The Doppler effect describes how the frequency of a wave (such as sound) is perceived by an observer who is moving relative to the source of the wave. When an ambulance approaches with its siren on, the observer hears a higher pitch tone due to compression of the sound waves. As the ambulance passes and moves away, the observer hears a lower pitch tone due to expansion of the sound waves. The Doppler effect can be calculated using an equation that takes into account the velocity of sound, velocity of the source, and velocity of the receiver.

1. The Doppler Effect
Victoria Nguyen LO5

2. Daily life scenario
 You’re sitting in the car when suddenly you hear a distant siren from behind
you. As the ambulance approaches your car, the sound of its siren gets
louder and louder. But once the ambulance passes your car, the sound of
its siren gets softer. Why?
 Well, its because of the Doppler effect!

3. Doppler effect
 The Doppler effect is a phenomenon that states that an approaching
source of sound will generate a high frequency and switch to lower
frequency as it passes by due to the compression and expansion of the
sound waves’ wavelength

5. Scenarios of the Doppler effect
1) If the source and the receiver are both
stationary, then the frequencies will remain the
same.
2) However, if either the source (S) or the receiver
(R) are moving, then the frequency at the receiver
end compared to the source will be different.
Let’s focus on the receiver’s end:
 If both source and receiver are approaching
each other = higher freq at R’s end
 If both source and receiver are moving apart =
lower freq at R’s end
This also explains why approaching sirens will sound
high pitch while a receding siren will sound low
pitch!

6. The Doppler effect equation
Now that we know the concept of the Doppler effect, we can
actually calculate the frequency at the receiver’s end using this
equation!
Frequency
measured at
receiver’s end
Velocity of
sound
traveling in air
(~343 m/s)
Velocity of
source
Velocity of
receiver
Frequency
measured at
source’s end

7. But what do these signs mean???
No need to panic! These signs just represent different scenarios of the
movement of the source and the receiver!

8. An end moving TOWARDS a stationary end
When either the source or the receiver is
moving TOWARDS the stationary end,
use the + sign on top and – sign on the
bottom.
This makes sense because by doing so,
we are increasing the numerator and
decreasing the denominator, which will
give us a higher R frequency, supporting
the Doppler effect.

9. An end moving AWAY from a stationary end
However, when either the source or the
receiver is moving AWAY from the
stationary end,
change the – sign at the bottom and
change it to a + sign.
This makes sense because it will
decrease the numerator and increase
the denominator, which will result in a
lower R frequency, supporting the
Doppler effect.

10. Now let’s answer the most important
question ever!!
 What costume is Sheldon (the character from Big Bang Theory) wearing in
this picture?

11. The Doppler effect!

12. Hope you enjoyed this presentation.
Now you know what the Doppler effect
is about!