Answer:
The Doppler effect
Step-by-step explanation:
As a fire truck with its siren blowing passes you, the sound of the siren will get lower and lower. This is because of the Doppler effect.
The Doppler effect is a phenomenon that occurs when a wave source is moving relative to an observer. When the wave source is moving towards the observer, the waves are compressed and the frequency of the wave increases, making the sound higher pitched. When the wave source is moving away from the observer, the waves are stretched out and the frequency of the wave decreases, making the sound lower pitched.
In the case of a fire truck with its siren blowing, the siren is the wave source and you are the observer. As the fire truck moves towards you, the sound of the siren will get higher pitched. As the fire truck moves away from you, the sound of the siren will get lower pitched.
The change in pitch of the siren is most noticeable as the fire truck passes you. This is because the rate of change of the frequency of the wave is greatest at this point.
Once the fire truck has passed you, the pitch of the siren will continue to decrease, but at a slower rate. This is because the rate of change of the frequency of the wave decreases as the fire truck gets further away.
The Doppler effect is a common phenomenon that can be observed in many different situations, such as when a car drives past you with its horn honking or when an airplane flies overhead.