Question

For a certain organ pipe, three successive resonance, frequencies are observed at $425,595$ and $765Hz$ respectively. Taking the speed of sound in air to be $340m/s$ the fundamental frequency of the pipe (in Hz) is

• A. $425$
• B. $170$
• C. $85$
• D. $245$

Answer 1

The correct option is C $85$

should take ratio of all three harmonics.

e.g.,$425Hz:595Hz:765Hz$

$\Rightarrow 5\times 85Hz:7\times 85Hz:9\times 85Hz$

$\Rightarrow (2\times 2+1)85Hz:(2\times 3+1)85Hz:(2\times 4+1)85Hz$

what you saw, each the successive harmonics is an odd number.

This is possible only when organ pipe is closed .

And $85$ is common for all harmonics ,

so fundamental frequency is $85Hz$