A closed organ pipe has length 'l'. The air in it is vibrating in 3rd overtone with rnaximum amplitude 'a'. The amplitude at a distance of l7 from closed end of the pipe.

The figure shows variation of displacement of particles in a closed organ pipe for 3rd overtone.For third overtone l = 7λ4 or λ = 4l7 or λ4 = l7Hence the amplitude at P at a distance l7 from closed end is 'a' because there is an antinode at that point.