In the lab investigation you saw that the wavelength and frequency of waves traveling through the same medium are inversely related. This means that as one increases, the other decreases. So, if the wavelength of a wave changes, then the frequency will change. When dealing with sound, that means that the pitch of the wave will change.
When objects giving off waves are moving, a phenomenon known as the Doppler effect is observed. The Doppler effect is a change in sound frequency caused by the motion of the sound source, the motion of the listener, or both.
The Doppler effect is based on the motion of one object relative to another. As a source of sound approaches, an observer hears a higher frequency. When the sound source moves away, the observer hears a lower frequency.
For example, if a person is standing still and an ambulance is approaching with the siren going, the siren appears to have a higher pitch as it approaches the person and a lower pitch as it moves away from the person.
Study the slideshow to learn how the Doppler effect occurs.
As the siren on an ambulance emits sound, the sound waves are given off at a certain frequency in concentric circles around the ambulance. The sound waves will reach an observer at the same frequency that they are given off, and a certain pitch based on that frequency will be heard. 
When the ambulance starts to move toward an observer, each consecutive wave that is given off comes from a position closer to the observer, so the waves travel a shorter distance, and the frequency of the arrival of the waves is higher than the rate at which the waves are produced. This decreases the wavelength and increases the frequency the observer perceives. A higher frequency is heard by the observer as a higher pitch. 
When the ambulance is going away from an observer, the waves must travel farther to get to the observer, so they will have a longer wavelength and a lower frequency. This is perceived by the observer as a lower pitch. |
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Question
If the frequency of a fire engine’s siren is 1320 Hz, describe what happens to the apparent frequency and pitch of the siren as it approaches you and then moves away from you.
The frequency will be above 1320 Hz and have a higher pitch as the ambulance approaches. As it goes away, the siren will have a frequency below 1320 Hz and have a lower pitch.
