From Inspired Acoustics Knowledge Base

1 Summary and General Description
2 Parts, mechanism, and sound production
3 History and development
- 3.1 Mechanical inventions
- 3.2 Electrical inventions
4 Construction of pipe organs
5 Tunings
6 Acoustic properties
7 Famous builders
8 Notable historical instruments
9 Pipe organ literature
10 Statistics of pipe organs
11 Virtual pipe organs
12 Further reading

Summary and General Description

A pipe organ is a keyboard instrument that produces sound by allowing forced air pressure (wind) to resonate through sets (ranks) of tuned flue- or reed pipes. Pipe organs are most commonly encountered in churches, and to a lesser extent in major concert halls; they are not simply large, majestic musical instruments, but beautiful pieces of art, as well. The pipe organ repertoire is particularly rich in solo music, but the organ is also frequently used to accompany choral and congregational singing, to accompany other solo instruments, plus participate in larger works specifically composed for organ and orchestra.

The pipe organ is the grandest musical instrument in terms of sheer size and acoustic scope, and has existed in essentially its current form since the 14th century (although other designs such as the hydraulis -- a hydraulic organ whose air pressure was ingeniously steadied by the weight of water) -- were already used in Antiquity.

Along with the mechanical pendulum operated clock, the pipe organ was considered one of the most complex human-made creations before the dawn of the Industrial Revolution. Organs (the "pipe" designation is generally assumed unless specifically noted otherwise) range in size from a single short keyboard at the small end, to huge instruments which may contain well over 10,000 pipes. A large modern organ typically boasts three- to five manuals of five octaves (up to 61 notes) each, with a 2-1/2 octave (up to 32 notes) pedalboard or pedalier.

Wolfgang Amadeus Mozart is credited for having declared the organ as "The King of Instruments." Some of the largest pipe organ installations boast pedal stops with 64-foot pipes. (A foot in this context refers to the "sonic foot" and the lowest note of a 64' stop sounds an 8Hz fundamental frequency, well below the threshold frequency of human hearing, but is a frequency that can be "felt" rather than heard.)

Perhaps the most distinctive feature of the organ is its dynamic range, which can span from the softest whispering sound to the most powerful "full organ" sound, with the special characteristic that such a sound can be sustained indefinitely by the organist. In contrast, sustained sounds of a piano tend to die out (decay in loudness) over time, wind instrument musicians eventually run out of air before having to take a breath, sustained sounds of string instruments eventually run out of "bow" and must stop, however momentarily. Not so with an organ; as long as the blower motors are functioning, at least one stop is drawn and at least one note is played, the organ's sound may be sustained essentially indefinitely, if desired.

Another interesting feature in the organ lies in its intrinsic "polyphonic" approach to creating sound in a given acoustic space: each set of pipes can be played simultaneously with those of any or all other stops.

The most unique feature of pipe organ sound is that the individual pipes sounds get truly mixed and interspersed only when they reach and interact with the acoustics of the listening environment, rather than from within the instrument itself. This latter feature is what differentiates true pipe organ sounds from their digital virtual organ and/or electronic organ counterparts -- in which the sound comes from loudspeakers which change the resultant electrical signal into a composite of tones being played.

Parts, mechanism, and sound production

Pipes

Flue pipes(*)

There are two main categories of organ pipes: flue pipes and reed pipes. Flue pipes (made of metal or wood) account for the majority of the stops of an average organ. The flue pipe consists of three main parts: the pipe foot, the mouth, and the pipe body or resonator. The pipe foot delivers compressed air, the mouth generates the sound and the pipe body defines the place for the air column to oscillate(**).

When a constant supply of compressed air is delivered to the mouth of the pipe, the speaking length of each pipe acts as an air resonator that develops standing waves in the column of air contained in each pipe. The oscillating air pressure is radiated as sound to the ambient air from the two openings of the flue pipe: 1) at the top end of the resonator, and 2) at the mouth of the pipe.

The flue pipe usually stands vertically on the wind-chest, with wind entering at the foot hole. The foot is separated from the speaking length by the languid, a flat plate; the only airway connection between the foot and the speaking length is a narrow slit called the flue. The wind emerges through the flue and strikes the upper lip, producing an audible resonant frequency of the air in the pipe, the pitch of which is determined by and amplified in resonance by the speaking length of the pipe.

(*) Please do not confuse the term "flue" pipes with "flute" pipes. Flue pipes are normally distinguished from reed pipes as described in the next footnote. "Flute" pipes are a subset of flue pipes, and describe a specific type of sound whose harmonic overtones mainly emphasize the fundamental pitch, but is relatively limited in terms of having few additional even or odd harmonic overtones. Even overtones refer to those overtone pitches that are pitched in octave intervals of the fundamental pitch. Stops that emphasize the even harmonics are mostly characterized as "Principal" stops. Odd harmonics (often emphasized in reed stops) usually sound at the distinctive pitches that are different than octaves of the original fundamental pitch of a given stop. Various "Mutation" stops may be of the flute family or of the principal family, both of which are largely classified as flue stops.

(**) There is a common misconception that the compressed air travels completely down the entire length of a flue pipe. In reality, the compressed air enters the pipe foot and exits at the mouth. The column of air residing in the pipe body is set into oscillation, also known as standing wave motion, and this motion is excited by the air exiting the mouth of the pipe. In direct contrast, the compressed air DOES TRAVEL the entire length of a reed pipe. This is different, because the compressed air is forced to pass by a metallic reed, and is not allowed to escape at the mouth of a reed pipe. Therefore, the compressed air continues its way out the end of the reed pipe.

The tone and sound power of a pipe is determined by many factors, including the pressure of the wind supply, the construction material used to make the pipe, the size of the foot hole, the width of the flue, the height and width of the mouth, and the scale, or the diameter of the pipe relative to its resonator. The construction material of which the pipe is made also exerts an influence on its final tone and power; it may be an alloy of lead and tin, wood, or, more rarely, pure tin or copper, and zinc for the bass pipes. The pipes may also vary in shape, a common variant being an upward taper in which the pipe is smaller in diameter at the top than at the mouth. Or, the top of the pipe may be completely closed by a stopper. Such a pipe is said to be stopped; a stopped pipe sounds an octave lower in pitch than an open pipe of the same speaking length^[1].

Reed pipes

Organ reeds were probably originally copied from instrumental prototypes. A reed stop may contain a beating reed like that of a reed contained in a clarinet's mouthpiece, or a free reed, which is allowed to flutter back and forth.

The shallot of a beating reed pipe is roughly cylindrical in shape, with its lower end closed and the upper end open. A section of the wall of the cylinder is cut away and finished off to a flat surface. The slit, or shallot opening, thus formed is covered by a thin brass tongue that is fixed to the upper end of the shallot. The tongue is curved and normally only partially covers the shallot opening. But, when wind enters the boot, the pressure of the wind momentarily forces the tongue against the shallot, completely closing the opening. Immediately, the modulus of elasticity (e.g., stiffness) of the brass asserts itself, and the tongue reverts to its curved shape, thus uncovering the opening. This process is repeated rapidly, and becomes the source of sound for the reed pipe.

The frequency of the pulsations of air entering the shallot is determined by the effective length of the reed and, in turn, determines the pitch of the note. From there, the air pulses pass into the tube, or resonator, which further stabilizes the pitch and refines the timbral quality of the note. Most reed resonators have a flared shape.

As in flue pipes, a wide scale (namely, a wide diameter in relation to a pipe’s speaking length) favors a fundamental tone, and a narrow scale favors a bright tone. Cylindrical resonators produce an effect similar to that of stopped flue pipes, the note being an octave lower than the equivalent flared pipe and the tone favoring the odd partials. Some reed pipes, such as the Voix Humaine, have very short resonators of quarter or eighth length. Those ranks of reed pipes whose resonators have no mathematical relationship to the pitch are known as regals; regal stops were popular in the 17th century, particularly with the North German school, and their use has been revived in modern times^[2].

Pipe foot numbering explained

QUICK SUMMARY

If you choose not to read this section in its entirety, the topic can be simply explained as follows:

When one selects an "8 foot stop" and plays "Middle C" on the organ keyboard, the pitch you hear is recognized as being in the same octave as Middle C of a Piano. 16' stops sound one octave lower, 4' and 2' stops sound one and two octaves higher, respectively, than Middle C.

BACKGROUND:

You may have noticed that speaking stops on an organ are labeled with numbers in addition to the names of the stops. For example: Bourdon 16', Trompette 8', Principal 4', Quint 2-2/3', Superoctave 2', Tierce 1-3/5', Larigot 1-1/3', Piccolo 1'. The numbers and their associated apostrophes refer to the "length" of the pipe in feet (1 foot = 12 inches = ~30 cm). In addition, you may have seen other speaking stops with Roman numerals associated with them. Examples include: Mixture IV, Cymbal IV-V, Sesquialtra II. These Roman numerals do not refer to the lengths of pipes, but rather the number of individual pipes associated with each note of the given stop.

When you physically look at a pipe organ whose speaking stops are exposed, one immediately recognizes that not all of the pipes are same length, and there are certainly many more lengths of pipes than those indicated on the names of the stops.

The purpose of this section is to clarify the mysterious numbers associated with the names of various speaking stops.

The pitch of any pipe is proportional to its speaking length. Most modern organs have a manual compass of five octaves (61 keys), from the second C below middle C to the third C above; an open pipe sounding the low C is about 8 feet (2.5 meters) in speaking length (64 vibrations per second). The shortest pipe in the same stop, is thus about 3 inches (8 centimeters) long (~2048 vibrations per second).

While large- and small-scale ranks often imitate the tones of flutes and bowed strings respectively, and are named accordingly, the most characteristic tone of the organ is produced by its Principal stops. These are of medium scale and moderate harmonic development – neither too dull nor bright.

From the earliest times, stops were arranged in "choruses", and the principal chorus is the very backbone of any organ. A chorus consists of stops of roughly similar tonal quality and power, but at a variety of octave-related pitches. A unison principal is known as Principal 8’ because of its longest (8-foot) C2 pipe. The next stop at an octave pitch would have the largest C2 pipe of 4 feet long. Next comes a 2-foot stop, while the sub-octave pitch is represented by a 16-foot stop. The top pipe of a 2-foot stop has a speaking length of only 19 millimeters (three-quarters of an inch), and this is about the practical upper limit to the speaking length of an organ pipe.

Because an organ with no stops higher in pitch than a 2-foot stop would be lacking in brilliance, most organs have so-called "mixture" stops, which have several high-pitched pipes assigned to each note; they are tuned in ways that reinforce the pitches of the natural harmonics of the regular stops. These mixture stops are so high in pitch that they cannot be carried right up to the top note (and they would be impractical to manufacture pipes with such tiny speaking lengths), so they break back an octave at some convenient point, sometimes even more than once. The result is a balance of power between bass and treble and a harmonious power that is completely characteristic of the organ, and can be produced in no other practical way. (As an aside, Maurice Ravel attempted to emulate mixture stops in his orchestral work Bolero, by having his flutes and piccolo double the melody line but at pitches that mimicked the natural harmonics (intervals of a twelfth and nineteenth of the regular melody line.)

Mixture stops also contain ranks sounding at pitches other than in octaves with the 8-foot Principal. In chorus mixtures, these normally sound at a fifth above the unison (e.g., G above C), although ranks sounding at a third above and even at a flat seventh can also be found. These quint- and third-sounding ranks reinforce the natural upper partials of the harmonic series (although they were included in organs long before this was well understood).

Off-unison ranks are also available as separate stops, mostly sounding at an interval of a 12th (an octave plus a fifth; 2-2/3’), 17th (two octaves plus a third; 1-3/5’), or 19th (two octaves plus a fifth; 1-1/3’) above the unison. These are used melodically to color the unison and octave stops, and they may be wide or narrow in scale. Such stops are known as mutation stops, as opposed to the mixtures, or chorus stops. Their use is essential for the historically correct performance of organ music^[3].

$f_{N'}=\frac{8}{N}\cdot f_{8'}$

For example, if the foundational tone of a 8' stop has a frequency of $f_{8'}=440\ \mathrm{Hz}$ then a Quint stop's frequency for the same note can be calculated as $f_{2\ 2/3'}=\frac{8}{\frac{8}{3}}\cdot 440=1320\ \mathrm{Hz}$ using $N'=2\ 2/3=8/3$ .

Stop and key mechanisms (action)

Wind system

Organ stops and ranks

A comprehensive encyclopedia of organ stops can be found here Encyclopedia of Organ Stops. The site contains many images showing the design of the different types of stop, as well as sound samples of the stops themselves.

Console and its tools

Keyboards

Couplers

Enclosure / swellbox / expression pedals

Crescendo wheel or crescendo pedal

A "crescendo" pedal is a large foot-operated pedal commonly found on medium-sized and larger pipe organs (as well as digital organs that imitate pipe organs), either partially or fully recessed within the organ console. The purpose of the crescendo pedal is to incrementally activate stops and couplers in a pre-determined order as the pedal is pressed forward; conversely, the crescendo pedal acts to retire (remove selected) stops in reverse order, as it is depressed backward.

The addition of stops, in order from quietest to loudest, creates the effect of a crescendo (and, likewise, a diminuendo, when the stops are retired). A "crescendo" wheel is usually found in larger organ installations where more travel is required to incrementally select a larger number of available stops in a pre-determined combination order.

The crescendo pedal is located directly above the pedalboard, to the right of any expression pedals that may be present. In actual use, the operation of the crescendo pedal usually does not physically move the draw knobs or stop tabs on the console; the stops are electronically activated inside the organ. Often an indicator light or lights will be present on the console to inform the organist when the crescendo pedal is activated and how far it is engaged. Ironically, when a crescendo wheel is featured in the console of a large organ, the crescendo wheel is often placed to the left of the traditional sweller pedals.

Combination action / Setzer

History and development

As its name implies, the pipe organ consists of pipes made of wood and/or metal. The first "pipes" that humans made were constructed from parts animals (such as a ram's horn or hollowed out leg bones of medium sized animals) and plants of hollowed-out bamboo, or small tree branches. Simple flutes made of bamboo could be blown, one at a time, or placed side-by-side, panpipe style, making possible the playing of simple tunes. Nevertheless, it is usually the bagpipe that is generally considered the organ’s most immediate ancestor, because of a wind reservoir (the so-called "bag" of bagpipe) of a sheep's bladder that was "inflated" by the exhaling lungs of the bagpipe performer. Its history goes back at least to the time of the Emperor Nero. Findings from the period prove that the pipe organ and its various ancestors did exist (e.g. the water organ [hydraulis] uncovered in 1931, Aquincum, Hungary). Many historical instruments still work today, the earliest surviving workable form of which, located in Sion, Switzerland dates to circa 1390.

Mechanical inventions

Electrical inventions

Construction of pipe organs

Processes

Materials

Intonation and its importance

Tunings

The history of tunings dates back to the ancient Greeks, more than a millenium before the first pipe organ was built. Various temperaments and tunings were developed for different instruments including pipe organs since the 12th century - based on empirical experiences and later human pitch perception studies.

Acoustic properties

Sound Pressure Levels of single pipes

According to a paper on the 2008 Internoise Conference in Shanghai, a single pipe of a flute stop measured one meter away in an anechoic chamber (i.e., soundproof room) produced sound pressure levels (SPL) between 88 dBW to 93 dBW depending on various wind pressures; a string stop excited 84-89 dBW while a diapason pipe made 89-97 dBW.

It is not easy to make accurate quantitative predictions (even within about +/- 10 dB) about the sound pressure levels of pipe organs in a given proposed musical space, because of the wide variation in size, geometry and acoustical absorption of the proposed space, as well as the power of the wind source and scale of the pipe design.

Overall sound power and dynamic range

Interaction with the room

Room influence on the player

Room influence on the organ pieces

Famous builders

Notable historical instruments

Pipe organ literature

French organ music

Thierry Escaich

Charles-Marie Widor

Statistics of pipe organs

According to a database containing around 3500 pipe organs of the world with around 50% of them in Germany, the majority of pipe organs available today were built in the 20th and 21st century.

The most common organs have ranks of between 30 to 85 speaking stops controlled by 2 to 4 manuals. Approximately 44% of the organs in the survey have at least one full-length 32' stop.

Virtual pipe organs

Hauptwerk, created by Martin Dyde is probably the most notable virtual organ software today. Using digitally recorded samples of actual sound of organs from all around the world, Hauptwerk gives the opportunity to anyone to enjoy the sounds of historic/famous organs while played at home or in a studio/classroom/church environment equipped with the minimum of a MIDI keyboard/pedalboard controller, a computer equipped with the Hauptwerk License and a given sound library, and a sound amplification system.

Pipe organ