The Doppler Effect - Example 1

Stationary Observer, Moving Source

A Garda car is speeding towards the scene of the crime at 70m/s with its siren blaring at 2000Hz. At what frequency do you hear the siren at when it's approaching, and when it has passed? Take the speed of sound in air to be 340 m/s.

1. Approaching

Apparent frequency will appear louder - use a minus on the bottom fraction 

f'=\(\frac{v}{v-vs}\)f = \(\frac{340}{340-70}\)2000 = 2518Hz

2.Passing

Apparent frequency will appear quieter - use a plus on the bottom

f'=\(\frac{v}{v-vs}\)f = \(\frac{340}{340+70}\)2000 = 1658Hz

Click here for a video example of the Doppler Effect

Example 1