A good expression box when fully open should not rob the pipes of clear projection and presence to any great degree, but when closed should reduce loudness from at least ff to p. To achieve this, a box must be reasonably sound-proof (with adequate density to control leakage of bass) and must be well sealed when closed; gaps are anathema to good expression-box control. The shades cannot be too thick; their bulk will not permit a full use of the opening. Shades should be able to open nearly 90°. They must be fast-acting and silent. Achieving smooth, continuous expression control is one of the greatest challenges in organ-building. View a Double Expression illustration.
To achieve a continuous dynamic range from fff to ppp we have developed a system of double expression, placing a box within a box. The inner box is placed at the rear of the outer main box so that there is a large air space between the two sets of shades. When both sets of shades are closed, the space contained between them provides a very effective sound-trap. We place the softest and most powerful sounds inside the inner box of the division: for example, a pair of ethereal strings and the Vox Humana; the high-pressure chorus reeds and a mixture. A balanced expression pedal is provided at the console for each box.
On large instruments a switching system allows the organist to select conveniently which shades are to be assigned to each balanced pedal. With the shades not quite fully open, the stops under the inner box are at a normal volume level to balance the rest of the division. With both sets of shades fully closed, the soft stops in the inner box are reduced to near-inaudibility and the chorus reeds are reduced to the level of color reeds. With all shades fully open, the chorus reeds and mixture are slightly louder than those of the Great. The Vox Humana usually has its own shades with a console switch to shift from pp to mf. There are many expressive possibilities with this system. For example, a crescendo may be started using the ethereal strings with both boxes closed, opening the inner box until the level is equal to the soft stops in the outer box, which are then added. The outer box is opened, adding stops in the normal manner while closing the inner box. The chorus reeds and mixture are drawn and the inner box re-opened to complete the crescendo. This is done with ease after a bit of practice.
Organs with double expression can be played in the traditional manner simply by disengaging the double expression feature.
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