The sound from an organ pipe is originally generated by air blown over a thin metal or wooden lip, or past a reed, causing it to vibrate. The sound that this vibration makes would have little impact on its own were it not for the air chamber of the pipe in which it sits.
When the dimensions of the lip or reed and the dimensions of the chamber are in mathematical sympathy, the air in the chamber can set up a sympathetic pattern of vibration. This effectively amplifies and transforms the original sound. In fact it is converting the energy of the lip or reedΓÇÖs vibration into more of the frequencies audible to the human ear. The resonating air chamber provides the body of sound that we hear. The source vibration is effectively only a spark that lights the gas.
If the relationship between the size of the lip and that of the chamber is imperfect, it may still be possible to force a note out of the pipe, but it will be a different note. By attacking the lip with a lot more air, and perhaps from a different angle, one can force it into a mode of vibration in which one of its natural partials becomes the dominating note. This may correlate with one of the inherent partials of the chamber. If so, new resonance is found.
You can experiment with the organ pipe in the laboratory to find the length of pipe which suits the reed and the pre-set air pressure. Listen to what happens on either side of this perfect length.
