Final answer:
The pitch or frequency of an ambulance siren increases as it approaches you and decreases as it moves away from you due to the Doppler effect.
Step-by-step explanation:
The pitch or frequency of an ambulance siren changes as it approaches you compared to its emitted pitch. This phenomenon is known as the Doppler effect. When the ambulance approaches you, the pitch of the siren will be higher than its emitted pitch, and as it moves away from you, the pitch will be lower than its emitted pitch.
This change in pitch occurs because the motion of the ambulance compresses or stretches the sound waves. As the ambulance moves towards you, the sound waves are compressed, resulting in a higher frequency or pitch. Conversely, as the ambulance moves away from you, the sound waves are stretched, resulting in a lower frequency or pitch.
For example, if an ambulance siren emits a sound wave at a frequency of 1000 Hz, when it approaches you, the frequency may increase to 1100 Hz. However, when it moves away from you, the frequency may decrease to 900 Hz.