you. This is because as the car moves towards you, the sound waves are compressed, causing an increase in the frequency that you hear.
Conversely, if the car is moving away from you, the sound waves will be stretched out, causing a decrease in the frequency that you hear. This is why the sound of a siren on an ambulance moving away from you seems to get lower and lower as it gets further away.
So in short, the Doppler effect is a change in the frequency of sound (or any other wave) that occurs when the source of the wave is moving relative to an observer.