Answer:
It is called Doppler's effect of sound.
Step-by-step explanation:
When there is a relative motion between the source of sound and the observer, then there is some change in the frequency of sound heard by the observer. This phenomenon is called Doppler's effect of sound.
Let v be the velocity of sound, vs be the speed of source and vo be the speed of observer and f be the original frequency. Then the apparent frequency heard by the observer is given by
![f'=\left ( (v-v_(0))/(v-v_(s)) \right )* f](https://img.qammunity.org/2020/formulas/physics/middle-school/7y209x38bl2q2c62capkxynhn897zqq6m8.png)
here, the observer and the source both are moving in rightwards direction.