Section 14.6

The Doppler Effect: A change in frequency (pitch)
due to relative motion between a source of sound
and its observer

The Doppler Effect can be produced by either
the source moving or by the observer
moving.
Doppler Effect for a Moving Observer
Actual Wave
Large wavelength
Small frequency (low pitch )
Small wavelength
Large frequency (high pitch)
Observed Wave

For a sound source moving towards a stationary observer with
speed vs , the perceived wavelength decreases and velocity remains
constant
◦ Therefore observed frequency will increase from f to f’

For a sound source moving away from a stationary observer with
speed vs , the perceived wavelength increases and velocity remains
constant
◦ Therefore observed frequency will decrease from f to f’

For an observer moving towards a stationary sound source with
speed v0 , the perceived velocity increases and wavelength remains
constant
◦ Therefore observed frequency will increase from f to f’

For an observer moving away from a stationary sound source with
speed v0 , the perceived velocity decreases and wavelength remains
constant
◦ Therefore observed frequency will decrease from f to f’
(new) observed frequency
speed of observer
(old) actual frequency
speed of sound
(usually 343 m/s)
speed of sound source
 Use the top set of
signs (+ in numerator &
- in denominator) when
the objects are moving
towards each other
 Use the bottom set of
signs (- in numerator &
+ in denominator) when
the objects are moving
away from each other

Some everyday examples are:
◦ Radar detectors
 Waves that strike an approaching car will bound back
with a higher frequency
 Waves that strike a car moving away will bounce back
with a lower frequency
◦ Loud fire engines/sirens moving past you
 As the fire engine/siren approaches, the pitch of the
siren sound (a measure of the siren's frequency) is
high
 As the fire engine/siren passes by, the pitch of the
siren sound is low.
 Virtual
Physic
Lab
Simulation
◦ Animals (bats)
 A) Stationary object: Waves will
reflect off surface with same
spacing as when they arrived; no
shift in frequency between cry
and echo
 B) Insect flying away: Echo waves
will have wider spacing (lower
frequency) than cry waves
because insect moves farther
from bat between waves
 C) Insect approaching: Echo
waves will be more compressed
(higher frequency) than cry
waves because insect moves
closer to bat between waves

How fast are the following aircrafts moving
with respect to the speed of sound?
(slower than, at, or faster than…?)
Aircraft moving at
the speed of sound
Aircraft moving faster
than the speed of sound

What is the “V” shape that forms behind an
aircraft traveling faster than the speed of
sound?
High pressure
regions result from
the overlapping of
compressions

What causes the “V” shape?
◦ Compressions interfere (overlap)
◦ Superposition of compressions
◦ One BIG compression all at once
What do you hear as the result
of this big compression?

When do you hear a sonic boom?
◦ As the high pressure region hits your ears all at
once!
◦ NOT as a plane crosses the threshold of sound

Where else in the real world do you see the
“V” shape occur?
◦ Water behind a duck
◦ Boats
◦ Wakeboarding/Waterskiing

http://www.youtube.com/watch?v=yWIMWqk
cRDU&feature=related