In this case, both the source and the observer moving with different speeds towards the wall so the frequency of sound heard by motorist is given as
f' = f0 [v + vm/v + vs]
While the sound reflected from the wall is moving towards the motorist. Hence the frequency heard by
the motorist will be f'' = fw [v + vm/v] . These two waves superimpose with each other to create beats and number of beats heard is given by n = f" - f' .
The frequency, f, of band heard by the motorist directly is given by f' = f[v + vm/vs] .
The frequency fw reaching the wall is
The frequency f″ reaching the motorist is given by