Have you ever heard an ambulance go by with its siren on? Have you watched a car race on TV and heard the roar of the engines as the cars travel past the microphones? If so, you may have heard the phenomenon known as the Doppler Effect. The sound that you hear in these examples changes as the object passes you. Click the play button below to hear the sound of a race car speeding past.
Consider an ambulance at rest with its siren blaring as depicted in the diagram below. For this example, we will assume that the sound from the siren is allowed to spread out equally in all directions. If the sound source (the ambulance) and the listener are both at rest, then the number of compressions passing by each second is the same, no matter where the listener is positioned.
This ambulance is not moving. Its siren is creating compressions and rarefactions, which move away at a consistent frequency. The frequency of the siren is the same no matter where the listener stands.
What happens when the ambulance begins to move as shown below? If the ambulance is moving to the left, it is essentially "catching up" to the compressions that its siren already emitted. This makes the compressions in front of the ambulance closer together. Alternatively, the compressions behind the ambulance are more spread out. Remember, the number of compressions heard each second is the frequency, or pitch of a sound. This makes the frequency, or pitch, of the sound heard by the listener higher as the ambulance approaches and lower after the ambulance has passed.
The ambulance is now moving to the left at a constant speed. Notice the difference in the spacing of the compressions in front of and behind the ambulance.
The Doppler Effect is defined as the apparent shift in frequency due to the relative motion of the sound source and the listener. Click the play button below to listen to the race car sound again. The shift in frequency due to the Doppler Effect is easy to detect.
Consider these questions:
What does the ambulance driver hear?
What if the ambulance stayed at rest, but the listener traveled past it?
Does the speed of the moving ambulance affect the frequency shift?