Vestavia Hills Baptist Church,
Reuter Organ

Flue Timbre and Stop Names

Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body

Introduction

Flue pipes and the sounds they produce are most often classified today in one of four categories. Although other systems of classification may be used, this one has the advantage of being based first on the listener's impression of the sound, supported by physical characteristics of the pipes that produce those sounds. These four categories are:

One additional category - - Compound Stops - - has been added to the discussion below, because to the ear, these stops produce a quality of sound that has unique properties. Compound stops are often omitted from such classifications because they contain component pipes of one of the other four categories. Because they present a different quality of sound to the listener, however, they are included here as an additional fifth category of tone color in flue stops.

Each description of timbre in the sections that follow are concluded with a selection of audio files which demonstrate the tonal properties of the stops that are described. Different systems will render the recordings with more or less fidelity to the sound of the original, of course. However, the listener should not rely on the recordings but should make every effort to hear the listed stops on a good instrument in a favorable acoustic.

Principal Timbre

Principal PipeThe sound of a principal, the foremost category of flue tone color, is unique to the organ and is that timbre which is most often associated with the instrument. In general, the timbre of a principal can be described as a strong fundamental with well-distributed strength in the first few overtones. Of course, not all principal sounds are alike, either on different organs or in different locations. In some there is more emphasis on the fundamental, in others on the first overtone. They all, however, form the basic sound of the instrument, one that distinguishes it from all other wind-blown tone colors.

Just as the sounds of principals can be found to differ from organ to organ, so do their names. Some of the stop names are language specific, others are in more general use. They are not entirely limitless in their range however, and it is possible to learn quite quickly to recognize the names of principal stops. Stop names found in this group include the following, 6 which are all produced by open pipes, most often of metal, similar to the one in the photograph to the right. 37


Sounds of principals

These sound files contain a short passage played on several different combinations of principal timbres.


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Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


Flute Timbre

The second most common category of tone color in flue pipes is flutes, a similarity in name that can at times be confusing. Most organ flutes produce a sound that is very similar to that of flutes found outside the organ. There is a strong fundamental tone with relatively weak overtones. Flutes may be further divided into four sub-categories, each of which is discussed below.


The sound of a partially stopped flute is demonstrated with a passage played on an 8' Flûte à cheminée.

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Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


String Timbre

Organ String PipeThe sound of an organ string is very bright, with strong overtones. The open pipes are generally narrower than those of principals and flutes, and they are sometimes fitted with slots for tuning. 7 The degree to which an organ string sound actually resembles that of a real string instrument varies greatly from instrument to instrument and may in fact be a case of sounds that are but distant relatives. At least the names of most string stops are taken from the names of string instruments. 42


Organ string sound is demonstrated on an 8' Viole de gambe.


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Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


Hybrid Timbre

Cylindrical and Tapered Flue PipesHybrid sounds, as their name implies, take on some of the characteristics of more than one basic color of sound. The hybrid sounds that are most often encountered seem to be a combination of principal and flute, or of principal and string timbre. Although it is not always the case, most hybrid stops have pipes with conical rather than cylindrical resonators. The photograph shows a principal and an Erzähler of the same pitch. 43


The sound of hybrid stops is demonstrated in recordings of two different tapered flues.


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Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


Compound Stops

Although individual flues can be classified ad indicated above -- as principals, flutes, strings or hybrids, there is one group of flue stop names that is not included. These are the compound stops, but that name itself is a possible point of confusion. In this case "compound" does not imply a combination of timbres in a single pipe, but a combination of more than one pipe.

Although it is generally true that a single pipe sounds when a single key is played with one stop engaged, this is not the case when that stop is a compound stop. These stops - - also often called mixtures - - have more than one pipe for each key. Compound stops most often have individual pipes of principal tone, but some of them use flutes. String tone is much less common in a compound stop.

The names of compound stops often include a Roman numeral that indicates the number of pipes that will sound with a single key. Although these number may vary from as few as two to as many as ten or more, most compound stops have no more than five pipes to each key.

The sound of compound stops is demonstrated in two short examples:

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Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


Although all flue pipes contain the same basic parts, whether they are made of wood or of metal, the way in which these parts are built makes the difference in the sound that is produced by the pipe. Elements of construction which affect the timbre of a pipe include these major items.

Some of these elements are determined at the time the pipe is built, while others are altered when the pipe is voiced. This process is the one that actually determines the timbre produced by any pipe. Without careful voicing, a pipe cannot produce a useful tone color, and if some of the elements mentioned above are not adjusted properly, the pipe might not speak at all. The sections below treat these elements of flue pipe construction and their relationship to timbre.


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Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


Flue Timbre: Pipe Scales

The scale of a pipe is the ratio of its length to its diameter (measured at the mouth). Two primary elements of sound are influenced by the scale of a pipe.

Stopped Wooden Flute PipesThe first characteristic is illustrated by the two stopped pipes in the photograph to the right. 44 Both are wooden stopped flutes, and both sound the same pitch. The first pipe is a Tibia clausa, the largest in scale of all stopped flutes and the one that produces the most sound. The second is a Fernflöte. The name means "distant" flute, and it produces a very quiet sound.

One other characteristic affected by scale can also be seen in this photograph: In wider scale pipes, the body is slightly shorter than a narrower scale pipe that sounds the same pitch. This is equally true of open and stopped flue pipes, of either metal or wood.

Three Flue Pipes, Different Scales The second point - - that scale has an influence on tone color - - is illustrated by the pipes in the photograph to the left. 45 The first pipe is a narrow-scale pipe from a Voix céleste, a string rank with a bright tone. The center pipe is a principal, of what is sometimes called Normalmensur, or normal measurement. The third pipe is a flute, of wider scale than the other two. All three pipes sound the same pitch, and the same difference in the length of the three bodies can be seen that was illustrated by the two stopped in the photograph above. In this case, however, although the amount of sound produced by the three pipes is indeed different, it is the timbre of their sounds that is the most noticeable difference.

It might appear from the discussion above that all pipes in a given rank will have the same scale. In fact, this is not the case, and the stated scale of a rank applies only to its first pipe, the one that sounds when the lowest C on the keyboard is played. Moving up through the rank, the diameter does not decrease in direct proportion to the length of the body. If it did - - if, for example, the pipe an octave higher were both one half the length and one half the diameter of the first pipe, the ear would hear a different timbre, one much brighter than that of the first pipe and one lacking the same emphasis on the fundamental.

In order for the ear to perceive a consistent timbre from a rank of pipes, the scale must increase slightly as the pipes decrease in length. In general, the pipe with a diameter one half that of the first pipe is found around the seventeenth or eighteenth one of the rank. The number of this pipe is identified as the one on which the rank "halves," meaning the diameter of that pipe is one half that of the first pipe. It is common to say that a rank of pipes of any scale "halves" on the seventeenth note, or on the eighteenth, and so forth.


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Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


Flue Timbre: Modifications to the Mouth

The mouth of a flue pipe is rectangular in its fundamental shape. In some cases, the upper lip may be cut in a curve rather than in a straight line in order to modify the speech characteristics of the pipe, but the overall shape remains rectangular.

Flute and Principal PipesThe width of the mouth is determined when the pipe is built, and in metal pipes it is usually measured as a fraction of the circumference. In wooden pipes, the width of the front of the pipe determines the mouth width. In general, wider mouths produce more sound from a pipe, because they present proportionately more surface to the wind stream. The photograph to the right shows two pipes that sound middle c. The pipe on the left is a flute, with a narrow mouth whose width is only a small fraction of the circumference of the pipe itself. The pipe on the right is a principal, with a mouth that is wider in relation to the circumference of the pipe, even though the scale is smaller.

Principal and String MouthsIn contrast to the mouth width, the height of the mouth - - the cut-up - - is not set until the pipe is voiced. Measured as a fraction of the width of the mouth, the cut-up is made higher to produce more sound. Raising the cut-up also decreases the emphasis on upper partials. In the photograph to the left, the scale of the two pipes (which both sound middle c) is similar. The pipe on the left is a principal, and the pipe on the right -- with a lower cut-up -- is a string. The principal pipe will produce more sound and have more emphasis on the fundamental than the string.

Given several open pipes of normal scale, a wider, lower mouth will produce a principal tone, and a smaller, higher mouth will produce a flute-like sound.

As the wind passes through the flue and strikes the upper lip, there is often some turbulence, producing a transient sound called "chiff." In some pipes, this transient tone is desirable, and the pipe is left unaltered. In other cases, often where a resonant acoustic is not present in the room, the chiff is unwanted and must be reduced. The most common means of reducing chiff is to alter the mouth by nicking the lower lip and languid. Small cuts are made perpendicular to the flue on both its boundaries. This allows the wind to strike the upper lip not in an unbroken stream, but with some irregularity. This reduces the sudden impact and thus the turbulence. The result is a more direct production of the primary tone of the pipe, with a reduced or eliminated chiff.

Three Pipe MouthsThe photographs to the right show three pipe mouths: 46


Pipe Mouth with BeardOther modifications to the mouth of a pipe can be made externally. Small vertical plates attached to the outside of a mouth, at the short ends, are commonly called ears. They serve to focus the wind stream on the upper lip, and may in fact be used in tuning the pipe. A harmonic bridge, a horizontal plate of metal or a wooden dowel commonly called a beard, may also be placed in front of and below the mouth to aid in focusing the wind on the upper lip. The photograph shows both ears and a wooden beard on a metal pipe.


Top of Flue Timbre Page

Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body


Flue Timbre: Modifications to the Body

The body of a pipe provides the resonating space for the sound produced at the mouth of a flue pipe. Its shape, particularly its scale, contributes to the development of a specific tone color, as indicated in the section on pipe scales above. The shape of the body - - whether is is cylindrical or conical - - modifies the strength of overtones and thus changes the quality of the sound. As indicated in the section on stopped flutes, a cap or stopper that completely or partially closes the opening at the top of a pipe body has an even greater effect on the sound. The first change is one in pitch, which is an octave lower than a sound produced by a similar open pipe of approximately the same length. The second change is in the tone color, because the odd-numbered overtones cannot develop in a closed pipe.

The second overtone sounds a twelfth above the fundamental, e.g., sounding g an octave and a fifth above a fundamental pitch of C. The prominent emphasis on this harmonic gives stopped flutes a characteristic sound that is sometimes described as "quinty." When other factors, including scaling and modifications to the mouth, further emphasize this overtone, the characteristic quint sound is even more pronounced. In these cases, the stop is often called a "Quintaton" or "Quintadena" in recognition of its particular timbre.

A cap modifies the pitch in one direction - - lowering it an octave, but another modification - - a hole placed at the mid point of the body - - raises the pitch by effectively shortening the length of the resonating space. As indicated in the section on harmonic flutes above, the additional length forms a secondary resonating space which emphasizes the fundamental tone of the higher pitch.

Another modification - - the presence of slots at the top of the body - - can have a similar effect on tone color by providing a secondary resonating space. Slots are not often found in organs of the late twentieth century, but were common earlier.

Slotted StringA slot is an opening made in the material of the pipe body itself, in effect reducing its functioning length as a resonating space for the sound wave, thus making the pitch higher. The presence of a slot can have an effect on timbre, because the extra length of the pipe can provide a secondary resonating space in a way similar to that found in harmonic pipes. This is most often found in the case of pipes from the earlier part of the twentieth century, and can be found in many organs of that period. In such cases, the slot might actually be in the front of the pipe because it would not be seen under normal circumstances. Such slots are most often quite narrow in relation to the diameter of the pipe so that the influence of the secondary resonance can be emphasized. The photograph shows the top of a resonator from a slotted string, with a narrow opening and a relatively short overlength. 47

Facade Principal with OverlengthIn some cases, particularly where these openings are found in newer instruments, the slot is placed at the back of the pipe, where it cannot be seen. The pipe thus appears to be longer than its actual sounding length. Where pipes are placed in a visible location, whether in an exposed position or in the front of a case, this might be done to improve the appearance of the instrument by providing a more pleasing display or even to further conceal pipes behind the façade. Under these circumstances, the slot is more often made quite wide and all the way to the top so that very little or no effect is had on the timbre, as in the case of the photograph to the right. 48 The pipe is from a visible display of principals in which each pipe is made with considerable overlength in order to provide a pleasing façade within a case. The lower arrow indicates the position of the languid, the upper arrow the tuning scroll; together they mark the limits of the speaking length of the body. The square brackets outline two large slots cut into the back of the pipe. If they were not there, the overlength of the body would create a secondary resonance chamber, and the tone or timbre produced by the pipe would be too dark, with less overtone development and more fundamental.


Top of Flue Timbre Page

Principals | Flutes | Strings | Hybrids | Compound Stops
Construction of Flue Pipes and Timbre:
Scale | Modifications to the Mouth | Modifications to the Body



© 1998, James H. Cook