Physical aspect of organ pipes

Arie de Wit

het ORGEL 94 (1998), nr. 6, 24-30 [summary]

The sound of an organ pipe depends on the fundamental (which defines the pitch of the tone) and the harmonics (which define the colour of the tone) that are produced by the pipe, as well as by the attack and the release. These aspects can be measured and analysed with the help of modern computer technology. The results can be displayed graphically, in order to make the frequencies (x-axis) and the amplitude of these frequencies (y-axis) visible. By repeating the measurements at short intervals of time and projecting the displays of the results behind each other, we get a three dimensional graph which shows which frequencies were sounding, how strong they were, what their attack was like and how they decayed.

Most organists think an organ pipe speaks because wind blows against the upper lip. This is a misunderstanding. The wind moves in and out of te pipe after leaving the windway. When this ‘sheet of air’ passes the upper lip inside the pipe, an air wave is created in the pipe. As in a stethoscope this wave is not damped quickly, so we get a standing wave in the pipe, the frequency of which defines the pitch.

The pitch depends on the temperature as well, for if the temperature changes, the speed of sound changes, too. Because the pitch of a reed pipe depends on the length of the tongue, it is much less influenced by changes of temperature. We tune, however, the reeds, because this is easier, and because they are fewer in number than the flue pipes.