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Q. The fundamental frequency of a closed pipe is $220 \,Hz$. If $\frac{1}{4}$ of the pipe is filled with water, the frequency of the first overtone of the pipe now is

Haryana PMTHaryana PMT 2010

Solution:

Fundamental frequency of closed pipe
$n=\frac{v}{4 l}=220 Hz $
$\Rightarrow v=220 \times 4 l$
If $\frac{1}{4}$ of the pipe is filled with water then remaining length of air column is $\frac{3 l}{4}$.
Now, fundamental frequency
$=\frac{v}{4\left(\frac{3 l}{4}\right)}=\frac{v}{3 l}$ and
First overtone $=3 \times$ fundamental frequency.
$=3 \times \frac{v}{3 L}=\frac{3 \times 220 \times 4 l}{3 l} $
$=880\, Hz$