How come? Stairs make weird echos

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If you've ever heard the doubled sound of your own footsteps in a long, empty hallway, you're familiar with echoes. Like an undeliverable letter, an echo is sound returned to sender.

Echoes are sound waves that bounce back at us from a hard surface. When you shout into a cave, you often hear your own voice a split second later. That's because the hard stone walls reflect sound waves back, like light reflecting from a mirror. Instead of seeing yourself, as in a mirror, you hear yourself, making an echo chamber a kind of ear mirror.

For the clearest echoes, a sound-reflecting surface should be flat, smooth, and perpendicular to the ground. Sound travels through sea-level air at about 1,100 feet per second. Stand too close to the echo-making surface, and the sound you make will shoot back too quickly, overlapping your original words or clap or musical note.

Then there are stranger echoes. Clap your hands near a wooden staircase and you may hear a drawn-out clap in response, its frequency dropping over time. Scientists call it the "picket-fence effect," since a long picket fence can produce the same odd echo.

How does it work? Physicist Jearl Walker, of Cleveland State University, suggests picturing a staircase from the side. When the sound waves from your clapping hands strike the stairs, waves reflect from the risers of the steps. But since each step is set behind the next, pulses from higher steps take a fraction of a second longer to return to you than those from lower steps.

Your ears perceive the widening gap between the "stepped" sound pulses as a gradually dropping tone. So what you hear, over less than a second, is a single, prolonged, almost musical echo. (Something similar happens with a picket fence; think of the fence as a vertical staircase.)

A famous example of the effect can be heard at the ancient Mayan Temple of Kukulkán in Mexico. Clap your hands at the bottom of the pyramid's 92 stone steps, and you'll hear a chirping echo.

To acoustics expert David Lubman, the chirps sounded eerily like those of the quetzal, a brilliantly colored bird, whose 2-foot-long tail feathers once adorned Mayan helmets.

Named after the serpent god Kukulkán, the temple displays a kind of homage to the god around the time of the spring and fall equinoxes. During those days, a shaft of sunlight creeps along the side of the pyramid. Triangles of light and shadow appear, stretching from a giant stone serpent's head at the bottom to the top of the pyramid, creating a serpentine body.

In one Mayan stone carving, the god Kukulkán is accompanied by a huge quetzal. Lubman suggests that the stairs' chirping echoes add a kind of sound track to the temple, the voice of the bird thought to be the messenger of the gods.

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