The Doppler effect describes how the frequency of a wave is altered by the motion of the source or receiver. There are three cases: a stationary source with a moving receiver, a stationary receiver with a moving source, and both moving. If the source and receiver move towards each other, frequency increases, and if they move apart frequency decreases. This is demonstrated by the Doppler effect equation. For example, an emergency vehicle's siren appears higher in pitch as it approaches a stationary listener due to the source moving towards the receiver.

1. The Doppler Effect
The Concept Explained!

2. ● The phenomena that the wave
frequency changes when the
distance between a source of
sound and a receiver of sound:
o If the objects are coming
closer to one another, the
frequency increases
o If they are moving farther
away from each other, the
frequency decreases.
Introduction
http://www.redorbit.com/media/upl
oads/2004/10/6_2794e0fcae75d8
49e10a80c3137aa8bc2.jpg

3. There are three cases related to the
doppler effect that can be modeled by a
simple equation.
● stationary source + moving receiver
● stationary receiver + moving source
● moving source + moving receiver
Cases of Doppler Effect

4. The equation used to model these 3 cases:
Equation
Vr = velocity of the
receiver
Vs = the velocity
of the source
fr = the frequency
of the receiver
fs = the frequency
of the source
Note the ± and ∓ in the numerator and denominator of the equation.
● Use the top sign of the numerator if the receiver is moving towards
the source. There is an observed increase in frequency
● Use the top sign of the denominator if the source is moving towards
the receiver. There is also an increase in frequency observed.

5. For the next couple of slides we will
be looking at how the doppler effect
occurs in the sounds of an emergency
vehicle siren.
Emergency Vehicle Siren and You

6. Let’s Pretend that the Source
was Stationary
● emits spherical waves with
the same speed in all
directions
● Receiver at any point will
detect the same frequency
because they are equally
spaced apart http://cfcpwork.uchicago.edu/kic
p-
projects/nsta/2007/sherman/dop
pler_files/image004.png

7. 1) Moving Source + Stationary Receiver
o Imagine that an emergency vehicle is
approaching you from the left and continues
to move right.
Special Cases when vr= 0 or vs = 0
http://mail.colonial.net/~hkaiter/aa_newest_images/doppler.effect.diagram.jpg

8. ● Distances between wave fronts at the
right side of the source are closer
together than the wave fronts at the left
of the source
● Waves are still travelling at the same
speed.
● Smaller distance between the wave
fronts on the right, a receiver would
detect more waves per second = higher
frequency
http://cfcpwork.uchicago.edu/kicp-
projects/nsta/2007/sherman/doppler_fi
les/image004.png

9. 2) Moving Receiver + Stationary Source
● receiver detects a reduction in the wave
speed
Special Cases when vr= 0 or vs = 0

10. First let’s think of it in this way:
Let’s pretend that the receiver was moving
towards the stationary source!
Let’s look at this from a different
approach:

11. ● If the receiver was one crest away from
away from the source, the distance
would be one wavelength away from the
source
● the speed from the perspective of the
receiver would be v+vr since it would
experience more waves per second
● The numerator would have a minus sign
when we plus this into the equation

12. In the end we get:
Stationary Source
Stationary Receiver
Upper sign = motion towards
Lower sign = motion away