9.2 CHAPTER 9. DOPPLEREFFECT
Example 2: The Doppler Effect forSound 2
QUESTION
What is the frequency heard by a person drivingat 15 m·s−^1 toward a factory
whistle that is blowingat a frequency of 800 Hz. Assume that the speed of
sound is 340 m·s−^1.
SOLUTION
Step 1 : Determine how to approach the problem basedon what is given
We can use
fL=
v + vL
v + vS
fS
with:
v = 340 m· s−^1
vL = +15 m· s−^1
vS = 0 m· s−^1
fS = 800 Hz
fL =?
The listener is moving towards the source, so vLis positive.
Step 2 : Calculate the frequency
fL =
v + vL
v + vS
fS
=
340 ,6 m· s−^1 + 15 m· s−^1
340 ,6 m· s−^1 + 0 m· s−^1
(800 Hz)
= 835 Hz
Step 3 : Write the final answer
The driver hears a frequency of 835 Hz.