Kukulkan Temple, at the Chichen Itza archaelogical site in Mexico, isn't just an impressive architectural achievement. It also uses physics to make bird calls. That's right, a stone temple has the ability to mimic the sounds a local bird. Let's see the pyramids at Giza do that!
The calls are all good imitations of the quetzal bird, a spiritually important animal to the 12th century Mayans. The staccato sound of two hands clapping together gets transformed, in echoes, to the drawn-out falling tone of the bird. How the hell does that happen? David Lubman, an acoustical consultant, took a look. It seems that the steps of the temple form a Bragg diffraction grating. Diffraction is what happens when a wave hits an object and spreads out.
For the most part, Bragg diffraction, and Bragg diffraction gratings, are studied by crystallographers. They send a wave of light into crystals, where the atoms cause diffraction, and because the atoms are arranged in an orderly pattern, they form a grating. The crystallographers then study the pattern of the returning light to get a sense of the structure of the molecules. Lubman is doing the reverse. Everyone knows the pattern of steps on the side of the pyramid, and he's studying the sound wave that comes off of them.
Bragg scattering refers to a particular pattern of wave scattering, which shows up acoustically in the sound coming from the pyramid. The two major features of the scattered sound from the pyramid are the extension of the sound, and the fact that the sound is tonal. The echo lasts longer than the sound, bounced off many steps, each at different distances from the original clapping sound. The scattering also lets lower frequencies lag behind higher frequencies, meaning that each of the "bird calls" starts out high and ends slightly lower.
[Via Ocasa.org]