2. The Doppler Effect
• Relative motion between source of the sound and
the receiver of the sound, the frequency at the
receiver is different from the frequency
transmitted.
• If two sources are moving towards each other, the
received frequency is lower.

3. Possible Scenarios
• Stationary source and moving receiver
• Stationary receiver and moving source
• Moving source and moving receiver
fs is the frequency of the source
fr is the frequency of the receiver
V is speed of sound in air
Vr is speed of receiver
Vs is speed of source
This equation shows the relationship between the source and the receiver if
they are moving along the line that connects them

4. Moving Source Stationary Receiver
• If a source moves to the right, while emitting spherical
waves, the wave fronts to the right of the source are
closer together, and the wave fronts to the left are farther
apart.
• A stationary receiver to the right of the source will detect
more waves per second than the receiver on the left.
• More waves per second relates to a lower frequency.
• The wavelength to the right of the source is shorter,
meaning it has a higher frequency according to
f=v/wavelength.

5. Moving Receiver Stationary Source
• When receiver moves away from source, receiver detects a
reduction in the wave speed
• A receiver moving toward the source detects and increase in
the wave speed.
• For a receiver moving towards a stationary source:
• For a receiver moving away from stationary source: we use
the same equation as above but has a negative sign on the
numerator

6. Question
• A race car travelling at 350 km/h honks it horn as it
approaches a stationary group of audience. The honk has
a frequency of 700 Hz. What frequency would be
perceived by someone standing at audience stand as the
train
a) Approaches the audience stand
b) Moves away from the audience stand

7. Solutions
• a) speed = 350 m/s = 97.2 m/s and the speed of receiver
is 0 m/s. Speed of sound in air = 343 m/s
• fr = 343m/s + 0 x 700 Hz
343m/s – 97.2m/s
fr = 976.8 Hz

8. Solutions
• b) When the race car is moving away from
audience stand, we use minus in numerator and
plus in denominator
• fr = 343m/s - 0 x 700 Hz
• fr = 545.4 Hz
343m/s + 97.2m/s