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.
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.
CHAPTER VIII.
A REVOLUTION IN WIND SUPPLY.
Prior to the construction of the above-named organ at Birkenhead, England, it had been the custom to obtain or regulate the pressure of wind supplied to the pipes by means of loading the bellows with weights. Owing to its inertia, no heavy bellows weight can be set into motion rapidly. When, therefore, a staccato chord was struck on one of these earlier organs, with all its stops drawn, little or no response was obtained from the pipes, because the wind-chest was instantly exhausted and no time was allowed for the inert bellows weights to fall and so force a fresh supply of air into the wind-chests.