In downtown Madrid stands a replica of a sculpture by minimalist artist Eusebio Sempere. It consists of an arrangement of polished tubes that look rather like organ pipes. As it rotates on its base, its mirrored surfaces reflect brilliantly in the sunlight.
But when acoustical scientist Francisco Meseguer first saw the sculpture he noticed something different about it--sounds passing through the structure were altered. In 1995, he and his colleagues from Universidad Politecnica de Valencia set up some microphones and ran a few tests. What they found was that some frequencies of sound became more dominant (were reinforced) and others less so (were attenuated).
As Meseguer's team soon reported in a letter to Nature, the sculptor had inadvertently created a "sonic band gap structure." Such band gaps were first observed in crystals where the arrangement of the atomic lattice permits only certain wavelengths of electromagnetic energy to pass. Recently, researchers have constructed synthetic "crystals" that can filter and channel certain wavelengths of light by creating photonic band gaps. Audible sound, with its longer wavelengths, responds in the same way to larger structures.
At the recent meeting of the Acoustical Society of America in Norfolk, Virginia, Meseguer reported his group had designed similar structures that could be manipulated and studied in the controlled environment of a sound-absorbing, echo-free (anechoic) chamber. By manipulating the diameter and spacing of the rods they found that some frequencies of sound could be amplified or blocked out altogether. He proposed that structures creating "deaf bands" could be used by architects to design screen out or filter noise.
One speaker at the conference who was not at all surprised by Meseguer's findings was acoustical consultant David Lubman of Westminster, Calif. Lubman is one of a small but growing number of researchers who are pioneering a new discipline that might be called "paleoacoustics" or "archaeoacoustics." These investigators are intrigued by the curious sound phenomena reported at many ancient sites. And, unlike many archaeologists, they do not believe they are accidental but proof that some ancient people had a sophisticated knowledge of acoustics and built it into their structures.
Lubman first became intrigued by reports of a curious echo from the Mayan pyramid of Kukulkan at Chichen Itza, in Mexico's Yucatan region. The odd "chirped" echo resounds from the pyramid's staircases in response to hand claps of people standing near its base. To hear for himself, Lubman packed up his recording gear and traveled to Chichen Itza last January.
After studying the staircases and analyzing his recordings and sonograms of the echoes, Lubman came back convinced that this was no architectural freak. In his paper, Lubman argued that the design of the staircases was deliberate and that the echo is an ancient recording, coded in stone, of the call of the Maya's sacred bird, the quetzal.
Like the tubes in Sempere's sculpture, the treads of the stairs at Kukulkan consist of elements that are repeated at regular intervals, or are "spatially periodic." "When periodic design elements are composed of sound reflective materials [such as stone], and if certain other conditions are met, odd echoes or other strange acoustical effects may result," says Lubman. He contends that the oddly narrow steps with abnormally high risers (an illogical configuration for people whose descendants are of short stature) were built to voice the call of the sacred bird.