Everything Science Grade 12

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