CK-12-Physics-Concepts - Intermediate

12.3. Doppler Shift http://www.ck12.org

source of the sound is stationary and the observer is moving (with a significant fraction of the speed of sound). The
following formulas have been developed for calculating the observed frequency:

When the source is moving toward you,fo= 1 −fvR
v

.

when the source is moving away from you,fo= 1 +fvR
v

.

In the equationsfois the perceived frequency,fis the emitted frequency,vRis the relative velocity andvis the speed
of sound.

Example Problem:The speed of sound is 340. m/s and a train whistle with a frequency of 512 Hz is emitted from
a train coming toward you at 40.0 m/s. What frequency will you hear?

Solution:fo= 1 −fvR
v
=^512 s

− 1

1 −^40340.^0 .mm//ss=^580 Hz

Summary

• The Doppler shift, or Doppler effect, occurs when a sound’s emission and the sound’s receiver travel relative
  to each other at a significant speed compared to the speed of sound.


• If the sender and receiver are getting closer together, the perceived frequency will be higher than the emitted
  frequency, given by the equation fo= 1 −fvR
  v

.

• If the sender and receiver are getting farther apart, the perceived frequency will be lower than the emitted
  frequency, given by the equationfo= 1 +fvR
  v

.

Practice

Questions

The following video demonstrates the doppler effect with car horns. Use this resource to answer the questions that
follow.

http://www.dailymotion.com/video/x3ogq9_doppler-shift_people

MEDIA

Click image to the left for use the URL below.

URL: http://www.ck12.org/flx/render/embeddedobject/82828

1. What is necessary for a wave source / receiver in order to observe the Doppler effect?


2. How are the waves made with his finger in water similar to the sound waves? How are they different?

Review

Questions

1. What is the frequency heard by a person driving at 15 m/s toward a blowing factory whistle if the emitted
   frequency is 800. Hz and the speed of sound is 340. m/s?


2. While standing near a railroad crossing, a person hears a distant train horn. According to the train’s engineer,
   the frequency emitted by the horn is 440 Hz and the train is traveling toward the railroad crossing at 20.0
   m/s. If the speed of sound is 340 m/s, what frequency will the observer hear?