have you observed with the sound of
these vehicles as they pass by at a certain
explanation can offer to account for
are the key variable involved?
apparent change in the frequency of a
source of wave due to the motion of the
source and/or the observer
by Christian Doppler in 1842
difference of the true frequency and the
apparent frequency is called the Doppler
The following symbols will be used in the
is the true frequency
is the apparent frequency
is the velocity of the sound (air)
is the velocity of the source
vo is the velocity of the observer
is the true wavelength
λ’ is the apparent wavelength
’ = [ ( v ± vo ) / v ] f
( + ) means the observer is moving towards
( - ) means the observer is moving away
from the source
’ = [ v / ( v ± vs ) ] f
( + ) means the source is moving away from
( - ) means the source is moving towards the
’ = [ ( v ± vo ) / ( v ± vs ) ] f
NOTE: please refer to case 1 and case 2
and analyze the situation of the observer
and the source respectively.
a wave source moves toward a
receiver, does the receiver encounter an
increase in wave frequency, wave speed ,or
the Doppler Effect occur for only some
types of waves or all types of waves?
after Austrian physicist and
philosopher Ernst Mach
Used to describe speed of objects that are
travelling close to, or above the speed of
The ratio of the speed of the object and the
speed of sound in the medium.
= speed of object / speed of sound
is heard as a sonic boom when an
airplane passes over; this is the result of the
compilation of sound waves generated by a
source that moves at equal or greater
velocity than the sound it creates
An ambulance traveling at 20. m/s sounds its siren
at a frequency of 256Hz. What apparent frequency
will you hear if the ambulance is:
( assume air temperature to be at 25°C )
a. approaching you?
b. moving away from you?
2. A car traveling at 30. m/s sounds its horn as it
overtakes and pass another car traveling at 20. m/s
in the same direction. If the frequency of the horn is
500. Hz, what is the frequency heard by the driver in
the slower car:
b.) after the car has passed?
Assume sound speed to be at 350. m/s.