No_3_March_1969 — Page 21

STAGE

TO DUNBARTON RO

First floor plan

purposes and the owners stipulated that its design must comply with these regulations.

Another factor affecting the span dimension was the inclusion of class- rooms beneath the hall. Considering only the classrooms, it was found that the ideal was a 51ft. 3in. clear span (54ft. 3in. structural span). This gave two 22ft. wide classrooms, a 5ft, corri- dor and a total of 2ft. 3in. in wall thicknesses. With this span the capacity of the hall would have been 860 seats.

Best solution

Finally a structural span of 55ft. was selected. While this increased the size of the classrooms slightly, with a small reduction in the size of the gang- ways it allowed the capacity of the hall above to be enlarged to 956 seats. The choice of a 55ft. span was also found to provide the best solution with regard to the percentage of the span used for seats, i.e. 14 seats per row, 31⁄2ft. side aisles and a 6ft. centre aisle.

In a hall of this type the ideal volume per person for a satisfactory reverberation time is 100 cu. ft. per person for voice and 400 cu. ft. for music. With these volumes minimum acoustic treatment is required and costs are therefore reduced. The hall is de- signed primarily for voices and use of

the steeply pitched roof, sloping to- wards the stage, resulted in an accept- able volume of 116 cu.ft. per person, excluding the volume of the stage.

The ceiling slope provides a good reflector of sound to the rear of the hall. It is left in fair-face concrete as this is a reflecting material.

The roof is also sloped in cross- section to reduce loss of sound on the side walls and reflect it back towards the audience.

The walls and ceiling of the stage are treated in reflective or absorbent plaster in various places to amplify direct reflections and reduce echoes. Absorbent plaster is also used on the rear walls of the hall and parts of the balcony ceiling. The front ceiling under the balcony slopes and is fair-faced concrete to reflect sound to this part, which is usually the worst seating acoustically.

Most of the hall is flat to permit dances and sports, but the seating under the balcony is stepped to give better sight lines and acoustics.

Materials used for finishing the hall are simple and require little or no maintenance. Parts near the existing building tie in with its decorative scheme to give unity between all sec- tions of the building.

The structure was designed to the

SCALE

s section

O

Sun grilles on east elevation improve interior comfort in the early mornings

Far East BUILDER, March 1969

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