Final answer:
The frequency of an ambulance's siren sounds higher as it approaches due to the Doppler effect, with the observed frequency matching the actual frequency when the ambulance is directly in front of you.
Step-by-step explanation:
The subject of the question is Physics, specifically relating to the Doppler effect. The frequency of the siren of an ambulance sounds higher than its frequency when the ambulance is next to you. This is because as the ambulance comes toward you, the sound waves get compressed, leading to an increased frequency. When it is in front of you, the observed frequency most closely matches the actual frequency of the siren because there is no relative motion along the line of sight. The principle behind this phenomenon is the Doppler effect, which also explains the characteristic shift in frequency for passing cars, airplanes, and trains.
When an ambulance passes you, there is a difference in frequencies you perceive before and after it passes due to the relative motion between you and the ambulance. The pitch of the siren changes as it circles around you, being higher as it approaches and lower as it moves away.