Question

A trained musician using an open flute of length $60cm$ produces third harmonic sound waves. A person runs towards the musician from other end of a hall at a speed of $18km/h$. If the wave speed is $340m/s$, the frequency heard by the running person shall be equal to_______ $Hz$

Answer 1

Answer: 862.5

$l=60cm=0.6m,v_p=18km/h=5m/s$
$v=340m/s$
Frequency of third harmonic.
$v=3\times \frac{v}{2l}=\frac{340\times 3}{2\times 0.6}=850Hz$
Frequency heard by person,
$v^{\prime }=\frac{v+v_p}{v}v=\frac{340+5}{340}\times 850$
$\therefore v^{\prime }=862.5Hz$