Everything Science Grade 12

9.2 CHAPTER 9. DOPPLEREFFECT

on the right of the source is constant. The distance between peaks onthe left is also
constant but they are different on the left and right.

1

2

− 1

− 2

2 − 1 − 1 2

This means that the timebetween peaks on the right is less so the frequency is higher.
It is higher than on the left and higher than if thesource were not movingat all.

On the left hand side thepeaks are further apart than on the right and further apart than
if the source were at rest - this means the frequency is lower.

When a car approachesyou, the sound waves that reach you have a shorter wavelength
and a higher frequency.You hear a higher sound. When the car moves away from you,
the sound waves that reach you have a longer wavelength and lower frequency. You
hear a lower sound.

This change in frequency can be calculated by using:

fL=

v + vL

v + vS

fS (9.1)

where fLis the frequency perceived by the listener,
fSis the frequency of the source,
v is the velocity of the waves,
vLthe velocity of the listener and
vSthe velocity of the source.

Note: Velocity is a vector and has magnitude and direction. It is very important to get
the signs of the velocities correct here:

Source moves towards listener vS: negative

Source moves away fromlistener vS: positive

Listener moves towardssource vL: positive

Listener moves away from source vL: negative

See video: VPnyt at http://www.everythingscience.co.za