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.