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THE PROJECTION ROOM 249
plaster-finished acoustical material. Such walls are light in weight and do not store heat or cold to any considerable degree.
(12) Hollow tile is light in weight, has excellent fireproofing qualities and fine soundproofing characteristics and does not store heat or cold appreciably. Hollow tile not less than three inches thick forms the best type of projection room wall, provided the front wall has a good underlying foundation and the tile is laid in good cement mortar tempered with lime — to make the mortar work well. (13) A good foundation for the front wall is necessary because the cutting of the ports weakens its strength considerably.
(14) All walls abutting upon the auditorium should be plastered on the inside with cement mortar to a thickness of at least three-fourths of an inch. A threequarter-inch-thick coating of acoustical plaster should cover the auditorium side. Walls treated thus will be strong enough, despite the port openings. (15) Conduit should be built into the wall.
The Ceiling
(16) Many theatres are paying high insurance rates because of improperly constructed projection room ceilings. (17) All requirements are met if the room walls join a concrete floor above. Otherwise either of the following two suggestions are practical : Use a 4-inch hollow tile supported by suitable T beams, laying the tile in cement mortar in the same manner as the tile walls. First apply a layer of cement mortar three-quarters of an inch thick and over that — for surface covering — lay a coat of acoustical plaster of the same thickness. Or use iron I beams, each not less than three inches deep, with metal lath above and below, laid in plaster. Twoinch beams are used frequently but these are too light for a deep room.
Tt is wise to consult local authorities before finally deciding upon any form of wall or ceiling construction. Frequently they insist on certain types of material and construction.