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I have gone through about each case of standing wave in open and closed organ pipe and understand antinode and node concepts. But i am confused that if all harmonics are possible in organ pipe, then which frequency do we hear the most. Is it fundamental frequency we hear when we play flute or any open or closed organ pipe or any particular frequency?

which frequency dominant other frequency so that we hear hear the particular frequency in high pitch in organ pipe.

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In the case of an organ pipe, it is usually designed to furnish you with the fundamental (lowest) note it is resonant at. Here's how.

An organ pipe which is excited by compressed air has a rather complicated assembly of bent and curved metal pieces situated at its blown end. The objective of that arrangement is to preferentially set the pipe into oscillation at its fundamental. This occurs when the compressed air being carefully blown past its end is set into back-and-forth motion right next to and then away from the pipe opening: when a positive pressure pulse exits the blown end of the pipe, it deflects the air stream away from the pipe end; when a negative pressure pulse follows it, it sucks the air stream back in towards the pipe opening.

That arrangement of metal sheets also provides a short burst of hissing noise from which the organ pipe filters out its fundamental and radiates it. clever design of the hiss maker can make it produce a whooshing noise (good for exciting the bass note pipes) or a sharper hissing noise (good for exciting the higher note pipes).

This whole business was worked out by trial and error by medieval craftsmen hundreds of years ago; what I have furnished here is just an extremely simplified picture of an extremely complicated business!

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    $\begingroup$ Don't different instruments sound different when playing the same fundamental note because of the overtones that are present along with the fundamental? $\endgroup$ Dec 6, 2021 at 0:06
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    $\begingroup$ @Not_Einstein, that is correct, and the presence or absence of overtones in an organ pipe will color its sound as well. My point is the organ designer can control that process, to make different pipes sound different even though they have the same length. $\endgroup$ Dec 6, 2021 at 0:16
  • $\begingroup$ @Not_Einstein you are correct indeed. The amplitude of the overtones (spectral content in general), their frequencies and their temporal envelopes are what constitute the timbre of each instrument and what makes the sound of, two instruments (even if they are both flutes, violins, cellos, etc.) different and distinguishable from each other. Niels Nielsen has focused more on the way the instrument designer can control various aspects of the aforementioned parameters/factors. $\endgroup$
    – ZaellixA
    Dec 6, 2021 at 23:35

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