Doppler Effect

1
Doppler Effect
2
Moving Source

• Sound moves equally in all directions from a
  source.
• Circular (or spherical) pattern
• If the source is moving the origin of successive
  circles moves.

3
Wavelength Shift

• The normal period is T and the observer moves at
  speed u.
• The wavelength ahead of the source is shorter.
• l l - uT
• The wavelength behind the source is longer.
• l l uT

4
Frequency Shift

• The period is related to the wavelength.

• This can be converted into a frequency shift.

Insert for to get the shift behind the source.
5
Ambulance

• An ambulance is approaching at 130 km/h with a
  siren at 1.2 kHz.
• What pitch do you hear?

• Change speed to m/s
• (130 km/h)/3.6 36 m/s
• Find the Doppler shift.
• u/v (36 m/s)/(343 m/s)
• u/v 0.105
• f f/(1-1.05) 1340 Hz

6
Moving Observer

• If the observer is moving toward the sound the
  effect is similar, but the wave crests have a
  different relative spacing.

A speed gun uses a double Doppler shift. There is
a shift for both the pulse out and reflected
signal.
7
Shock Wave

• A moving source can exceed the speed of sound.
• The sound waves constructively interfere on a
  front at an angle to the motion.
• This is called a shock wave.

8
Sonic Boom

• The speed of sound is called Mach 1.
• Faster than that speed causes a shock wave a
  sonic boom.

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