A whispering gallery is usually a circular,
hemispherical,
elliptical or
ellipsoidal enclosure, often beneath a
dome or a
vault, in which
whispers can be heard clearly in other parts of the gallery. Such galleries can also be set up using two parabolic dishes. Sometimes the phenomenon is detected in caves.
Theory
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.[1] The extent to which the sound travels at St Paul's can also be judged by clapping in the gallery, which produces four echoes.[2] Other historical examples[3][4][5] are the
Gol Gumbaz mausoleum in Bijapur, India 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,[4] 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,[3] and irregularly shaped smooth-walled galleries in the form of caves, such as the
Ear of Dionysius in Syracuse,[4] also exist.
The
Mapparium at The Mary Baker Eddy Library in Boston, Massachusetts allows visitors to enter the interior of a reflecting surface forming a nearly complete sphere