What is this name of the sound affect that Danny hears?

When a fire truck rushes past Danny, he hears the pitch of siren fall as it goes by. The effect Danny heard is called effect.

When a fire truck rushes past Danny, he hears the pitch of siren fall as it goes by. The effect Danny heard is called 'The Doppler effect'.

Explanation:

The Doppler effect is a very important phenomenon in physics, which is very interesting and important to astronomers. It is said that the frequency of a mechanical or electromagnetic wave has a different value observed by the receiver compared to the value emitted by the transmitter, if the source and receiver move in relation to each other.

If the receiver and transmitter move towards each other, the frequency moves higher (increases), and if the receiver and transmitter move away from each other, the frequency moves down (decreases).

For example, we can observe the Doppler effect on the highway: the noise the car engine makes as it approaches us is different from the one we hear as it moves away from us.