Question

Unless specifically stated otherwise, you may assume the speed of sound for all the scenarios below is 350 m/s.1. A police car is driving at 14 m/s towards a café. His siren is on blasting a frequency of 720 Hz. What frequency will people in the café hear as he approaches and what will they hear after he’s passed?

Answer 1

750 Hz and 692.3 Hz

Explanation:

Given:

f0 = 720 Hz

vs = 14 m/s

vr = 0 m/s

v = 350 m/s

The corresponding formula for the Doppler effect is as follows:

`f_s=f_o\left((v\pm\:v_o)/(v\pm\:v_s)\right)`

The first scenario the police car is approaching the cafe, therefore:

`\begin{gathered} f_s=720\cdot\left((350+0)/(350-14)\right) \\ \\ f_s=750\text{ Hz} \end{gathered}`

The second scenario the police car is moving away from the cafe, therefore:

`\begin{gathered} f_s=720\cdot\left((350+0)/(350+14)\right) \\ \\ f_s=\:692.3\text{ Hz} \end{gathered}`

Therefore, the frequency that will be heard when approaching is 750 Hz and when passing the frequency would be 692.3 Hz.