The fundamental frequency in an open organ pipe is equal to the third harmonic of a closed organ pipe. If the length of the closed organ pipe is 20 cm the length of the open organ pipe is

For closed organ pipe, third harmonic is 3v4l For open organ pipe, fundamental frequency is v2l'Given, third harmonic for closed organ pipe = fundamental frequency for open organ pipe.∴  3v4l=v2l'⇒l'=4l3×2=2l3where l and l' are the lengths of closed and open organ pipes respectively.∴  =2×203=13.33 cm