Imagine driving home along a road that has a surprisingly small amount of traffic. Suddenly, you hear the distant wail of a siren. Before you even turn your head to see, your mind has already registered that an ambulance is approaching your position from behind. The siren sounds louder and more shrill as the ambulance approaches your car, but then changes its pitch as soon as the ambulance overtakes you, almost like an opera singer can change the shrillness of her voice. You can’t help but notice the unique, shifting sound of the siren.
The Doppler effect is observed when the source of a particular set of waves is moving with respect to the observer. Suppose you are standing on the sidewalk, waiting to cross the street. On the far left side of your position, you see an ambulance racing in your direction. In this case, the siren on the ambulance is the source of the waves (sound waves, in this case) and you are the observer.
Apparent Increase in Wave Frequency
As the ambulance approaches you, the distance between the source of the waves and the observer decreases. Consequently, the siren sounds more shrill as the pitch of the wailing siren ‘sounds’ higher than its original value, as sound waves reach you ‘more frequently’. It should be noted that there is absolutely no difference between the frequency of the original sound waves and the sound waves approaching you; you simply perceive the sound to be higher pitched as the sound waves reach you at a more frequent rate, because the distance between you and the ambulance is closing.
