How Sounds Change?
Did you ever notice the sound of a fire
truck change as it drove past you?
You might have noticed the intensity
increase as it got closer, and decrease
as it drove away
But did you ever notice a change in
pitch of the siren?
If you have, you have first hand
experience with the Doppler Effect
The Doppler Effect was first
discovered by Australian Scientist
Christian Johan Doppler (1803-1853)
People behind
the fire truck
hear a lower
pitch than the
firefighters in the
truck hear.
People
standing in
front of the fire
truck hear a
higher pitch
than the
firefighters in
the truck hear.
What is the Doppler Effect?
The Doppler Effect is the apparent change in
frequency of a wave as its source moves in relation to
an observer
The term “apparent” is used because the actual
frequency emitted from the source is not changing
It only appears to change by the observer as he or
she changes position
Since changing the frequency of a sound wave
changes the pitch, an observer is noticing a change in
pitch as their distance from the sound source changes
How does the pitch sound to the train
conductor? _________________
A plane is passing an observer,
going from point C to point D.
1. At point C, the pitch will be
___________________than normal, and at point D,
the pitch will be ____________________ than normal.
What causes the Doppler Effect?
When the sound source moves toward a listener, the frequency of the waves is
higher than it would be if the source were stationary
When the sound source moves away from the listener, the frequency of the
wave is lower than it would be if the source were stationary.
When a sound source moves, the frequency of the waves changes because
the motion of the source adds to the motion of the waves
Imagine that you are standing still and throwing a tennis ball at a wall in front of you
at rate of one per second
Now suppose you walk toward the wall while still throwing one ball per second
Because each ball has a shorter distance to travel than the one before, each
takes less time to get there
The balls hit the wall more often than one per second, so the frequency is higher
On the other hand, if you throw a ball at the wall as you back away, each ball has
farther to travel and the frequency is lower
Doppler Effect with Light
Astronomers use the
Doppler Effect to
calculate precisely how
fast stars and other
astronomical objects
move toward or away
from Earth.
If you are referring to the
frequency of a light wave,
you would notice a
difference in color
The colors of the visible
spectrum are arranged
by increasing frequency:
R.O.Y.G.B.I.V.
•If an object is moving toward
Earth, it is said to be blue shifted
•If an object is moving away from
Earth it is said to be red shifted
Breaking the Sound Barrier
1. Slower than
the speed of
sound
2. Approaching
the speed of
sound
3. Faster than the
speed of sound
What causes Shock Waves?
At high speed, the Doppler Effect can be pretty spectacular
When a plane travels almost as fast as the speed of sound
(700m/h), the sound waves pile up in front of the plane
This pile up is the “sound barrier”
As the plane flies faster than the speed of sound, it moves
through the barrier
A shock wave forms as the sounds overlap
The shock wave releases a huge amount of energy, and a
change in pressure causes a small cloud to form
People nearby hear this loud noise as a sonic boom when the
shock wave passes by them
http://www.brainpop.com/science/energy/s
ound/fyi/
Visualizing the Doppler Effect
http://lectureonline.cl.msu.edu/~mmp/appli
st/doppler/d.htm
http://archive.ncsa.uiuc.edu/Cyberia/Bima/
doppler.html