The number of possible natural oscillation of air column

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The number of possible natural oscillation 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 ms^-1)

1. 4
2. 5
3. 7
4. 6

Correct Option: D

In case of closed organ pipe frequency,
f_n = (2n + 1)
for n = 0, f₀ = 100 Hz
n = 1, f₁ = 300 Hz
n = 2, f₂ = 500 Hz
n = 3, f₃ = 700 Hz
n = 4, f₄ = 900 Hz
n = 5, f₅ = 1100 Hz
n = 6, f₆ = 1300 Hz
Hence possible natural oscillation whose frequencies < 1250 Hz = 6(n = 0, 1, 2, 3, 4, 5)