Question

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

Answer 1

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