Download - Doppler Effect
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The Doppler EffectThe Concept Explained!
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● 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
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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
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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.
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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
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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
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1) Moving Source + Stationary Receivero 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
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● 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
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2) Moving Receiver + Stationary Source
● receiver detects a reduction in the wave
speed
Special Cases when vr= 0 or vs = 0
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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:
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● 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
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In the end we get:
Stationary SourceStationary Receiver
Upper sign = motion towards
Lower sign = motion away