A musician using an open flute of length $50\,cm$ produces second harmonic sound waves. A person runs towards the musician from another end of a hall at a speed of $10 \,km/h.$ If the wave speed is $330\,m/s,$ the frequency heard by the running person shall be close to...... $Hz$
JEE MAIN 2019, Medium
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$f=\frac{2}{2 \ell} V_{s}=\frac{330}{0.5}=660 \mathrm{Hz}$
$ \therefore \quad f^{\prime}=f\left(\frac{v_{s}+v}{v_{s}}\right)=(660)\left(\frac{330+\frac{50}{18}}{330}\right)=660\left(1+\frac{50}{18 \times 330}\right)$
$=666 \mathrm{Hz}$
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