To understand the cicada song, Navy researchers are using lasers to track the parts of cicadas that make sound. Cicadas' internal noisemaker consists of two parts: a pair of plates on their thorax, each of which are attached to ribs that bulge outward. Cicadas contract the plates, making the ribs click inward. A large air sac behind the ribs makes the sound reverberate in the same way that blowing air over an empty bottle creates a deeper sound. Cicadas bulge the pair of ribs over their air sac 300 to 400 times a second. Because the resultant sound waves are out of phase with each other, they somehow combine and amplify, thus creating the discordant rattle that animates the East Coast every 17 years.