The Doppler Effect
Ch. 25.9-25.11
Objectives
1. Describe doppler effect
2. Describe bow waves
3. Describe sonic booms
Stationary Source
• Bug bobbing at
constant frequency on
surface of water
• What do you notice
about f & ?
• Bug forms concentric
circles.
• Waves encounter
point A as frequently
as point B.
The Doppler Effect
Moving Source
• Bug begins to move across
surface @ less than wave
speed.
• Which observer (A or B) will
encounter a higher frequency
of waves?
• Answer: B
A
• Doppler Effect:
– change in FREQUENCY (f)
due to the motion of the source
• What about wave speed, does
it change?
• NO!!!!
A
B
Doppler Effect
Moving Source
• Water waves spread over flat
surface, but Sound waves travel in
3-D and expand like balloon
• What do you notice about the
waves in front of train compared
to those in back when train is
stationary? In motion?
• Wave crests ahead of moving
source are closer together than
behind source
• What does this mean?
• Higher frequency in front, lower
frequency in back.
– i.e. car/train horn
– Demo with string and noise
Doppler Effect
Radar
• Bounce radar waves
off of moving vehicles
• Compares f of radar
waves from gun with
the f of reflected
waves
• Q: When a source moves towards you, is
there an increase or decrease in wave
speed?
• A: Neither! It is the frequency of the wave
that undergoes a change, NOT the wave
speed.
What happens when the speed of the source is
is EQUAL to the speed of waves it produces?
• The waves pile up and
superimpose on each
other
• This is a barrier wave:
– Source and waves are
equal speeds
What happens when the speed of the source is
is GREATER than the speed of waves it
produces?
• Produces a Bow Wave:
– Speed of source is greater
than wave speed
– The wave crests overlap at
edges creating a V shape.
Bow Waves
• This can be seen by
speedboats
• Increased speed
produces a narrower V
shape.
• Notice that the boat
down below is
outrunning the waves
it produces
Patterns made by an object moving at
successive speeds
Draw these please
F-14 Tomcat
Do Planes actually “break the
sound barrier?”
Here’s what happens. . .
• Overlapping wave crests
disrupt flow of air over
wings --> harder to control
plane when flying close to
speed of sound (761mph)
• But, the barrier is not real
• Think of it this way…
• Just like a boat travels
faster than speed of water
waves…a plane can travel
faster than the speed of
sound
• Supersonic!!
Shock Waves
• Shock Wave:
– produced from overlapping
spheres that form a cone
• Jet “pushes” sound waves
in front of it
• Sound waves must
“obey” the speed limit-->
pile up against each other
• These “piled up” waves
are called shock waves
Shock Waves
• Sonic Boom:
– the “crack” you hear when the
compressed air of a supersonic
object reaches the ground
• Sound waves hit you all at once
(not one at a time)
• Boat (one wave at at time) vs. jet
(all waves at once)
• A bullet passing overhead
produces a “crack” which is a
small sonic boom
• Bigger the object --> more air is
displaced --> louder the boom
• Sonic Boom Description
Shock Waves
• Shock Waves spread
until they reach the
ground.
•Crests overlap and
encounters listener in a
single burst
•So all listeners hear
the boom at the same
time?
•Shock wave and sonic
boom are swept
continuously behind
aircraft
•Bullets, whips, and
towels