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CHICAGO (WGN-TV) — They’ve begun to emerge. And they’re going to make their presence very much known.

In Illinois, two broods of periodical cicadas are coming out. In central and southern Illinois, its the 13-year cicadas. In central and northern parts of the state, it’s the 17-year cicadas. (More on what this means is down below). Together, it means billions of them will be making that signature buzzing and clicking.

When they emerged three years ago, readings put the high-pitched sound in the 90 to 100 decibels range. By comparison, that can be as loud as a lawnmower, hairdryer or motorcycle. Still, your exposure to them is unlikely to cause hearing loss, according to the Center for Disease Control and Prevention.

And Illinois isn’t the only place that’s about to be buzzing.

What exactly IS that sound?

In short, the buzz is the male cicadas looking to find a mate. Arizona State University explains it this way:

“Both sides of their thoraxes have thin, ridged areas of their exoskeletons called tymbals. Tymbals are made of a rubbery substance called resilin. The cicadas vibrate their tymbals very fast using muscles in their bodies. With every vibration, a sound wave is released, and cicadas can send out 300-400 sound waves per second!”

ASU’s Ask a Biologist further explains that the abdomens of male cicadas are almost completely hollow. So when sound waves from the tymbals enter the hollow area, they “bounce around,” which can alter the sound to be louder.

Females also emit a sound to attract males, but it’s a clicking sound from their wings.

When are they the loudest?

It depends on the weather conditions. There’s a just-right Goldilocks zone when you can expect them to be at their loudest.

Generally speaking, you’ll hear them the loudest on warm, sunny, dry days with a calm wind.

(WGN-TV photo)

As explained by the U.S. Environmental Protection Agency, there are generally two kinds of cicadas that emerge each year — annual ones, which come out every year, and the periodical cicadas, which emerge every 13 or 17 years to mate.

In Illinois, as explained above, one area of the state is seeing 13-year cicadas come out and another will be seeing cicadas that have been underground for 17 years.

All-in-all, cicadas will be active above ground for about eight weeks, says the EPA. Then, nymphs will hatch from eggs (most likely laid on twigs and brush) in about six weeks before heading underground for their waiting period.