An ambulance is moving with blowing horn towards a person standing on the road with a speed of $54\…
An ambulance is moving with blowing horn towards a person standing on the road with a speed of $54\ kmh^{-1}$. What frequency does the person hear, if the velocity of sound in air is $330\ ms^{-1}$ and frequency of horn is $450\ Hz$?
Solution
Sol. Given, frequency of horn, $f = 450\ \mathrm{Hz}$
Speed of ambulance, $v_s = 54\ \mathrm{kmh}^{-1} = 15\ \mathrm{ms}^{-1}$
$v_o = 0$ and speed of sound in air, $v = 330\ \mathrm{ms}^{-1}$
As source is moving towards a stationary observer, therefore frequency heard by the observer,
$f' = \left(\dfrac{v}{v - v_s}\right) f = \left(\dfrac{330}{330 - 15}\right) 450\ \mathrm{Hz}$
$= \dfrac{330}{315} \times 450\ \mathrm{Hz} = 471.43\mathrm{Hz}$
Answer: 471.43 Hz