The swell box is an old device (seventeenth century Spain), and through several centuries of development it has become a widely used method for controlling organ sound. Expression devices were rudimentary until the nineteenth century, and even Franck had only an inconvenient open-or-closed hitch-down pedal for the single enclosed division at Ste. Clotilde. It was incapable of the smooth, controlled expression that is available with the balanced swell pedal. Expressive divisions became larger. Soon, not only the Swell but also the previously unenclosed Choir and Great divisions disappeared into boxes, and eventually the entire organ vanished behind shutters and into organ chambers, the ultimate enclosure having a double set of shutters, one inside the other, for reducing the sound to inaudibility.
Lately, with organ chambers out of fashion, the swell box has returned in a wide variety of forms. A tiny Brustwerk may have a set of hand operated doors. Larger expressive divisions may have shutters of wood, glass, plastic, steel, aluminum, hardboard or other materials, with either a direct mechanical connection or an electric, pneumatic or hydraulic motor to control the position of the shutters.
Several different musical goals are available to the designer of an expressive enclosure. In the era of the organ chamber the desire was to achieve the greatest reduction in sound so that the organ could be made as distant and soft as possible. Large, deep chambers with small tone openings, severe acoustical traps and even sound absorbent materials inside the chamber were common.
With the emergence of the exposed organ and the tracker revival, the goal was maximum projection and minimum Interference or alteration of the sound. Shutters were as thin and "aerodynamic" as possible, as large as possible, and the box was shallow and close to the compact chest.
These two extremes of design criteria (maximum reduction or maximum projection) reflected the great change in organ design. In practice, however, few expression boxes were "maximum" since many factors come into play in both extremes to make the ideal impossible. Most expression boxes provide a range of projection from medium soft to medium loud.
But a third style of expression box is also possible, and many organbuilders are seeking a goal of maximum dynamic range from an enclosed division. In this maximum dynamic box, the builder tries to get the fullest possible projection with shutters open, but maximum reduction with shutters closed. Such an enclosure can never have the perfect projection of the thin-shuttered, neo-tracker box, nor the disappearance to inaudibility of the old organ chamber, but It does have as wide a range from loud to soft as possible. Such a box requires the application of thoughtful practical acoustics, using thick, hard walls, tight fitting shutters with no gaps, and having maximum sound power loss through walls and shutters when the box is closed. The small, tight swell box does produce a noticeable flattening of the pitch of the enclosed pipes when the shutters are closed, but the return to normal pitch when opened is an effect which can be musically useful and interesting.
Having discovered that pipe shades on an unenclosed division can produce an interesting modification to the sound of that division, many builders now have no fear of using a thicker and tighter shutter for the enclosed division. There is no expressive division with the same acoustical and musical qualities possessed by an unenclosed division and vice versa, and the wide variety of current design styles in expressive divisions indicates a freedom and diversity of outlook toward the different uses and goals of musical expression and control of pipe organ sound that is healthy and constructive.