George Bozeman contributes Pipe Scaling Primer

I’m not sure if my excitement about PipeCAD is contagious, but perhaps a little of it has rubbed off on at least one other person. George Bozeman has been a tremendous asset as I have familiarized myself with the art of pipe scaling, and he has written a short primer on organ pipe scaling for inclusion in the documentation for PipeCAD. Rather than hoard this valuable info just for users of PipeCAD, I am also posting the information here for the interested reader. -MirthMaker

A SHORT PRIMER ON ORGAN PIPE SCALING

contributed by George Bozeman

Töpfer PipeCAD is becoming a wonderful tool for use in scaling organ pipes, and also a helpful aid in voicing procedures. However, if you are new to the concept of pipe scaling, or are not sure exactly what is involved, this primer will help you.
The Töpfer Normalmensur posits an ideal Principal stop in an ideal acoustic environment. But let us imagine a much smaller environment in which a Normalmensur Principal would be too loud, although of the proper timbre. One could then simply scale the pipes somewhat smaller and achieve the same timbre at a proper dynamic level. Or, perhaps the environment is generally in agreement with Töpfer’s ideal, except that the room is too efficient on the lowest C, and is not friendly to the frequencies around middle C. Then one could devise a variable scaling in which the lowest notes were made smaller in scale, and with a smooth rise in scale around middle C, in order to result in an even dynamic and timbre throughout the compass of the stop. This is the basic rationale behind scaling differences. If one assumes the proportion of the mouth width to the circumference of the pipe remains constant, then the larger pipe not only has a larger scale, but also a larger mouth which gives the pipe greater energy and thus maintains the timbre even though the scale is larger.
Factors Influencing the Speech and Tone of Flue Pipes:
These are the factors of speech and tone to be influenced and how scaling affects them:

• Loudness: The strength of the vibrations. Wide-scaled pipes are capable of greater loudness than narrow-scaled ones. Other factors which affect loudness include the effective wind pressure (the regulated pressure of the bellows, the adjustment of the size of the toe hole and the width of the windway or flue), and the width of the pipe mouth.
• Timbre: (sometimes called ‘tone’, ‘color’, etc.). The presence or absence and relative strengths of the overtones, partials, or harmonics in the tone. Wider scales emphasize the lower pitched partials whereas narrow scales emphasize the higher pitched partials. Higher effective wind pressure emphasizes the higher pitched partials. Lower cut-ups (height of the mouth) emphasize higher pitched partials.
• Pitch: The number of vibrations per second. Basically the pitch of an organ pipe is determined by the number of air molecules influenced by the pipe resonator. Longer and/or fatter pipes have lower frequencies. Colder pipes have lower frequencies. Increasing the wind pressure will increase the frequency. Töpfer PipeCAD will provide several means for determining the proper lengths of pipes.
• Speech: This word has at least two, somewhat different meanings:
  • The transitional sounds the pipe makes when beginning to speak, ‘chiff’, for example;
  • The angle of the wind sheet as it enters the pipe body at the onset of the speech; various adjustments of this can cause the pipe to begin its tone slowly or quickly; hence we use the terms ‘quick’ and ‘slow’ speech. Slower speech encourages higher partial development. This makes it possible sometimes to correct pipes that are not ideally scaled.

  However, speech is not a factor generally considered in the scaling of organ pipes.

• Formant: Coloration of the tone cause by vibrations of the walls of the pipe, rather than by the distribution of harmonics in the normal tonal output of the pipe. It is not a factor considered in scaling pipes.
• Non-Harmonic Noise: Essentially white noise inherent in the tone of flue pipes. Voicing techniques can lessen or enhance this. It is not a factor considered in pipe-scaling.

These are factors which influence the speech and timbre of flue pipes:

• Wind Pressure: The effect of wind pressure in general is that higher pressures cause greater development of higher partials. Thus larger scales or techniques are employed to maintain the same timbre that would be achieved with lower pressures. Although it is a general factor affecting the results of scaling, there are no formulas for it, and will not be considered in Töpfer PipeCAD.
• Toe Hole Size: If the toe hole is smaller than the amount of air the windchest can deliver and/or can issue from the flue, it will decrease the effective pressure of the pipe. Töpfer PipeCAD will provide a means for generating smoothly graduated toe hole sizes.
• Lenght & Shape of Foot: This is not a factor considered in scaling for tonal purposes, but Töpfer PipeCAD will provide a means of smoothly graduating foot lengths for visual or construction purposes.
• Shape & Size of Flue: The width of the flue (i.e., front to back) can determine the effective wind pressure of the pipe. The graduation of this in a set of pipes is usually determined visually by the voicer, but Töpfer PipeCAD can provide precise graduations, if desired.
• The Mouth Width: The mouth width determines how large the ‘motor’ of the pipe is which generates the sound. Increasing the mouth width without changing the actual height of the cut-up will increase the power of the pipe (assuming the effective wind pressure remains the same) without changing any other factor, including timbre. Töpfer PipeCAD will provide several ways to calculate mouth widths.
• The Cut Up: The distance from the lower to upper lip of the pipe mouth is the cut up. It is actually a factor of the speaking length of the pipe, but is often expressed as a fraction of the mouth width. Principals often have a mouth width that is 1/4 of the circumference and a cut up that is 1/4 of the mouth width. Obviously this can go awry if the mouth width is instead 2/7 of the circumference, because the cut up height should remain the same in either case. Töpfer PipeCAD will offer several ways to calculate cut ups.
• Shape of Pipe Body: Flue pipes can be cylindrical and either open at the top end or stopped. They can be tapered either smaller at the top or at the mouth. There can also be combinations of tapered and cylindrical sections. Stopped pipes can be fitted with chimneys or simply a hole in the top cover. Töpfer Pipe Cad will provide a means for calculating all of these variables.
• Tuning Devices: Pipes can be cut to exact length, fitted with tuning sleeves in which case the pipes are cut slightly too short, fitted with cut out slots, etc. Töpfer PipeCAD will provide ways to calculate pipe lengths according to these various factors.

If one wishes to learn how to scale organ pipes it is necessary to listen to pipes that one has scales for in the actual room where the pipes are playing. Only this way can one listen for the peculiarities of the room’s acoustics and how the scale of the pipes (and their voicing) reacts to this. It is also helpful to listen to recordings of organs that one has scalings for, but microphone placements and recording techniques can make accurate conclusions very difficult. Obviously a great deal of listening and careful analysis is necessary to become an expert pipe scaler.

The Blank Voicing Graph: Many voicing operations need a means to produce smooth gradations of dimensions. It would be unwieldy to use a computer for many of these operations, particularly where proportional dividers are used to mark the dimensions on the pipe. Therefore a blank voicing graph based on the Normalmensur has been provided which can be printed out in as many copies as required. One can enter dimensions on desired points of the compass and connect the points with straight lines. A useful hint is to multiply the dimensions by 10 on the graph and set your proportional dividers to 10:1 to read the points, especially on smaller dimensions. This will improve your accuracy. (Note: Please feel free to download the Graph Template from this website for use in your voicing work - MirthMaker)

George Bozeman is a native of Texas. He majored in organ performance under the late Dr. Helen Hewitt at North Texas University in Denton. Following college he apprenticed as an organbuilder with Otto Hofmann of Austin, one of the pioneers of the tracker revival in the United States. He worked with the late Joseph E. Blanton in an effort to develop a standard model one-manual organ, and at the same time received a thorough grounding in architectural matters relating to the organ. Then he was employed by the firm of SipeYarbrough which later became Robert L. Sipe & Company of Dallas. He was a vice president of the latter firm when he left in 1967 for a Fulbright grant in Austria. There he studied organ with the late Anton Heiller, harpsichord with Isolde Ahlgrimm, and organbuilding with Joseph Mertin. He also began the study of European organs, new and old, which now encompasses those of some 13 western European nations.

On returning to the United States, Bozeman was employed by the Noack Organ Company in Massachusetts. In 1971 he founded his own firm in Lowell, Massachusetts. In addition to organbuilding, he has continued an active career in musical performance. For many years a practicing church musician, he now frequently substitutes for organists in New England, and has performed as a recitalist all across the United States and in Canada and Mexico. He is an active member of the Boston and New Hampshire chapters of the American Guild of Organists, the Organ Historical Society, the American Institute of Organ Builders, and the International Society of Organbuilders. He also is a frequent contributor to The Diapason, The American Organist and The Tracker.