The Elderly Novice Organist

The Pipe Organ Explained

This page is intended to give the uninitiated reader a good understanding of the basic principles of the pipe organ.

0 Introduction

The pipe organ is a wind instrument that produces sound by driving pressurized air through selected pipes of various shapes and sizes. It is also a keyboard instrument played on manuals (normally one to five), plus usually a pedalboard. The console also has controls to enable and facilitate selection of pipes of specific types.

A large pipe organ has thousands of pipes, which can produce a wide variety of sounds covering the full range of human hearing. However, each pipe is voiced to work at a specific wind pressure, and (unlike an orchestral instrument) has a fixed pitch, volume level, and sound quality (timbre). In other words, a pipe is either ON or OFF, depending on whether its airway is open or closed.

As the sound made by each pipe is fixed, volume and sound quality are set by selecting appropriate combinations of pipes. This art, called registration, is a fundamental part of the interpretation of organ music. The quietest sounds of the organ are produced by using only a single soft pipe; the loudest by deploying numerous pipes simultaneously (which on a large organ could be several hundred).

The pipe organ is perhaps the most complex device built by man before the telephone exchange. Each is built individually according to user requirements and the building in which it is to be used; there is no mass production. Each pipe is unique, and voiced in its location to achieve a proper balance with the others. This is one reason pipe organs are so expensive; the cost of a large instrument is measured in millions of dollars.

Pipe organs vary considerably in size, age, style, nationality, and numerous other design factors. The smallest organs have only a few tens of pipes; the largest has over 33,000. The pipe organ is not only the largest, most complex, and most expensive musical instrument; it is also the most diverse.

1 Console

The main controls on the console can be divided into the following areas:

1 to 7 manuals, plus usually a pedalboard.
Stop Controls
Typically located to the left and right of the manuals.
Located under the keys of the manuals.
A vertical panel located above and behind the pedalboard.

Keyboards and Stop Controls are present on all organs, and select which pipes are to sound. Pïstons and controls on the Kneeboard may not be present, especially on older instruments.

The organist sits over the pedalboard on a wooden bench, whose slippery surface permits the pivoting necessary to reach all the pedals. The height is critical, and normally adjustable to suit people of different physical stature. Shoes are of a special flexible type with leather soles (but many organists play in socks).

In organs with mechanical action, the console must be positioned close to the pipes. With electric action, the console may be in a completely different location. See Actions (Mechanical and Electric).

1.1 Keyboards

On the organ, there are two types of keyboard, with different names to distinguish them. Manuals are played by the hands, the Pedalboard (or pedals) with the feet. Organists normally use one of these two specific terms; on this page, the word keyboard is used to cover both types. Each keyboard normally operates on a Division of the organ.

Organ keyboards have the same layout as piano keyboards, with the 12-note octave comprising five raised keys in two groups (called sharps) and seven other keys (called naturals). The lowest note in both manuals and pedals is always C, which at a given stop pitch are at the same frequency; however, higher stop pitches are typically used in the manuals.

The keys of the manuals and pedals operate the key action; this and the stop action determine which pipes will sound - see Actions and Wind. As keys open and close airways rather than operate hammers, each note is sustained at full volume for as long as the key is held down, and terminates immediately when the key is released. This makes organ keyboard technique quite different from that of the piano - see Comparison with the Piano.

1.1.1 Manuals

A manual is similar to a piano keyboard, except that that it normally covers no more than 5 complete octaves (61 notes), compared to the 7 octaves or more of the piano. In older organs and some modern organs, there may be significantly fewer keys. However, with stops of different pitches (see Pipe Lengths and Pitches), the compass is potentially greater than that of a piano. At standard (8') pitch, the middle C on the manual produces the same note as the middle C on the piano.

It is important to use contrasting colors for the sharps and naturals. The sharps are most commonly black, with white naturals. However, on some organ manuals, the naturals are black, and the sharps a light wood color (as in the above photo); this is a purely cosmetic matter.

Having multiple manuals makes it quick and easy to change from one division to another, for each hand to play on a different division (for example, to distinguish between solo voice and accompaniment), and it is even possible (to some extent) to play two manuals simultaneously with the same hand.

1.1.2 Pedalboard

A compass of a pedalboard is about half that of a manual; normally 30 or 32 notes, or two and a half octaves. 30 pedals are required even for some baroque music, although many historical organs have fewer.

A pedalboard may be either flat or concave. In a concave design, the outer pedals are higher than those in the middle, making them easier to reach. Most modern pipe organ pedalboards are concave, as this is generally preferred; however, the pedalboards of baroque organs are flat.

A pedalboard may also be either rectangular or radial. In a radial design, the pedals are further apart by the kneeboard than by the bench, providing a more natural angle for the feet; the downside is the variable spacing between the pedals. Radial designs tend to be more common in the US and UK, rectangular designs more common in continental Europe.

Sharps in the pedals are much more elevated than those in the manuals. Although they are often black, the use of contrasting colors for the pedals is unnecessary, and wood finish may be used throughout.

1.2 Stop Controls

Most of these are for speaking stops that open and shut the airways to complete ranks of pipes, thus enabling the sounds of the organ to be selected - see Stop.

Apart from speaking stops, in this area may be found accessory stops, including:

Combine Divisions of the organ.
Create a wavering effect by varying the wind supply.
Toy Stops
Sound effects (such as drums) and semi-musical effects (such as bells) that do not involve pipes.

The traditional type of stop control is a drawknob, which is pulled out to select the stop, and pushed in to deselect it. Another commonly-used type of stop control is the tilting tablet (tab). Although stop controls are most commonly located on jambs to the left and right of the manuals, some organs have one or more rows of tabs positioned above the manuals.

1.3 Pistons

These controls (located under the keys of the manuals) are mainly playing aids that make it easy to draw a large number of stops. Pressing one of these pistons normally recalls a pre-recorded combination, thus enabling that combination to be selected with a single press. There will be a Set piston to enable a combination to be saved. There is usually also a Cancel piston to cancel any drawn stops, thus facilitating a fresh selection.

When Set is active, the recording mode is entered; pressing a piston then saves the current combination to that piston. Pressing Set again exits this mode (pressing a combination piston then recalls the saved combination). A piston may be be either general (applying to all divisions) or divisional (applying only to the keyboard on which it is located). Some pistons may have fixed combinations set by the organ builder.

An important function is the combination stepper, which allows the organist to step through a sequence of general combinations. This is pre-programmed with all the combinations required for the music to be played, in the order in which they appear. Only two pistons are required (typically duplicated on each manual and kneeboard for convenience); one to select the next combination, and the other for the previous one. This approach is increasingly being used in preference to managing large numbers of divisional and general pistons.

During the baroque period, there was no provision at all for storing and recalling combinations; this limited the frequency with which registrations could be changed. Later developments allowed mechanically-operated presets. But with an electric stop action, and especially with computer electronics, it is easy to provide a large number of memories and various means of using them. On many new organs, different sets of presets can be loaded for different organists. MIDI makes it possible to record a performance in terms of stop, key, and expression pedal activity.

1.4 Kneeboard

This vertical panel above and behind the pedalboard (also called kickboard) may contain the following types of control:

Expression Pedals
Allow dynamic change of the volume of sound.
Toe Studs (Pistons)
Have similar functions to pistons on the manuals. Some may be divisional pistons for the pedals, others duplicate some of the pistons on the manuals, to enable activation when the hands are busy.
Coupler Controls
Used on organs with mechanical key action; they are latched down by means of a spring - see Coupler. Similar controls may be used for Tremulants.

1.4.1 Expression Pedals

These are of two types:

A swell pedal exists for each enclosed Division, to gradually open and close the shutters.
This is available on some organs to successively add and remove stops in a predetermined sequence.

Both types of pedal enable the volume to be changed continuously while playing; they affect previously keyed and held notes, not just those played after the pedal is operated. However, neither is a true volume control, as they also change the sound quality.

Expression pedals did not exist during the baroque period, and are not present on modern organs that are purely neo-baroque. The swell pedal was introduced in England around the beginning of the 18th century; it is a normal feature of romantic-period and later organs that are not specifically neo-baroque. The crescendo pedal was introduced in the mid 19th century, and is found mainly on the larger organs.

2 Pipe Organization

2.1 Rank

This is a row of pipes producing the same type of sound. Each rank contains a separate pipe for each note on its corresponding keyboard; thus a manual rank requires typically 61 pipes, a pedal rank 30 or 32 pipes.

Pipes within each rank vary greatly in size; for example, on a 61-key manual (5 complete octaves), the speaking length of a flue pipe for the lowest note is 32 times (25) greater than that for the highest note. See Pipe Lengths and Pitches.

A large organ may have well over one hundred ranks; the smallest possible organ has only a single rank.

2.2 Stop

The term stop properly refers to the valve that opens and closes the airway to a set of pipes that forms one of the sounds of the organ. However, it is commonly used to refer to any of:

There are hundreds of types of stop with many names and sounds, which vary according to the country of origin and also historically. The sections Pipe Classification and Special Stops describe a few illustrative examples. They also have different pitches - see Stop Pitch.

The majority of stops are designed to blend with other stops; they change the sound without being heard distinctly in themselves. Registration is then akin to mixing paint. However, certain stops (often designated solo) are designed to stand out from the others.

Most stops comprise only a single rank, but as some stops are multi-rank, an organ will typically have more ranks than the number of speaking stops on the organ console.

2.3 Division

Stops are grouped into divisions, each of which is normally associated with a keyboard. Thus an organ with three manuals and pedals normally has four divisions. In organ specifications and registrations, each manual is often identified by a Roman numeral (I, II, III, ...), and the pedalboard by the letter P. Each division has a particular characteristic, and can be thought of as a separate instrument. However, two divisions may be combined using a Coupler, causing notes played on one keyboard to also be applied to another.

Many organs have at least one enclosed division, often with the name Swell (or the national language equivalent). The pipes in such a division are enclosed in a chamber with shutters (similar to a Venetian blind) that may be opened and closed using a swell pedal (one of two types of expression pedal). This also changes the sound quality.

As with stops, division names and characteristics vary with the country of origin and organ type. An English or American organ with three manuals might have divisions named Great (usually played from the middle manual), Choir (usually played from the lower manual), Swell (usually played from the upper manual), and the Pedals. The Great organ is meant to sound broad and majestic, while the Choir (a corruption of chair) organ is likely to have a smaller, brighter sound.

In many cases, the same stop name appears in more than one division; for example, a Principal 8' may appear in both Great and Choir divisions. These will normally be separate ranks, each voiced to be compatible with its division (that in the Great is likely to be stronger than that in the Choir). However, in some cases, the same rank is used; this is called borrowing (borrowed stops should be clearly indicated as such).

2.4 Coupler

This enables the stops selected on one division to be played from a keyboard corresponding to another division (usually in addition to those that would otherwise be played). For example, if a manual is coupled to the pedals, pressing a pedal has the effect of also pressing the corresponding key in that manual. This is common practice in music with an elaborate bass line (such as in J S Bach), to give an incisive sound that could not be achieved using stops in the pedal division alone.

Organs typically provide a set of couplers enabling each manual to be coupled to one of a lower number, or to the pedals. For example, on an organ with three manuals and pedals, the basic set of couplers is: III → II, III → I, III → P, II → I, II → P, I → P (6 couplers). These can be combined as required. Then (for example), if couplers III → I and II → I are selected, pressing a key on manual I will have the effect of also pressing that key on both of manuals II and III.

On some organs, it is also possible to couple at a different pitch, normally an octave higher or lower; such couplers are known as super-octave or sub-octave couplers, respectively. This will result in keys on the linked division an octave higher or lower being sounded in addition to the unison keys.

Many organs also provide a Unison Off option so that only the coupled keys sound, not those from which they were coupled. This facility is particularly useful in combination with the ability to couple a division to itself (via super-octave or sub-octave couplers), and may also be used to reassign manuals.

On some large organs, there are more divisions than there are keyboards. A division not normally connected to a keyboard is known as a floating division; this may be connected to a keyboard as required by means of a coupler.

With mechanical coupling, the mechanical linkage results in the keys on the coupled keyboard being physically moved in tandem with those played. For example, if the Choir division is coupled to the Great, when the player presses a key on the Great, the same key in the keyboard for the Choir division will also be depressed. There are also special latchable mechanical coupler controls on the Kneeboard.

With electric coupling, there is no visible effect (the connection is internal), but the end effect is the same as in the physical connection. The controls are normally located with the Stop Controls.

3 Pipe Classification

Pipes used in nearly all organs fall into two classes:

Produce sound by blowing wind through an aperture, in the same way as a flute.
Produce sound using a vibrating metal tongue, with amplification by a resonator.

Both are voiced to work only at a specific wind pressure; the sound from each pipe is fixed in pitch, timbre, and volume.

3.1 Flues

These have no moving parts, and produce sound by blowing air through an aperture (in the same way as a flute or recorder). There is a foot through which air is blown, a body above that comes in various shapes and sizes, and in between a narrow hole. The air blown in causes the air column in the body to vibrate.

The pitch of the note depends on the length of the body containing the air column (the foot is not a speaking part of the pipe). The timbre depends on the shape of the pipe, and especially its diameter in relation to the speaking length.

Flue pipes may be either open at the top end or stopped, the latter being only half the length for a given pitch. They may be of either metal (normally with a circular cross section) or wood (with usually a square or rectangular cross section).

Good-quality metal pipes are made of a tin/lead alloy. These metals are expensive and dense, but are soft and easy to work; this allows the very fine adjustments necessary to achieve high-quality voicing. Tin is the more expensive, but has a brighter appearance than lead; alloys higher in tin are therefore likely to be used for pipes at the front. Zinc is also used, but for economic reasons.

The majority (usually more than three quarters) of organ ranks are of flue pipes. For convenience, they are divided into three categories according to their scale (diameter in relation to the length):

Medium Scale
Large Scale
Small Scale

3.1.1 Principals

These produce the sound characteristic of a pipe organ (similar to an "ah"). They tend to be equally strong from low notes to high. Many such stops are named simply Principal; other commonly-used names for principal-scale stops include:

Another name for Principal in English and American organs.
Principal an octave above standard pitch, typically used with the standard principal.
Principal two octaves above standard pitch, again often used with the 8' principal.
A strongly-voiced Octave principal in the pedal division, used for cantus firmus.
A principal often mounted at the front of the organ case (also Montre).

3.1.2 Flutes

These have a broad scale, and produce sound that is softer and fuller-bodied than a principal (similar to an "oh" or "oo"). They tend to sound stronger in the high range, and weaker in the low range. Illustrative examples are:

A wide-scaled conical or stopped flute, named after the recorder, with similar sound.
A stopped wooden flute of large scale.
An open manual stop first made of wood, later of metal.
Perhaps the most common stopped flute stop.
Harmonic Flute
An open metal flute made to sound an octave above its length by means of a small hole at its midpoint.
Describes a variety of flute stops; may be metal or wooden, open or stopped.
Wide scaled with a relatively small mouth. Produces a soft but penetrating sound.
Stopped flute with a prominent twelfth.
A semi-capped metal pipe with a narrow open-ended tube extending from the top that resembles a reed.
An open flute stop of metal with pipes tapered to a point at the top.
Tibia Clausa
A stopped wood flute pipe of large scale, usually with a leathered tip.
A soft flute stop, usually made of wood.

3.1.3 Strings

These have a narrow scale, and produce sharper but weaker sound than a principal (similar to an "ee"). They tend to be stronger in the low range. They include those with names containing Viol or Geigen, and:

Metal string stop; usually the softest on the organ.
Open metal or wood pipes, often tapered; tone has a sharp "stringy" quality.
One of the earliest string stops, named after the baroque instrument.
Nearly always of open cylindrical metal pipes; softer in tone than the Gamba.

3.2 Reeds

These produce sound using a vibrating metal tongue, which is amplified by a resonator. Air is blown into a boot containing the tongue, which vibrates against a shallot, both held in place by a wooden wedge. A tuning wire enables the pitch to be adjusted by changing the vibrating length of the tongue.

Unlike flues, reed pipe resonators simply reinforce certain Harmonics of the sound wave, and there is no fixed relationship between pitch and pipe length. However, the length and shape of the resonator greatly affect the volume and timbre (longer resonators give greater power). Timbre is also affected by the thickness and curve of the tongue.

Reed pipes vary greatly in quality, and include both the loudest in the organ and quiet ones. They tend to be strongest in the middle of the range. Means of classifying include:

Conical reeds include both strong-toned ones and imitative ones. Cylindrical reeds have prominent even-numbered harmonics and include baroque reeds and imitative reeds. Reeds with long resonators tend to be the strong-toned ones. Chorus reeds are intended to be used in combination with other voices, solo reeds to be heard as separate voices.

Some commonly-found examples are:

Powerful conical chorus reed with a brassy timbre.
Cylindrical imitative reed.
Trumpet-type conical chorus reed.
Cor anglais
Imitative conical reed.
Chorus and solo reed like a large-scale Trumpet.
Cylindrical baroque solo reed with a short resonator that has a buzzing or bleating sound.
Cylindrical baroque reed with tone similar to a bassoon.
Inverted conical construction, softer than Tumpet or Trombone.
French Horn
Imitative conical reed.
(also Hautbois) Used as both a solo and a chorus reed. Larger scale than Orchestral Oboe.
High-pressure chorus reed with powerful sound, much like the Bombarde or Trombone.
Orchestral Oboe
Imitative conical reed.
Has fractional-length resonators; produces a buzzy sound with little fundamental tone.
(shawm) Baroque "color" reed. Usually cylindrical; some are conical.
Chorus reed simulating the trombone; similar tone to Bombarde and Ophicleide.
Trompette en Chamade
Solo trumpet laid horizontally; can often be heard over full organ.
Trompette Militaire
Powerful solo reed of the trumpet family, with a brassy, penetrating tone.
(also Trompette) A loud chorus reed stop, with inverted conical resonators.
Large-scale, high-pressure smooth solo reed. Not named after the orchestral tuba.
Vox Humana
Organ stop of the Regal class, supposedly intended to imitate the human voice.

4 Pipe Lengths and Pitches

4.1 Stop Pitch

The pitch produced by a flue pipe depends on the speaking length of the pipe (the resonating column of air, which excludes the foot). Halving the length doubles the pitch, thus producing a note an octave higher.

The pitch of an organ stop is given in feet. This is the speaking length of the longest (and thus lowest pitched) open flue pipe, which produces the bottom C in both manuals and pedals. Although the use of feet to indicate pitch may seem incongruous, it is in many ways convenient, as this length is close to a power of 2. Each single-rank stop is usually marked with this number, which on a large organ may range from 1' to 32'.

For example, a stop marked as Principal 8' on a 61-key manual (five octaves) has pipes ranging in length from 8 feet to 3 inches (a factor of 32, or 25). 8' pitch is the standard, giving a lowest note on the manual or pedalboard of just over 64 Hz; the middle C on the manual is then at the same pitch (around 256 Hz) as the middle C on a piano. Octave 4' sounds one octave higher, Principal 16' sounds one octave lower, and so on.

The range of stops from 1' to 32' on a large organ gives fundamental frequencies that span the entire range of human hearing, from 16 Hz (the lowest 32' note) to over 16 kHz (the highest 1' note). In other words, the organ can produce sounds from an earthquake to a dog whistle, and everything in between.

The pipe length / pitch relationships indicated above hold only for open flue pipes. Stopped flue pipes sound an octave lower than open flues of the same length. There is no direct relationship between pipe length and pitch with reeds. Nonetheless, in all cases the pitch is indicated by the length in feet of an open flue. So, for example, the lowest note of a stop indicated as 16' will always have a pitch of about 32 Hz, regardless of the type of pipe.

4.2 Harmonics

A musical instrument produces notes consisting of a fundamental tone, plus overtones (harmonics) that are integer multiples of it. For example, when an oboe sounds concert A=440 Hz, in addition to the fundamental tone of 440 Hz, it produces a 2nd harmonic at 880 Hz, a 3rd at 1320 Hz, a 4th at 1760 Hz, and so on. This is referred to as the harmonic series, and the timbre of an instrument is determined by the proportions of its harmonics in this series.

A tuning fork produces a dull tone that is almost entirely fundamental, the flute (both instrument and organ stop) has a full sound containing a large proportion of fundamental, whereas a violin has a thinner and brighter sound much richer in upper harmonics (especially the 7th) with less fundamental. Harmonics that are odd multiples of the fundamental tend to give a harder/edgier sound than those that are even, but all harmonics add color.

All these harmonic components are fused by the ear into a single note, whose pitch is the fundamental frequency. In fact, given only the harmonics, the ear can often reconstruct the fundamental. For example, if an organ pedal note is played on loudspeakers incapable of reproducing the fundamental frequency (as will often be the case), the ear can nonetheless perceive the depth of the note if there is sufficient harmonic content; this would be the case with pedal reeds, but not necessarily flues.

This is of especial relevance to the organ, as flue pipes tend to be heavy in fundamental, and it is often necessary to give them more brilliance. For this reason, the 8' principal is often combined with 4' and 2' principals to brighten the sound (by adding 2nd and 4th harmonics). This "principal chorus" with added octave harmonics may in turn be enriched with other stops that add more harmonics. The next section covers stops that add high and non-octave harmonics to give extra brilliance to the sound. They include base pipe lengths are not powers of two, but fractions derived from the harmonic number.

The names of many of these stops refer to the interval in the musical scale, not the number in the harmonic series. For example, the name Terz denotes a third, but generates the 5th harmonic (which is E, the major third in the musical scale based on C). Similarly, Quint indicates a fifth, but generates the 3rd harmonic (G in the scale based on C). These are usually indicated as fractions in relation to the base pitch; for example, a Terz is indicated as 1 3/5' (= 8' / 5), and a Quint as 5 1/3' (= 16' / 3).

4.3 Special Stops

Mutations and mixtures are used to enrich the harmonic content of unison stops, and are found on nearly all organs. They are always tuned pure, in line with the harmonic series. This may result in beating with pipes that are tuned according to the organ Temperament. With equal temperament, these are greater with a stop such as Terz that adds a major third (5th harmonic). Conversely, with meantone temperament, there is a greater discrepancy with a stop such as Quint or Nasard (perfect fifth, or 3rd harmonic).

4.3.1 Mutation

Comprises a single non-octave rank, which may be either principal-scale or flute-scale. Intended to be used only with octave-pitch stops to add harmonics.

Principal-scale mutations include Terz (1 3/5') and Twelfth (2 2/3'). Flute-scale mutations include Nasard (2 2/3'), Terz (1 3/5'), Larigot (1 1/3'), Septième (1 1/7' or 2 2/7'), None (8/9'), and Quint (5 1/3' or 10 2/3').

4.3.2 Mixture

Comprises multiple ranks that typically include non-octaves (most commonly fifths); all are principal-scale. Again, they are to add upper harmonics to octave-pitch stops. The number of ranks is often shown by a Roman numeral (e.g. IV for a 4-rank mixture).

They may be described simply as Mixture, or have special names (e.g. Plein jeu, Scharf). Mixtures differ from all other stops in that in the manuals they "break back"; that is, they repeat at intervals of usually either an octave or a fifth.

4.3.3 Cornet and Sesquialtera

Both these are multi-rank stops; however, unlike mixtures, they are wide scale and do not "break back". They may be present only in the upper part of the manual.

Cornet (has nothing to do with the modern brass instrument) comprises five ranks giving the first five pitches in the harmonic series: 8', 4', 2 2/3', 2', 1 3/5'. It may thus be used alone, and with a soft but prominent sound, may dominate the registration.

If the stop contains fewer than five ranks, one or more other similar stops must be drawn with it to complete the series: hence Cornet IV requires an 8' flute to be drawn, and Cornet III requires both 8' and 4' flutes.

Sesquialtera usually comprises two ranks at 2 2/3' and 1 3/5' (in other words, the non-octave pitches of the Cornet).

4.3.4 Undulating

Comprises two similarly-voiced ranks, one tuned slightly sharp or flat of the other. When played together, a beat frequency effect is produced. The stop may be either two-rank containing both detuned and normal ranks, or else contain only the detuned rank and require another similar stop to be drawn with it.

Unda Maris is an undulating flute stop. Aeoline and Voix céleste are undulating string stops.

4.3.5 Resultant

Provides a stop of lower pitch than the ranks involved, using difference tones. For example, a Gravissima 64' stop can be produced from 32' and 21 2/3' ranks (only two organs in the world have a true 64' rank).

4.3.6 Toy

Unlike all others in this section, these are accessory stops, not speaking stops. They include percussions (melodic such as Chimes, and non-melodic), Bird Whistle, and others. They are especially abundant on Theatre Organs (from which the term "toy" originates), but baroque organs may also feature this type of stop.

4.4 Temperament

Temperament refers to adjustments to deal with discrepancies that occur between:

The octave is always untempered (pure), with a constant frequency ratio of 2. However, other intervals depend on the temperament used. The two basic intervals considered (here, and historically) are:

Perfect Fifths
with a pure frequency ratio of 3/2 (3rd harmonic over 2nd)
Major Thirds
with a pure frequency ratio of 5/4 (5th harmonic over 4th)

Temperaments involving these intervals also affect the perfect fourth (the remainder of the octave after taking out the perfect fifth), and the minor sixth (an octave minus a major third). This extends to other intervals (for example, a tone is half a major third).

Temperament is a compromise that perturbs pure intervals to make them fit the scale; if excessive, it can lead to obvious sour "wolf" notes. Some temperaments (meantone) are aimed at preserving the major thirds (and thus also tones). Others (Pythagorean) are based on perfect fifths (and thus perfect fourths). Except for equal temperament, the intervals differ with the key; "wolf" intervals may occur with some keys, while other keys give purer results.

Temperament is pertinent to keyboard instruments, which must be tuned to give the required pitches before they are played. Singers and players of most other instruments can make pitch adjustments on the fly (this is referred to as intonation). To understand why temperament is necessary, we compare the frequency ratios from pure intervals with those that arise from the 12-note octave.

4.4.1 Perfect Fifths

The circle of fifths is generated by starting with a note (here C), and successively adding perfect fifths. This gives the following sequence:

C, G, D, A, E, B, F♯, C♯, A♭, E♭, B♭, F, C

Thus, after stepping through all 12 notes, the sequence reverts to C, but 7 octaves higher.

The frequency ratio of this 7-octave interval is 27, or 128. However, this interval is also 12 perfect fifths. If these are pure, the ratio is (3/2)12, or about 129.75.

As pure perfect fifths are too large to be congruent with the octave, they must be tempered downwards.

4.4.2 Major Thirds

Successively adding major thirds to the note C gives:

C, E, A♭, C

Thus the sequence reverts to C an octave higher after stepping through 3 thirds.

The frequency ratio of this octave interval is 2. However, this interval is also 3 major thirds. If these are pure, the ratio is (5/4)3, or about 1.9531.

As pure major thirds are too small to be congruent with the octave, they must be tempered upwards.

4.4.3 Equal Temperament

This is the standard used almost universally today, except in performance of early music. It is unique in providing the same intervals in all keys. This is achieved by dividing the 12-note octave into exactly equal intervals (ratios). Thus the ratio of frequencies in successive semitones is 21/12, or about 1.05946.

So in equal temperament, the ratio of a perfect fifth (7 semitones) is 27/12, or about 1.4983. This is less than the pure ratio of 1.5, but is actually quite close. However, the major third, being 21/3 (about 1.2599) deviates much more from the pure ratio of 1.25. Equal temperament thus provides quite pure perfect fifths and fourths, at the expense of major thirds and other intervals.

4.5 Tuning

The pitch and temperament of the organ are determined during its construction; it would not normally be practical to change these later. So tuning is confined to correcting deviations from the original pitch and temperament. Nowadays, equal temperament with a pitch of A=440 Hz is the norm, although some modern instruments use unequal temperament.

Tuning an organ is a critical and involved process. All the perhaps thousands of pipes in the organ must be in tune with each other; those out of tune are likely to produce a noticeable beating effect. Each pipe is adjusted to achieve zero beating with the pipe to which it is tuned.

The tuning process begins with a tuning stop, to which most of the other stops will be tuned in turn. This is usually a principal stop at 4' pitch. The middle octave is tuned first, using an electronic tuning device to generate the required frequencies. Then the rest of the stop is tuned to itself in octaves, working outwards to the ends of the keyboard. Once the tuning stop is in tune with itself, it is used as a basis for tuning most of the remaining stops.

Adjustments are made by tapping the tuning mechanism of the pipe with a tuning knife, to avoid touching the pipe with the hands. There are several means of adjusting the tuning, depending on the construction of the pipe. For example, metal open flue pipes usually have a sliding collar, and for stopped flues the stopper is moved up or down. Different methods apply to reeds, all of which may also affect the tonal properties; thus tuning reeds is more problematic than tuning flues.

Organ pipes are very sensitive to temperature; so much so that the body heat of the organ tuner can affect the tuning. In addition, wooden pipes are affected by humidity. It is therefore desirable to maintain even temperature and humidity to reduce pitch fluctuations, and to minimize the frequency with which tuning is necessary.

5 Actions and Wind

Wind refers to the pressurized air fed to the pipes, which stand on one or more wind chests. Actions are concerned with directing the wind to the required pipes by opening and closing valves. The organ contains two separate actions that allow selection of both notes to be played, and the stops to be engaged for each note.

5.1 Stops and Keys

The two mechanisms that control switching of the wind supply to the pipes are:

Thus air enters a pipe and the pipe sounds only when both:

  1. its stop mechanism has been opened by means of a drawknob or other control on the console, and
  2. the corresponding key in the applicable keyboard is depressed (notes are sustained until key release).

The following popup images show that the stop and key actions correspond to two dimensions, giving a matrix of pipes that are open or shut.

This shows a single one-rank stop pulled, with a single key depressed. The effect is to open the airway to a single pipe.

This shows three stops pulled, one of which is a 3-rank mixture, giving a total of 5 open ranks. There are 4 keys depressed, and thus airways to 5x4 = 20 pipes are opened.

Opening and closing stops affects immediately any held keys; thus the registration may be changed mid note.

5.2 Mechanical and Electric

In old organs and some modern reproductions, both actions are carried out by mechanical linkages. The stop mechanism is activated by drawbars that move a plank to align holes in it with the pipes, thus requiring a long range of draw. Keys open the airway via a complex tracker mechanism using rods of wood or metal; as a result of this, and especially having to work against the wind pressure, key action can be very stiff and heavy, making the organ in some cases almost unplayable. Mechanical linkages also mean that the console must be located close to the pipes.

From the twentieth century, organs began using electric action in which the switching is done via electromagnets. This provides short stop draw action and light key action. Also, as pipes can be located well away from the console, both pipes and console can be placed as desired. Electric action consoles are often remotely located in the auditorium via either wired or wireless connection, thus enabling the organist to hear the sound more as the audience would.

But despite its disadvantages, many organs made within the last few decades have mechanical key action (even some quite large ones, which present more difficulties with wind resistance). This is because many organists prefer its tactile feel, with the ability to control the transition between on/off states by varying the speed at which the key is depressed/lifted. This enables the initial transient (known as "chiff") to be varied. However modern organs usually have electric stop action, thus making it easy to provide convenient features such as memory capable of holding numerous registrations that may be easily recalled via by a piston or other means. Some organs have both electric and mechanical consoles, thus getting the best of both worlds.

This shows an animation of a key action on a single open pipe, with two closed pipes.

This shows the layout of a slider wind chest.

5.3 Wind Supply

This used to be provided by men treading huge bellows in a side chamber of the organ, but is nowadays provided by an electric blower motor (old instruments still in use will normally have an electrified wind supply). For consistent sound, a constant pressure must be maintained; however, the amount of wind required varies considerably in accordance with the stops drawn and keys depressed. To deal with this, the organ has huge reservoirs and compensation mechanisms.

With electric blower motors, it has become possible to produce a rock-steady wind supply. However, this may be detrimental as it robs the sound of irregularities that give naturalness to the sound. With too even a wind supply, the sound may seem synthesized and artificial; therefore organ builders allow some "give".

Wind pressure is usually from 60mm to 100mm of water, but some stops (for example powerful reeds) may require significantly higher pressure (say 250mm). The highest wind pressure employed on any pipe organ is 2500mm for the Grand Ophicleide (and three other stops) of the Boardwalk Hall Auditorium Organ; however, this produces sounds better suited to fog signaling or security purposes than a musical instrument.😆

5.3.1 Tremulant

This is a wavering effect created by the wind channel being periodically partly opened and closed. Although tremulant controls are usually found among the console stop controls, they are accessory stops that modify the sound of speaking stops; they do not produce sound in themselves.

Tremulant can typically be applied independently to each manual Division. However, in some cases it can be applied to individual stops; in other cases, the same tremulant is applied to two or more divisions. It is not usually possible (nor is it generally considered desirable) to apply tremulant to the pedal division.

Tremulant is an important function, available in most organs. It has been around since the early 16th century. In some of the more recent organs, the rate of beat is adjustable from the console.

6 Some History

Instruments that employ the basic components of an organ (wind-raising device, wind chest with pipes, and a keyboard with valve mechanism admitting wind to the pipes) date back to antiquity. The Greek engineer Ctesibius of Alexandria is credited with inventing the organ in the 3rd century BC. This instrument, called the hydraulis, used water pressure. From the 2nd century AD, wind supply was by an inflated leather bag, and true bellows appeared in the 6th or 7th century.

Portable organs (portatives) with real keyboards are depicted in illuminated manuscripts from the 13th century. Large organs with chromatic compass, multiple manuals, and pedalboard date from the middle of the 14th century. The first documented permanent organ installation was in 1361 in Halberstadt, Germany. It used twenty bellows operated by ten men, and wind pressure was so high that the full strength of the arm was required to hold a key down.

Stop controls were introduced in the mid-15th century; before then, each manual controlled a fixed set of ranks at multiple pitches (known as the Blockwerk). But around 1450, controls allowed the ranks of the Blockwerk to be played individually, leading to the development of modern stop actions. However, the higher-pitched ranks of the Blockwerk remained grouped together under a single stop control; this arrangement evolved into the mixture.

During the renaissance and baroque periods, more sounds were introduced to the organ, including imitative ones such as the Krummhorn and the Viola da Gamba. By the 17th century, most of the sounds of the modern pipe organ were available. From this time, distinct national styles of organ building also began to emerge.

The baroque period is often considered the golden age of organ building, manifesting both exquisite craftsmanship and beautiful sound in ornate cases. There are many original baroque organs still being played. Also, many modern organs are neo-baroque, or at least incorporate elements of the baroque style. One reason for this is the importance of J S Bach to the organ repertory. Baroque organs of northern Germany, Holland and France were particularly highly developed, frequently having several divisions including an independent pedalboard. Those in England were less substantial, but the swell box appeared in English organs near the beginning of the 18th century.

The Classical period was a relatively inactive one for the organ, as the piano was preferred. During the Romantic period, the organ became larger and more expressive, sometimes featuring a crescendo pedal as well as one or more enclosed (swell) divisions. A warmer and richer sound was preferred, created by more 8' and 16' stops and less use of mixtures and other high-pitched stops. Higher wind pressures were used, leading to greater key resistance; the Barker lever was developed to alleviate this.

The early 20th century saw the construction of monster organs, instruments preoccupied with imitating orchestral sounds, and some mediocre factory-produced organs. However, by the mid-20th century, there was a return to the mechanical key action, lower wind pressures, narrower pipe scales, and greater use of mixtures of the baroque period.

7 Theatre (Cinema) Organs

Theatre organs (known as cinema organs in the UK) were derived from British and American church organs by Robert Hope-Jones, and built from 1911 to 1940 to accompany silent movies and for popular music. But although theatre organs have not been built for several decades, there are a number of extant examples, and they continue to have a following. They also demonstrate certain features not usually present in pipe organs found in churches and concert halls.

Consoles are located remotely via electro-pneumatic action. They are in the form of a horseshoe, and feature gaudy decoration with color-coded stop control tabs, plus numerous couplers and toy stops (such as sleigh bells, snare drum, klaxon, marimba). They often entertained audiences by rising dramatically from the orchestra pit to a thundering fanfare and brilliant illumination.

Double touch enables the player to play with different stops by applying extra pressure to the keys; thus solo voices and accompaniment could be played on the same manual. But perhaps the most notable feature of theatre organs is unification, which uses rank extension to create additional stops at different pitches.

For example, a 4' stop can be created from an 8' rank by adding an octave above; the 8' stop then excludes the top octave, and the 4' stop the bottom octave. Similarly, a 16' stop can be created from an 8' rank by adding an octave below. Using this means, more stops can be created from a given number of pipes, albeit with some limitations; this is particularly important with the limited space in which such organs operated.

Although more economical in the number of pipes than a classical organ, in theatre organs these tend to be of larger scale and operate under higher wind pressures. They include the diaphone (invented by Hope-Jones, and subsequently used in fog horns). However, pipework is hidden behind decorative grilles and enclosed in swell boxes, which affects both quality and quantity of sound.

The characteristic sound is the Tibia Clausa (a hooting flute stop, also introduced by Hope-Jones), rather than the Principal. Other pipe stops are mainly imitative of orchestral instruments, and there are also many percussion stops and other sound effects. Theatre organs have a heavy Tremulant, giving them their characteristic exaggerated vibrato.