Swell Boxes
The invention of the Swell is generally attributed to Abraham Jordan. He exhibited what was known as the nag’s head Swell in St. Magnus’ Church, London, England, in the year 1731.
The “nag’s head” Swell, with its great sliding shutter, rapidly gave place to the “Venetian” Swell shades, used almost universally to this day. At the beginning of the period under consideration Swell boxes were almost invariably made of thin boards and their effect upon the strength of the tone was small. Willis was one of the first to realize the artistic possibilities of the Swell organ and in almost all his organs we find thick wooden boxes and carefully fitted shutters, and often an inner swell box containing the delicate reeds, such as the Vox Humana and Oboe.
Many of the leading organ builders now employ this thicker construction, and it is no uncommon thing to find Swell boxes measuring three inches in thickness and “deadened” with sawdust or shavings between the layers of wood of which they are formed.
A few organs of Hutchings and other makers are provided with a double set of shutters, so that sound waves escaping through the first set are largely arrested by the second. The crescendo and diminuendo are thus somewhat improved.
By the adoption of scientific principles Hope-Jones has multiplied the efficiency of Swell boxes tenfold. He points out that wood, hitherto used in their construction, is one of the best known conductors of sound and should, therefore, not be employed. The effects produced by his brick, stone and cement boxes (Worcester Cathedral, England; McEwan Hall, Edinburgh, Scotland, Ocean Grove, New Jersey, etc.) mark the dawn of a new era in Swell-box construction and effect. It is now possible to produce by means of scientific Swell boxes an increase or diminution of tone amounting to many hundred per cent.
We have heard the great Tuba at Ocean Grove, on 50-inch wind pressure, so reduced in strength that it formed an effective accompaniment to the tones of a single voice.
The Hope-Jones method seems to be to construct the box and its shutters (in laminated form) of brick, cement or other inert and non-porous material, and to substitute for the felt usually employed at the joints his patented “sound trap.” This latter is so interesting and of such import in the history of organ building that we append, on the next page, illustrations and descriptions of the device.
If a man should stand at one end of the closed passage (C) he will be able to converse with a friend at the other end of the passage (D). The passage will in fact act as a large speaking tube and a conversation can be carried on between the two individuals, even in whispers (Figure 12).
This passage is analogous to the opening or nick between Swell shutters of the ordinary type.
If a man should stand in room 1 at A, he will be able to see a friend standing in room 4 at B, but the two friends will not be able to converse. When A speaks, the sound waves that he produces will spread out and will fill room 1. A very small percentage of them will strike the doorway or opening into room 2. In their turn these sound waves will be diffused all through room 2, and again but a small percentage of them will find access into room 3. The sound waves will by this time be so much attenuated that the voice of the man standing in room 1 will be lost. Any little tone, however, that may remain will become dissipated in room 3, and it will not be possible for a person standing in room 4 to hear the voice.
This plan illustrates the principle of the sound trap joint.
Figure 13 shows in section the joint between two Swell shutters. A small proportion of the sound waves from inside the Swell box striking the sound trap joint, as indicated by the arrow, will pass through the nick between the two shutters, but these sound waves will become greatly weakened in charging the groove A. Such of the sound waves is pass through the second nick will become attenuated in charging the chamber B. They will be further lost in the chamber C, and practically none will remain by the time the chamber D is reached.
It is Hope-Jones’ habit to place the shutters immediately above the pipes themselves, so that when they are opened the Swell box is left practically without any top. It is in such cases not his custom to fit any shutters in the side or front of the Swell box.
To relieve the compression of the air caused by playing for any length of time with the shutters closed, he provides escape valves, opening outside the auditorium. He also provides fans for driving all the cold air out of the box before using the organ, thus equalizing the temperature with the air outside—or he accomplishes this result through the medium of gas, electric or steam heaters, governed by thermostats.
The Hope-Jones Vacuum Swell Shutters, with sound-trap joints, are shown in Figures 14 and 15.
It is well known that sound requires some medium to carry it. Readers will doubtless be familiar with the well-known experiment illustrating this point. An electric bell is placed under a glass dome. So long as the dome is filled with air the sound of the bell can be heard, but directly the air is pumped out silence results, even though it can be seen that the bell is continuously ringing. As there is no air surrounding the bell there is nothing to convey its vibrations to the ear.
That is why the hollow swell shutter, from the interior of which the air has been pumped out, is such a wonderful non-conductor of sound.
The shutters shown in Figures 14 and 15 are aluminum castings.
Ribs R1 and R2 are provided to support the flat sides against the pressure of the atmosphere, but each of these ribs is so arranged that it supports only one flat side and does not form a means of communication between one flat side and the other. Thus R1 supports one flat side whilst R2 supports the other. The aluminum shutters are supported by means of pivot P.
They are very light and can therefore be opened and closed with great rapidity.
A very thin vacuum shutter forms a better interrupter of sound waves than a brick wall two or three feet in thickness.
When partially exhausted the aluminum shutters are dipped into a bath of shellac. This effectually closes any microscopic blow-hole that may exist in the metal.
The use of Swell boxes of this vastly increased efficiency permits the employment of larger scales and heavier pressures for the pipes than could otherwise be used, and enormously increases the tonal flexibility of the organ.
It also does away with the need for soft stops in an organ, thus securing considerable economy. Where all the stops are inclosed in cement chambers (as in the case of recent Hope-Jones organs) and where the sound-trap shutters are employed, every stop is potentially a soft stop.