Standing waves in a cylindrical pipe have a pressure antinode (point of maximum pressure fluctuation) at a closed end of the pipe. To a first approximation, an open end can be treated as a pressure node (point of no pressure fluctuation), but in fact the node is displaced beyond the physical end of the tube by a short distance known as the end correction. The effective length of the air column, which is used in calculating the wavelengths and frequencies of the standing waves, is the sum of the physical length and the end correction. If the pipe is open at both ends, two end corrections are required. The end correction at the open upper end of a thin-walled organ flue pipe is close to 0.61 times the pipe radius at low frequencies; it decreases as the frequency increases, and increases with increasing wall thickness. Partially closing the end increases the end correction, lowering the sounding pitch. The end correction at the mouth of an organ flue pipe depends on the details of the mouth, and is normally greater than the pipe diameter.
See also Speaking length.
MURRAY CAMPBELL