The Doppler Effect - Example 2

Stationary source, moving observer

A pupil is running towards school at 5m/s as they are late. The bell sounds at a frequency of 350 Hz. What frequency does the pupil hear the bell at? Take the speed of sound in air to be 340m/s. 

Solution

Use the formula f'=$\frac{v±vo}{v±vs}$f

Because the source is stationary, vs =0

So now our formula looks like f'=$\frac{v±vo}{v}$f

The apparent frequency will sound louder as the pupil is approaching the source

f'=$\frac{v±vo}{v}$f = f'=$\frac{340+5}{340}$350 = 355 Hz

Example 2