Flue Pipes have no moving parts. The sound is made when the wind enters the foot of the pipe and is directed outward against the upper lip of the pipe mouth. This lowers the air pressure in the pipe, causing the wind to be sucked back in. Then the cycle starts again. This process causes the column of air in the pipe to vibrate. The number of vibrations per second, and therefore the pitch of the sound, is determined by the length of the pipe and whether it is open at the top or closed by a cap or stopper. (Stopped pipes sound an octave lower than open pipes of the same length.) The tone color of the pipe is affected by its scale (ratio of diameter to length), the material from which it is made (wood or various metal alloys), and by modifications that can be made to its mouth or shape. Flues are divided into groups according to their tone colors.
The basic flue groups are Principals, Flutes, and Strings.