There’s no end to the public fascination with Brood II cicadas that emerge every 17 years on the U.S. East Coast, so there will be plenty of noise about their noise Monday at the International Congress on Acoustics in Montreal.
A U.S. Navy research team has been studying the insects in an effort to replicate their acoustical feat of producing big sound with a tiny instrument – small plates on the sides of the insect’s thorax.
Why would the Navy care about cicadas? It’s all about underwater acoustics used in signaling and remote sensing. Getting a lot of sound from a small device using little power is a triple-coupon for the Navy.
The team from the U.S. Naval Undersea Warfare Center in Newport, R.I., has been chipping away at the physics of the cicada thrum for some time, trying to figure out a way to re-create it mechanically. Lately they’ve been using lasers to measure the movement of cicada tymbals, kinds of plates on the thorax of male cicadas (whose sole mission over the next few months is to use that thrum to attract a mate before dying).
But the physics is complicated, as previous researchers have found out. The cicada’s two tymbals, which look like scales or plates, have convex ribs on the underside. The insect can compress the tybal plate, which flexes the ribs inward, creating a loud click.
An Australian and British team in the 1990s theorized that an air sac that fills most of the insect’s thorax acts as a type of resonator.
If you ever blew across the open neck of a bottle, you’ve held a Helmholtz resonator, named for German polymath Hermann von Helmholtz, who perfected them in the 1850s. You’ve also heard one from that booming car stereo or the growl of Dodge Ram pickups and many vehicles with customized exhausts.
When you blow across the lip of a bottle neck, you’re pushing a mass of air down from the neck into the container, and the warbling sound you get is from a pressurization "spring" that bounces that column up and down until pressure equalizes again.
The cicada’s tymbals create a similar effect as they flex some 300-400 times per second on each side of its thorax, pressing the air sac.
The sound waves that propagate from the tymbals, however, are out of phase with each other, the research team has found. Somehow, they combine and amplify into the familiar loud thrum heard on every 17th summer.
The Navy is a long way from replicating the feat, but the model of how it works is becoming more clear. At least to the researchers. They'll explain it to the conference Monday.