The Doppler effect describes how the observed pitch of a sound changes as the source of the sound and the observer move relative to each other. If the source and observer are approaching each other, the observed frequency is higher than the actual emitted frequency. If they are moving away from each other, the observed frequency is lower. This change in observed frequency occurs because the wavelength of sound waves is compressed when the source and observer approach, increasing frequency, and stretched when they recede, decreasing frequency.

1. DOPPLER EFFECT

2. You walk on the street, and notice the change in
the sound pitch as an ambulance passes by
you…
and this is because of
the Doppler effect…

3. If an observer (you) and
the source (ambulance)
are approaching
eachother, the frequence
that the observer receive
will be higher than the
actual sound emitting
If the distance between
observer (you) and the
source (ambulance) are
increasing, the frequence
that the observer receive
will be lower than the
actual sound emitting

4. But… why?
f = v l
Remember this equation?
- When the source and the observer are approaching each other, the sound
waves are compressed and cause the wavelength to become smaller,
which the frequency will increase as the wavelength decrease
- When the source and the observer are moving away fro each other, the
sound waves stretch and become longer, which the frequency will
decrease as the wavelength increase

5. Fig 14.8, p. 435
Slide 12
f ' = (v + vobs ) l
Which is equal
to…
When approaching
…
ƒ' = ƒ
v+ vo
v
æ
è
ç
ö
ø
÷

6. Fig 14.9, p. 436
Slide 13
ƒ' = ƒ
v - vo
v
æ
è
ç
ö
ø
÷
When moving
away…

7. Doppler Effect Equation