A fire truck siren siren getting louder as it braces towards you and quieter as it moves away is called

As distance from the sound source increases, the area covered by the sound waves increases. The same amount of energy is spread over a greater area, so the intensity and loudness of the sound is less. This explains why even loud sounds fade away as you move farther from the source

Doppler effect.

Explanation:

Of course, to an observer on the truck, the pitch does not change at all.

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