Courtesy of Gonzalo Viramonte
The Metropolitan Brasilia Cathedral is one of the great works of Brazilian architect Oscar Niemeyer, often well-known for his constructivist designs in reinforced concrete. It is a hyperboloid structure with a rather intriguing history.
The construction lasted more than ten years on this cathedral with the cornerstone being laid in September 1958, which marked the architect’s first work in what was to be the nation’s new capital- Brasilia. The piers (which are in close resemblance to pointed peaks) rise up like hands seeing the sky. In the interior, there are sixteen panels of stained glass. The cathedral almost seems like a piece of heaven beamed down into the Brazilian capital at the hands of Niemeyer.
The form’s unity in appearance and feel is clearly presented through its broad spaces, with the volume being generated by its structural elements. This structure and volume interact with each other to create larger generated spaces. The nave is the main element of the cathedral and is located 3 metres below the esplanade floor. It creates a unique sense of daylight and colors, which adds to the acoustical effect of the structure.
The twenty one 40-metre tall hyperboloids (in geometry, a hyperboloid of revolution, sometimes called a circular hyperboloid, is the surface generated by rotating a hyperbola around one of its principal axes) have been reduced to sixteen 30-meter tall piers- keeping in mind the extent of the incident: reflected ray ratio, while a source of sound comes in contact with the piers. Likewise, the concrete ring forming the base was initially 70 metres in diameter, but reduced to 60 metres in diameter.
The sixteen curved-piers are tied together with glazing panes which reduces noise by absorbing the sound energy and preventing the sound vibrations from travelling through the glass. Whereas, the linkage between the piers is a space frame in steel, covered with glass panes to control noise problems with the tempered glass being mounted in neoprene mullions; thus allowing the glazing to be displaced. Due to the roof slab sheathing, which allows for natural ventilation through the central opening, the walls radiate at the same frequency, thus reducing the wall system’s acoustical effectiveness.
Since the source of sound is at a distance of 20m from the edge of the piers touching the ground, the rays of reflected ray tend to not reach the zones touching the piers, hence leading to a sound defect, which makes the space less usable.
At a height of 20.4m from the floor, the piers touch each other – this accounts for an acoustical defect and hence the rays do not travel leading to a time delay. Lastly, since there is a certain amount of reflection of a sound wave at the barrier, at an equal distance behind the barrier, this gives rise to an echo.
AC of concrete: 0.04
AC of glass: 0.3
AC of glazing: 0.15
AC of neoprene glass: 0.10
AC of smooth marble (flooring): 0.01
AC of 0.8mm unperforated metal panels: 0.5
AC of double sheet steel skin: 0.35
Hence, absorption coefficient of all materials= 1.45
Volume of hyperboloid= 1/3 π h * (2a²+r²)
Where h= height
r= base radius
Hence, volume= 29.845 m3
RT 60 for worship rooms should ideally be: 1.8-3.2 seconds
Calculation of RT 60
RT 60= 0.161 x Volume of Hyperboloid
Total absorption coefficient of all materials
= 0.161 x 29.845 = 3.31s
Courtesy of Reine Fernandes