Broadly speaking, the pipes of an organ are divided into two classes called flue pipes and reeds respectively. They may be either closed (stopped) or open. Open pipes have a more brilliant tone than stopped pipes, as they are able to produce more overtones. In the case of a flue pipe a jet of air impinges on a sharp edge called the lip. Eddies are thus set up in the air-stream which causes compression and rarefaction to travel up the pipe.
These small impulses set up resonant vibrations in the pipe of a large amplitude, and hence a loud sound is given out. “Voicing” an organ pipe is-a. highly skilled craft, and consists of shaping the mouth and lip of the pipe so as to emphasize desirable overtones which enhance the beauty of the tone.
Reed pipes may be recognized by their conical shape. In these the air column is set in vibration by means of a strip of springy metal called the reed. The reeds are set in vibration by the air-stream. Tuning of the reeds is carried out by adjustment of a stiff wire, the end of which presses against the reed and controls its effective length (Fig. 29.11).
A large organ contains many hundreds of pipes, ranging in length from a few centimeters to over 9 meters in length. Certain ranks of pipes have two pipes for each note. One of each pair is de-tuned slightly so that beats are produced. The resultant tremolo effect is characteristic of those stops labelled “Vox human a” or “Vox angelica”.