Whispering wall, London, 1950
A whispering gallery is most simply constructed in the form of a circular wall, and allows whispered communication from any part of the internal side of the circumference to any other part. The sound is carried by waves, known as whispering-gallery waves, that travel around the circumference clinging to the walls, an effect that was discovered in the whispering gallery of St Paul's Cathedral in London. The extent to which the sound travels at St Paul's can also be judged by clapping in the gallery, which produces four echoes. Other historical examples are the Gol Gumbaz mausoleum in Bijapur and the Echo Wall of the Temple of Heaven in Beijing. A hemispherical enclosure will also guide whispering gallery waves. The waves carry the words so that others will be able to hear them from the opposite side of the gallery.
The gallery may also be in the form of an ellipse or ellipsoid, with an accessible point at each focus. In this case, when a visitor stands at one focus and whispers, the line of sound emanating from this focus reflects directly to the focus at the other end of the gallery, where the whispers may be heard. In a similar way, two large concave parabolic dishes, serving as acoustic mirrors, may be erected facing each other in a room or outdoors to serve as a whispering gallery, a common feature of science museums. Egg-shaped galleries, such as the Golghar Granary at Bankipore, and irregularly shaped smooth-walled galleries in the form of caves, such as the Ear of Dionysius in Syracuse, also exist.