Case I When train is moving towards a stationary observer.
According to Doppler's effect of sound,
apparent frequency heard,
n′= where,
n= frequency of source,
v= speed of sound and
vs= speed of source.
⇒vs=v[1−].....(i)
Case II When train is leaving the standing observer.
Again, according to the Doppler's effect of sound, Apparent frequency heard,
n′=⇒vs=v[−1] .......(ii)
From Eqs. (i) and (ii), we get
+=2 Given,
n′=216Hz and
n′=184Hz Substituting these values in the above equation, we get
n=≈200Hz