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.
Figs. 14-15. The Vacuum Shutter
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.