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THE DOPPLER EFFECT

After reading this section you will be able to do the following:

  • Observe the experiment below and discuss why you hear a difference when an object is moving, but the sound itself is not changing.

Questions

  1. If the noise the object makes is not changing, why do you hear a change?

Sound and motion

When we are moving, or a source producing a sound is moving, we hear things differently. You may have noticed that a train whistle gets lower as it passes you. The whistle is not changing pitch, but you are hearing a change. This principle is known as the Doppler effect. The Doppler effect is named after the Austrian physicist, Christian Johann Doppler, who discovered it.

What did Christian Johann Doppler discover?

Doppler claimed that if a sound is getting closer to you, either because its source is approaching you or because you are going towards the source, the sound will seem higher than it really is. If you are heading away from a source or it is going away from you, he believed the sound would seem lower than its actual pitch. To test his theory, scientists hired 15 trumpeters to play on a moving train. As the train passed by them, they heard a drop in pitch, just like Doppler predicted.

The Doppler effect happens because distance affects the amount of time it takes you to hear the sound. Imagine you are playing in the park and your friend rolls a ball to you. The ball would reach you sooner if you walked towards it and later if you moved away from it. The same is true for sound. Remember that frequency is wavelengths per time. If you hear a frequency in a shorter amount of time, it seems like you are hearing a higher frequency. For example, say you heard a sound that had 50 wavelengths by the time it reached you, it would have taken it 5 seconds to reach you. The frequency of that sound is 50 divided by 5, or 10 Hertz. Imagine you heard the same sound, but this time you were moving towards its source and it only took 2 seconds for 50 wavelengths to reach you. Now the frequency you hear is 50 divided by 2, or 25 Hertz. The frequency seemed higher because you were moving. If you were not moving, after 2 seconds, only 20 wavelengths would have reached you and the frequency would still sound like 10 Hertz.

The opposite happens when the distance between you and a source of sound widens. Now it takes longer for you to hear a certain amount of wavelengths. Therefore, the frequency seems lower. The Doppler effect makes a pitch appear to change when you, or the source, are in motion.