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In an open organ pipe $\nu_3$ and $\nu_6$ are $3^{\text{rd}}$ and $6^{\text{th}}$ harmonic frequencies, respectively. If $\nu_6 - \nu_3 = 2200 \text{ Hz}$ then length of the pipe is _________ mm.
(Take velocity of sound in air is $330 \text{ m/s}$.)

The correct answer is
225

Open Organ Pipe Harmonics Calculation

For an open organ pipe, the frequencies of the harmonics are given by the formula:

$ \nu_n = \frac{n v}{2L} $

where $n$ is the harmonic number ($n=1, 2, 3, \ldots$), $v$ is the velocity of sound, and $L$ is the length of the pipe.

Harmonic Frequency Analysis

  • The 3rd harmonic frequency ($\nu_3$) is: $ \nu_3 = \frac{3v}{2L} $
  • The 6th harmonic frequency ($\nu_6$) is: $ \nu_6 = \frac{6v}{2L} $

Calculating Pipe Length

We are given the difference between the 6th and 3rd harmonic frequencies:

$ \nu_6 - \nu_3 = 2200 \text{ Hz} $

Substituting the formulas for $\nu_6$ and $\nu_3$:

$ \frac{6v}{2L} - \frac{3v}{2L} = 2200 $

Simplify the equation:

$ \frac{(6-3)v}{2L} = 2200 $

$ \frac{3v}{2L} = 2200 $

Now, we solve for the length $L$. Rearranging the equation:

$ L = \frac{3v}{2 \times 2200} $

Given the velocity of sound $v = 330 \text{ m/s}$:

$ L = \frac{3 \times 330 \text{ m/s}}{4400 \text{ Hz}} $

$ L = \frac{990}{4400} \text{ m} $

$ L = \frac{99}{440} \text{ m} = \frac{9}{40} \text{ m} $

To convert the length from meters to millimeters, multiply by 1000:

$ L = \frac{9}{40} \times 1000 \text{ mm} $

$ L = 9 \times 25 \text{ mm} $

$ L = 225 \text{ mm} $

The length of the pipe is 225 mm.

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