An Unplanned Sound Mirror
The Cathedral of Brasília (Catedral Metropolitana Nossa Senhora Aparecida), designed by architect Oscar Niemeyer, is defined by its dramatic, crown-like structure. This form is visually striking; it produces an unusual acoustic effect known as a "whispering gallery." A person standing near the curved interior wall can whisper and be clearly heard by someone standing on the opposite side of the 70-meter (230 ft) diameter nave. This happens because the smooth, concave concrete acts as an acoustic mirror, focusing sound waves that travel along the circumference with minimal energy loss.
This sonic behavior was not part of Niemeyer's original plan. His design, a hyperboloid of revolution formed by 16 concrete columns each weighing 90 tons, was intended to create a sense of openness and upward movement, representing two hands reaching toward heaven. The structural engineer Joaquim Cardozo performed the complex calculations needed to realize this ambitious form. The resulting geometry, however, creates the conditions for whispering-gallery waves, a phenomenon first identified in St. Paul's Cathedral in London. These high-frequency sound waves "cling" to the concave surface, allowing them to travel long distances.
The Physics of the Form
The cathedral's foundation stone was laid in 1958, and the main structure was completed by 1970. The building stands 40 meters (131 ft) high and can accommodate up to 4,000 people. Its primary structure consists of the 16 parabolic concrete columns that form the hyperboloid shape. The smooth interior surface of these columns is important for the acoustic effect, as it minimizes sound absorption and scattering, allowing the sound waves to propagate cleanly.
Between the concrete columns are vast triangular stained-glass windows, created by French artist Marianne Peretti in 1990. These pieces, in shades of blue, green, white, and brown, are suspended under an outer roof of fiberglass. While the glass dominates the visual experience inside, the underlying concrete skeleton governs the building's acoustic properties.
Visitors enter the main nave through a dark, subterranean tunnel, emerging into the brightly lit space. Inside, three angel sculptures by Alfredo Ceschiatti and Dante Croce are suspended by steel cables. The largest of these is 4.25 meters (13.9 ft) long and weighs 300 kilograms (660 lb). These elements, along with the altar donated by Pope Paul VI, exist within the open volume of the nave but do not interfere with the sound waves traveling along the perimeter walls.