Fig. 13. Sound Trap Joint
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.
