The Secret of the Cicada's Chirp
The song of the cicada has been romanticized in mariachi music, used to signify summer in Japanese cinematography, and cursed by many an American suburbanite wishing for peace and quiet. Despite the bugs' ubiquity, scientists haven't uncovered how they sing so loudly—some are as noisy as a jet engine—and why they don't expend much energy doing it. But researchers reported in Montreal yesterday at the 21st International Congress on Acoustics that they now have the answer.
The detailed mechanism of the cicada's song is far from fully understood, says Paulo Fonseca, an animal acoustician at the University of Lisbon who was not involved in the project. The work by the researchers "is innovative and paves our way to a better understanding of this complex system allowing such small animals to produce such powerful sound."
Cicadas aren't just a natural curiosity. Small devices that produce extremely loud noises while requiring very little power appeal to the U.S. Navy, which uses sonar for underwater exploration and communication. Derke Hughes, a research engineer at the Naval Undersea Warfare Center in Newport, Rhode Island, says that the loudest cicadas can make a noise 20 to 40 dB louder than the compact off-the-shelf RadioShack speaker in his office using the same amount of power.
Intrigued, he and his colleagues used microcomputed tomography)—a kind of CT scan that picks up details as small as a micron in size—to image a cicada's tymbal, which helps the insect make its deafening chirp. Each cicada has two tymbals, one located on each side of its abdomen. The tymbal is made of a thin membrane connecting thicker sections known as ribs, each of which is thinner than a human hair. Hughes's team scanned the intact tymbal as if it were in a series of cross-sections just a few microns thick and then assembled those cross-sections via a computer to give an extremely detailed picture of the tymbal.
A computer model developed from the image shows that a cicada makes noise by buckling the tymbal. A muscle pulls all the tymbal ribs inward and together. The ribs make a short, sharp noise when they draw together and again when they snap apart. The cicada repeats the action 300 to 400 times per second, creating the characteristic crescendoing drone. Producing noise by buckling is unusual in the insect world, Hughes says. Crickets and katydids, for example, rub their legs to create their characteristic chirps.
But the researchers discovered that buckling isn't the whole story when they mapped each cross-section of the tymbal with a grid and carefully tracked each square on the grid as their model tymbal ribs pulled together and snapped apart. Instead of acting like a single speaker, the cicada's left and right tymbals may act like two speakers that produce sound waves that constructively combine. To see how that happens, imagine two waves traveling on the same string. Where the peaks of the two waves perfectly overlap, they add together and spike much higher than the peak of either wave alone. If the waves are sound waves traveling through the air, the peaks would be spots where the volume is very high. The cicadas may be artfully using this effect to pump their volume to deafening levels without expending as much energy as if a single tymbal had to do it alone.
Hughes says that he eventually hopes to be able to replicate the cicada's sound propagation efficiency, sending loud sounds long distances at low power with a small device. Such a tiny noisemaker could be lighter and more efficient than a typical sonar setup. Right now, Hughes is modeling ribbed shells that are lung-shaped like the body of a cicada, but he has a hunch that the shape might not be essential. "It doesn't need to look like a cicada," Hughes says. It just has to sound like it.