According to the doppler effect, the sound of a police car siren is and pitched as it approaches the observer.

On the basis of the Doppler effect, the sound of a police car siren is louder and higher pitched as it move towards the observer.

Explanation:

Whenever the police car with a siren moves towards an individual, the frequency of the siren will increase and the reverse happens when the police car moves away from the person. The individual can listen to the sound of the siren louder and clearer when the car approaches the individual, as the sound is highly pitched when it moves towards the observer. This phenomenon is known as the Doppler effect.

explanation:

on disadvantage of hydroelectric power is the cost. another disadvantage of using hydroelectric power is finding a water source powerful enough to harvest hydroelectric power.