The number of possible natural oscillations of air column in a pipe closed at one end of length 85 cm whose frequencies lie below 1250 Hz are (Velocity of sound = 340 m s-1)

Fundamental frequency of the closed organ pipe isv=v4L Here, v=340 m s-1,L=85 cm=0.85 m∴  v=340 m s-14×0.85 m=100 HzThe natural frequencies of the closed organ pipe will bevn=(2n-1)v=v,3v,5v,7v,9v,11v,13v,…       =100 Hz,300 Hz,500 Hz,700 Hz,900 Hz                                                 1100 Hz,1300 Hz,… and so on. Thus, the natural frequencies lies below the 1250 Hz is 6.