Doppler Notes 1

The Doppler Effect is the apparent variation in frequency of any emitted wave, such as a wave of light or sound, as the source of the wave approaches or moves away, relative to an observer.

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.
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!

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.

IMPORTANT
Assume that a police car emits a siren at a frequency of 400Hz and the speed of the sound is 340m.s-1.

The speed of the sound would be 340m.s-1 whether the car is at rest or travelling at any speed.

The speed of the sound is INDEPENDENT of the speed of the car.
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!

The Doppler Equation

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

In Doppler 2 you will learn how to use this equation.

Easy Questions

1. What is meant by the Doppler Effect?
The Doppler Effect is the apparent variation in frequency of any emitted wave, such as a wave of light or sound, as the source of the wave approaches or moves away, relative to an observer.

2.1. Does the Doppler Effect occur only for sound waves?
No. The Doppler Effect occurs for all types of waves.

2.2. Does the Doppler Effect occur for transverse waves?
Yes. It occurs for both transverse waves and longitudinal 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.