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The choir division of the organ at St. Raphael's Cathedral, Dubuque, Iowa. Wood and metal pipes of a variety of sizes are shown in this photograph.

Organ pipes are the sound-producing elements of the pipe organ. These resonators are made of either wood or metal that produce sound when air under pressure (commonly referred to as wind) is allowed through it. Each pipe is tuned to a specific note of the musical scale.

Flue pipes and Reed pipes are by far the most common organ pipes, and variations on these are found on every instrument. Diaphone pipes are also made, but these are very rare, and found on only a handful of pipe organs around the world. Organ pipes are categorised according to the construction style of each pipe, variables of which include the mechanism, materials, and structure of the pipe.

Main article: Flue pipe

Flue pipes (or flues), also referred to as labial pipes, contain no moving parts. They produce sound by the use of whistles or fipples (in a similar manner to a recorder. The majority of pipes in most organs are flue pipes. Flue pipes are themselves divided into three broad classes:

• Flute pipes have the purest tones, and are generally the widest-scaled flue pipes.
• Diapasons are intermediate in tone, and represent the fundamental sound of the pipe organ.
• String pipes have the richest harmonics, and tend to be the narrowest pipes.

Main article: Reed pipe

Reed pipes (or reeds), also referred to as lingual pipes, are driven by a beating reed.

The diaphone is a unique and uncommon organ pipe. Invented by Robert Hope-Jones around 1900, it has characteristics of both flue pipes and reed pipes. The pipe speaks through a resonator, much like a reed pipe, but a pallet instigates the vibration instead of a reed. The pipe is generally of wooden construction (although some examples of metal diaphones exist) and can be voiced on various wind pressures. The diaphone is usually found at 16′ and 32′ pitches, however there are a few examples of 8′ diaphones, and a full-length 64′ Diaphone-Dulzian is installed in the Boardwalk Hall Auditorium Organ in Atlantic City (which is one of only two full-length 64′ stops in the world, the other being the Contra Trombone reed stop in the organ at the Sydney Town Hall in Sydney, Australia).

Hope-Jones also developed an imitative version called the diaphonic horn, which had a more reed-like quality than the diaphone and was voiced on lower wind pressures. Wurlitzer built a version for their theater organs at 32′ and 16′ pitches with huge wooden resonators as extensions of its Diaphonic diapason, and at 16′ with metal resonators as an extension of its smaller-scale Open diapason. The Austin Organ Company also developed a metal diaphone at 16′ pitch known as a Magnaton. In addition to its usage in the pipe organ, the diaphone has also been used in foghorns and fire signals due to its penetrating tone.

Organ pipes are generally made out of either wood or metal. Metal flue pipes are usually made of varying mixtures of lead and tin, depending on the requirements sought for that particular pipe. The more lead used in the alloy, the darker the resulting tone will be. Conversely, if a pipe has a high proportion of tin, it will have a brighter tone. In addition, high amounts of tin give a gleaming and long-lasting polish which may be required if the pipe is clearly visible. Pure lead is too soft to be used alone; the best strength ratio is approximately 60% tin to 40% lead. The cost of each metal is also a factor, with tin being of far greater expense than lead. The usual exceptions to tin-lead alloys are very lowest pipes in a rank, which are sometimes made of rolled zinc. In addition, pipes have been made of many metals, including gold, silver, aluminum, brass, copper, and iron. Very rarely, glass, porcelain, plastic, or even stone pipes may be seen.

Metal pipes are made by first casting a metal alloy onto a long flat surface. This alloy is usually a mixture of lead and tin, with trace amounts of other metals. Once the metal dries, it is cut into pieces, which are then rolled into shapes around molds (known as mandibles) and welded together. Wooden pipes are made by cutting wood into the required shapes and sizes and assembling the pieces together.

Organ pipes are generally made in three shapes: cylindrical, conical, and square/rectangular. There are some irregular shapes that appear as well (the Flûte triangulaire, for example, is a triangular-shaped flue pipe). In addition to this, a pipe can be tapered from top to bottom.

An organ pipe can be either open or closed (also known as stopped) at the end opposite the reed or mouth. An open pipe produces a tone in which both the even-numbered and the odd-numbered partials are present, while a stopped pipe produces a tone with only the odd-numbered partials.

A stop tuned to unison pitch is known as an 8′ stop ("eight foot"). This refers to the approximate speaking length of the longest open flue pipe of that stop, sounding C (the C two octaves below middle C). It is these 8′ stops which sound at the same pitch as a piano. A 4′ stop (so called because its C pipe is approximately four feet long) speaks an octave above an 8′ stop.

A stopped pipe is one in which the top end of the pipe is blocked ("stopped") meaning the air travels up and back down the pipe, doubling the length of the sound column. This means a stopped pipe of 4′ length will sound as an 8′ stop. This has many space saving advantages as well as varying tone qualities available. This would mean that the longest pipe in an 8′ Stopped Diapason is actually only 4′ long. A gedackt is another example of a stopped pipe.

Stopped pipes tend to be gentler and sweeter in tone than open pipes, though this is largely at the discretion of the voicer. Stopped pipes are also used in situations in which there is limited space and/or money available, and a full-length rank of pipes (especially bass pipes of 16′ and 32′ length) would take up too much space and/or be too costly to build. It is quite common for the lowest few notes of an otherwise open rank of pipes to be stopped for this reason.

See Flue pipe#Variations in timbre and Reed pipe#Variations in timbre for more information on this topic.

A pipe's tone color can be affected to certain degrees by many variables. The degree to which a particular variable affects a pipe's tone color depends upon whether the pipe is of the flue or reed variety. More important than any of these considerations, however, is the way in which the organ builder voices the pipes during installation. A skilled pipe voicer can coax a stunning variety of sounds out of one pipe.

List of pipe organ stops