MCQ
The closed and open pipes have same length. When they are vibrating simultaneously in first overtone, produce three beats. The length of open pipe is made $\frac{1}{3}^{\text {rd }}$ and closed pipe is made three times the original, the number of beats produced will be
  • A
    8
  • B
    14
  • 17
  • D
    20

Answer

Correct option: C.
17
(c) : For open organ pipe, first overtone $v_1=\frac{v}{L}$ For closed organ pipe, first overtone $v_1^{\prime}=\frac{3 v}{4 L}$
Given : Beats $=3 Hz$
$
v_1-v_1^{\prime}=3=\frac{v}{L}-\frac{3 v}{4 L}=3=\frac{v}{4 L}=3 \Rightarrow \frac{v}{L}=12
$
For open organ pipe, fundamental frequency $v=\frac{v}{2 L^{\prime}}$ Given $L^{\prime}=\frac{L}{3} \quad \therefore v=\frac{v}{2\left(\frac{L}{3}\right)}=\frac{3 v}{2 L}$
For closed organ pipe, fundamental frequency $v^{\prime}=\frac{v}{4 L^{\prime}}$ Given $L^{\prime}=3 L \quad \therefore \quad v^{\prime}=\frac{v}{4(3 L)}=\frac{v}{12 L}$
$\therefore \quad$ Beat produced $=v-v^{\prime}$
$
=\frac{3 v}{2 L}-\frac{v}{12 L}=\frac{17 v}{12 L}=\frac{17}{12} \cdot 12=17
$

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