In air and room temperature: Speed of sound is constant: 340 m/s
What happens if the sound source moves?
The sound itself does not go faster!
Imagine going by while you are standing aside oberving. What do you hear?
When the sound source moves, the wave in front gets compressed.
===> the wavelength becomes smaller.
===> the frequency becomes higher.
When the sound source moves, the wave behind gets `extended".
===> the wavelength becomes larger.
===> the frequency becomes lower.
The phenomenon of the Doppler effect can be quite easily explained with wave front graphs. To get the quantitative change in frequency, we need to some math.
Here v is the speed of sound and v s
the speed of the souce.
The frequency emitted by the souce is given by f, and the frequency heard by the observer
is f'.
Here v is the speed of sound and v o
the speed of the observer.
The frequency emitted by the souce is given by f, and the frequency heard by the observer
is f'.
In the most general case the source and the observer are moving, either towards each other or away from each other.
Here v is the speed of sound, v s
the speed of the souce, and
v o
the speed of the observer
The frequency emitted by the souce is given by f, and the frequency heard by the observer
is f'.
Example how to determine the relative speed
Consider an ambulance driving along the road at 35 m/s. Two cars drive towards the ambulance on the opposite lane, one car parks in the lane of the ambulane and one car drives as fast as the ambulance.
Example problems for Doppler Effect
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