What this means is that if a siren emits a sound at a frequency of 400Hz, you will hear the frequency of 400Hz ONLY IF you were at rest compared to the siren.
Therefore we hardly ever hear things at their correct frequencies! If everything was at rest relative to you, only then would you hear the correct frequency.
The speed of the sound would be 340m.s-1 whether the car is at rest or travelling at any speed.
If the siren was MOVING TOWARDS YOU, you would hear the frequency HIGHER than 400Hz!
If the siren was MOVING AWAY FROM YOU, you would hear a frequency LOWER than 400Hz!
This means that as the car gets faster, the sound doesn't!
Hence the car can "catch up" with the very sound that it itself is making!
This is why if the car approaches you, you hear a higher frequency.
Each successive pulse has to travel a shorter and shorter distance to reach you as the car approaches, creating a higher listening frequency!
fL is the apparant frequency, what you actually hear (Listener)
v is the speed of the sound
vL is the speed of the listener
vs is the speed of the source (the object making the sound)
fs is the actual frequency of the sound
2.1. Does the Doppler Effect occur only for sound waves?
No. The Doppler Effect occurs for all types of waves.
3. A police-car is moving towards you with its siren on.
3.1. Would you expect to hear a higher or a lower frequency of the siren sound? Explain.
Higher frequency. As the car approaches, each new sound has a shorter and shorter distance to travel, creating a higher frequency of pulses for the listener. Hence a higher frequency is heard.
3.2. Would the wavelength of the sound that you hear be greater or lesser than at the source?
The wavelength would be lower since the speed of the sound is constant, but the frequency is higher.