Fig. 12. The Principle of the Sound Trap
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.
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.