Final answer:
A siren on an ambulance sounds different as it approaches due to the Doppler effect, which causes a higher pitch as it approaches and a lower pitch as it moves away. The pitch changes continuously if the ambulance circles around you. The actual frequency of the siren matches your perception when the ambulance is directly in front of you.
Step-by-step explanation:
The siren on an ambulance sounds different as it approaches you due to the Doppler effect. As the ambulance comes towards you, the sound waves compress in front, leading to a higher frequency, and therefore, we perceive a higher pitch. Then, as the ambulance moves away, the sound waves stretch out, decreasing in frequency, causing a lower pitch. This shift in frequency is why a siren's sound changes as it moves relative to an observer.
If you are standing in the center of a circular road and an ambulance drives around with its siren blaring, the pitch of the siren will continuously change. It will increase in pitch as the ambulance approaches you and decrease as it moves away, repeatedly, until the ambulance has completed its loop around you.
When you perceive the frequency of the siren to be the same as the actual frequency, it will be when the ambulance is directly in front of you, neither moving towards nor away from you.