Information about Organ Pipe

An organ pipe is a sound-producing element of the pipe organ that resonates at a specific pitch when pressurized air (commonly referred to as wind) is driven through it. Each pipe is tuned to a specific note of the musical scale. A set of organ pipes of similar timbre tuned to a scale is known as a rank or a stop.

Construction

Materials

Organ pipes are generally made out of either metal or wood. Very rarely, glass, porcelain, plastic, or even stone pipes may be seen.

Metal

Metal pipes are usually made of an alloy of lead and tin, along with trace amounts of antimony and copper for increased rigidity. The percentage of each metal in the alloy depends on the desired characteristics of the resulting 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 desired if the pipe is clearly visible. The cost of each metal is also a factor, as tin is more expensive than lead. The usual exceptions to tin-lead alloys are very lowest pipes in a rank, which are sometimes made of zinc. In addition, pipes have been made of many metals, including copper, aluminium, gold, silver, brass, and iron.

Metal pipes are made by first casting a metal alloy of the desired metallurgical composition onto a long flat surface. Once the metal cools, it is cut into pieces, which are then rolled into shapes around molds called mandrels and soldered together. Thus, the cross-section of a metal pipe is usually circular.

Wood

The body of a wooden pipe is made of either a coniferous wood or hardwood, although the lower section of the pipe (comprising the foot, cap, block and mouth) will nearly always be made from hardwood to provide a precise edge for the pipe's mouth. Using screws and glue, the pipes are assembled from wooden pieces of various shapes and sizes. In contrast with the circular cross-section of a metal pipe, the cross-section of a wooden pipe is most commonly square or rectangular.

Shapes

The bodies of organ pipes are generally made in three shapes: cylindrical, conical, or rectangular. Cylindrical pipes are simple cylinders, while conical pipes form the shape of a cone, widening at the top. Rectangular pipes form cuboid shapes with either a square or rectangular cross-section when viewed from above. There are some irregular shapes as well: the Flûte triangulaire, for example, has a triangular cross-section when viewed from above). In addition, a cylindrical or rectangular pipe can be tapered: that is, it can be made to be wider at the bottom than at the top.

The end of the pipe opposite the reed or mouth may be either open or closed (also known as stopped). An open pipe produces a tone in which both the even-numbered and the odd-numbered partials are present, while a stopped pipe, such as a gedackt, produces a tone with only the odd-numbered partials. In addition, the wind travels both up and down the body of the pipe, doubling the length of the column of sound; thus, a stopped pipe sounds an octave lower than an open pipe of the same length. The tone of a stopped pipe tends to be gentler and sweeter than that of an open pipe, though this is largely at the discretion of the voicer.

Pitch

For more about the pitch of organ stops, see Organ stop: Pitch and length.

The pitch produced by an organ pipe is a function of its length. The nomenclature of a rank of pipes, for example "8′ Open diapason" refers to the length of the longest pipe in the rank. Thus the longest pipe, C, (two octaves below middle C) is 8 feet long. An 8′ stop is sounds at unison pitch, like a piano. Much like a vibrating string, the frequency of the tone of a pipe half the length of another, will sound one octave higher. If the longest pipe, C, is 8′ in length, the pipe one octave higher will be 4′ long, and two octaves above (middle C) will be 2′ long.

Varieties

Flue pipes

Main article: Flue pipe

The sound of a flue pipe is produced solely from the vibration of air molecules, in the same manner as a recorder or a whistle: wind from the "flue", or windway is driven over an open window and against sharp lip called a Labium. This produces a siphon effect just below the window. When the vacuum under the window reaches a critical stage, the Airstream is pulled under the Labium lip to fill the vacuum and pressurize to the opposite value. This creates high and low pressure waves within the pipe's aircolumn. 1 high and 1 low pressure wave form a single "cycle" or 1 Hertz. The Labium produces a type of Von Karman vortex street. There are no moving parts in a flue pipe.

Flue pipes generally belong to one of three tonal families: flutes, diapasons (or principals), and strings. The basic "foundation" (from the French term fonds) sound of an organ is composed of varying combinations of these three tonal groups, depending upon the particular organ and the literature being played.

The different sounds of these tonal families of pipes arise from their individual construction. The tone of a flue pipe is affected by the size and shape of the pipes as well as the material out of which it is made. A pipe with a wide diameter will tend to produce a flute tone, a pipe with a medium diameter a diapason tone, and a pipe with a narrow diameter a string tone. The reason for this is the strength of the soundwaves. A large diameter pipe will favor/not restrict weak high frequncy harmonics, while a narrow diameter pipe will suppress/restrict these high harmonics to favor lower frequency tone color. The science of measuring and deciding upon pipe diameters is referred to as pipe scaling, and the resulting measurements are referred to as the scale of the pipe. Ranks of all three tonal families can be either open or stopped, although flutes are by far the most common of the three to be stopped.

Reed pipes

Main article: Reed pipe

The sound of a reed pipe is produced by a beating reed: wind is directed towards a curved piece of brass (the reed). A partial vacuum is created by higher velocity air flowing under the Reed which causes it to be pulled closed against a hard surface called the shallot. This shuts off the vacuum and allows the Reed to spring open again. A tuned resonator extends above this assembly and reinforces the sound produced. The principle is the same as that of the orchestral clarinet. The pitch of a reed pipe is determined primarily by the length of the reed but the cubic volume of air in the resonator supports that frequency. Some Reed pipes have a slide to adjust the vibrating length of the Reed to fine tune it. Because of the precision required in the making of the vibrating reed,resonator pipe and its accompanying parts, reed pipes are more complicated to manufacture than flue pipes.

By altering any of several parameters (including the shape and cubic volume of the resonator, as well as the thickness and shape of the reed), a reed pipe can produce a wide variety of tonal colors. This allows reed stops to imitate historical musical instruments, such as the krumhorn or the regal. Because the resonator is partially stopped/closed by the Reed, odd-numbered partials/harmonics are dominant (in the hollow tones of Krumhorn and Clarinet stops, for example). If the resonator pipe expands outward to conical, the geometry allows the production of both even- and odd-numbered partials, resulting in the fuller tones of Trumpet and Oboe stops.

Diaphone pipes

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 spring-loaded pallet instigates the vibration instead of a reed. The pipe is generally made of wood and can be voiced at 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.

Hope-Jones also developed an imitative version of the diaphone 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 of the diaphonic horn 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. Due to its penetrating tone, the diaphone has also been used in foghorns and fire signals.

References