Q.

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)    [2014]

1 4  
2 5  
3 7  
4 6  

Ans.

(4)

Fundamental frequency of the closed organ pipe is υ=v4L

Here, v=340 m s-1, L=85 cm=0.85 m

∴  υ=340 m s-14×0.85=100 Hz

The natural frequencies of the closed organ pipe will be

υn=(2n-1)υ=υ,3υ,5υ,7υ,9υ,11υ,13υ,…

=100 Hz,300 Hz,500 Hz,700 Hz,900 Hz,1100 Hz,1300 Hz,…

and so on

Thus, the number of natural frequencies lies below 1250 Hz is 6.