Theatre Design & Technology - Fall 1979 - 38

tors. The enclosures are designed to hold the projectors as close together as possible to minimize the difference in optical path to the
projection surface.
The box consists of a 520MM x 600MM X 650MM box of 12MM ply-

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wood with holes for air intakes. exhausts. and projection windows
(Fig. 3). The door is cut into the right side and front. and has two small
notches to clear the power and control cables. Two small barrel bolts
are used to lock the door. The entire box should be cut and fitted, except for the toP. and lined with 50MM of fire resistant foam. Thin plate
glass windows should be fitted. mounted on foam pads.
The interior baffles are constructed as a unit from 12MM plywood
and 3MM masonite, and the unit is slid into the foam-lined box from the
top. lt is vital that the fit be airtight. The exhaust filling is made from a
piece of sheet metal carefully shaped to fit tightly around the exhaust
vent on the back of the projector. This metal filling is fastened to a
piece of 3MM plywood which is. in turn, fitted inside a piece of 100MM
dryer vent hose about 200MM long. The hose is fastened to the bottom
of the back partition with a further piece of plywood and a foam gasket. The air intake holes in the side partition are spaced to feed air to
the projector inlets below the lens and behind the side slide control
buttons.
A flexible upholstery-type rubber is used for two reasons: First. it
provides a tight gasket between the inner and outer boxes. and provides a tight air seal. The second is that it does not transmit vibration
from the fan motor or changer mechanism as would a rigid foam. The
key point is that there must be an air tight seal between the boxes.
The flexible foam used must be a sound absorbing type.*
To use the enclosure, set it in the correct location. and insert a projector with the appropriate lens and a test slide. You will probably
have to shim the projector up or down. Spike the projector and shims

and fasten the exhaust vent fitting over the vent. TAPE IT SECURELY
IN POSITION WITH GAFFERS TAPE. This is a tricky job, but must be
done; if it comes loose. the slide tray will melt. Finally. install the remote changer cord and position the projector and shims on the spike
marks. Repeat for the second projector, and close and lock the door.
With the projector fans running. there should be a perceptible air flow
from both the exhaust vents. A slide change should be inaudible less
than a meter from the box. Check the exhaust temperature after the
projectors have run for a few minutes. In AV Equipment Memo S-ti3.
Kodak recommends 72°C (160°F) as the maximum exhaust temperature. Our projectors ran at 35°C (95°F) after about ten minutes of
operation.
The enclosures as built and described are rather large and bulky.
Slightly smaller and considerably lighter enclosures could be made in
a variety of ways; however the two essential features of (1) long air
paths for intake and exhaust air and (2) complete vibration isolation
between the interior and exterior boxes should be retained in any
modifications. These enclosures should not be used for extended periods with older Kodak projectors that use 500w lamps. as the higher
wattage lamp causes too high an operating temperature. A solution to
this problem is the installation of a small 'Pamotor' or other quiet
electronic ventilating fan in each compartment in the back wall of the
inner box to the left and above the exhaust vent fitting. The fan should
be masked so that it draws only enough air to remove the exhaust air
and not so much so as to also remove the fresh make up air entering
the enclosure. The fans can be wired into a receptacle on the rear of
the box. which can also serve as a non-dim supply for the projector
fans.
These enclosures have been used on several productions at Dalhousie and The Neptune Theatre. The only problem occurred when
one of the exhaust vent fillings was not fastened properly and came
loose during a performance. The internal temperature rose high
enough to warp the carousel tray and melt several slide mounts. Curiously. the slides were undamaged and were used in new mounts for
the next performance. All projectors in enclosures should be monitored constantly during use. Since the projectors will probably be inaccessible. the image should be watched. If a slide starts to go out of
focus. then the projector is overheating and should be turned off.
If the enclosures are used for long runs. or reused after sitting in
storage. they should be vacuumed to remove dust. If your theatre is
very dusty. you may need to filter the intake air.D

* Sound transmission is stopped by the plywood and masonite and is absorbed within the box by the foam. The two-inch thickness is necessary to absorb sound at
low frequencies. Any foam used should be checked for its sound-absorbing properties over the range of octave bands 125Hz-4000Hz.-€ds.

David Mardon was Technical Director at Dalhousie University; Sharon Wade is Lighting and Sound Supervisor at Dalhousie University; and Art
McKay is a cinematographer with the Atlantic Filmmakers Coop.

Theatre Design & Technology

USITI /Fall. 1979

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