Everything Science Grade 12

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.