Almost any enclosed volume of air can be made to oscillate and produce sound if given the proper energy input. There are many thousands of shapes and sizes of organ pipes which have been tried, and hundreds of designs are currently in use. The organ builder has the job of specifying the shape and size of each of the many volumes of air (pipes) which will be energized to produce musical sound by the slightly compressed air of the organ.
The truth is that the organ builder rarely thinks about the volume and shape of the air contained inside the pipe resonator, but rather about the pipe itself. The pipe is the visible, substantial entity which may easily be mistaken as the source of the sound. In actuality, the pipe is merely the package, the container, the means of confining the invisible, intangible air which is the real source of sound. That the pipe walls themselves vibrate and radiate sound in sympathy with the enclosed oscillating air is a secondary but musically important effect. A pipe with rigid, vibration less walls would still produce a similar, if musically less interesting sound.
Musically interesting and useful sounds are produced if the shapes and sizes of the oscillating volumes of air are skillfully designed, built and voiced. The art of designing these enclosed volumes of air is called scaling. To an architect, scale is a description of how the size and shape of any building element compares with the other elements and to the complete structure. An item which is too large for its use or position is overscaled, and an item which is too small is underscaled. In organ pipes, scaling is the process of deciding what shape and size to give each pipe and its components so as to produce the right character and volume of sound. The very small-scaled keen string and the large-scaled dull flute of a few decades ago are examples of extremes of scaling which are not much used today.
It is the current goal of most builders to avoid the scaling extremes of the recent past and to capitalize on the subtle refinements of scalings which are now available through the use of variable scaling and constructional variations in the pipes which have been emphasized since the second World War. Even though extremes of scale are generally avoided, there is available today a wider variety of tonal styles, voicing and scaling treatments than at almost any time in the past. The earliest builders scaled by trial and error, often building pipes in the church. In the late 19th and early 20th centuries, mathematics was applied to the organ pipe problem in order to evolve a rational basis for design. Many organ builders scaled by formula and chart rather than by creative intuition. The results were less than ideal in many cases. Today, every organ builder has his own outlook on scaling which has developed from very personal and indescribable concepts of organ sound and function. No amount of study can ever completely answer our questions as to why any ancient organ builder scaled in a particular way. Copying older pipes is interesting and instructive, but most great art is the product of a creativity above the level of copying. No two organ builders scale alike, nor should they. No dogmatism or dictating of style should ever require conformity by builders to some formalism. The ongoing development of organ sound is important enough to require that the organ builder be encouraged to express his best ideals rather than adjust or modify them to suit others.
Scaling is the blueprint upon which the edifice of organ sound is constructed. Other factors such as voicing, casework, action, acoustics and performance practice will add color, shadow and texture to the final structure, but the basic foundation is created by the architecture of the stoplist and the scaling of the thousands of oscillating volumes of air which create the sound.