CHAPTER 9. DOPPLEREFFECT 9.2
Example 1: The Doppler Effect forSound 1
QUESTION
The siren of an ambulance has a frequency of 700 Hz. You are
standing on the pavement. If the ambulance drives past you at
a speed of 20 m·s−^1 , what frequency will you hear, when
a) the ambulance is approaching you
b) the ambulance is driving away from you
Take the speed of soundto be 340 m·s−^1.
SOLUTION
Step 1 : Determine how to approach the problem basedon what is given
fL=
v + vL
v + vS
fS
fs = 700Hz
v = 340 m· s−^1
vL = 0
vS =−20 m· s−^1 for (a) and
vS = +20 m· s−^1 for (b)
Step 2 : Determine fLwhen ambulance is approaching
fL =
340 + 0
340 − 20
(700)
= 743,75Hz
Step 3 : Determine fLwhen ambulance has passed
fL =
340 + 0
340 + 20
(700)
= 661,11Hz