Scientists Have Made the Loudest Sound Humanly Possible

Sound pressure can reach much higher decibels underwater than it can in the air. Ezume Images/Shutterstock

Think humans couldn't possibly make more of a racket than they already do? Scientists at the SLAC National Accelerator Laboratory beg to differ — in the noisiest way humanly possible.

The team in Menlo Park, California, claims to have created the loudest sound ever recorded underwater — an ear-splitting blast that would make a rocket's roar sound like elevator music.

As you might imagine, it required more than just banging a few seashells together — more like a battery of X-rays firing in short bursts at micro-jets of water. In a report published this month in the journal Physical Review Fluids, the researchers say the resulting sonic siren exceeded 270 decibels (dB).

Just how loud is that?

A lawn mower revs up to around 90 dB; a Metallica concert might crack 120 dB. You don't have enough fingers to stuff in your ears at 150 dB.

The sound — or more accurately, sound pressure — was the result of shockwaves that formed when the X-ray laser struck and vaporized the water jet.

The shockwaves rippled through the jet, spawning copies of itself along the way, with each segment alternating between high and low sound pressures. Scientists gave that phenomenon the appropriately disco designation, "shockwave train."

This groove continued until the intensity of the submerged sound literally reached a breaking point. That's when the water broke into vapor-filled bubbles that finally collapsed.

It popped, literally, and ate itself.

But what a trip it was. Researchers noted not only the skull-splitting sound pressure, but also the point at which sound appeared to reach its utmost limit underwater.

The volume dial, they noted, can't be cranked much higher than 270 dB, at least underwater. In the air, sound may be even more limited.

That's because, as David Szondy explains in New Atlas, sound is a "pressure wave."

"At zero decibels, there is no pressure wave, but at the other end, the medium that the sound is traveling through starts to break down, so it can't get any louder."

It turns out water is a much sturdier medium than air for keeping it together under a barrage of sound. That's why a rocket can only roar so loud, while an underwater "shockwave train" can blow the doors off a submarine.

But what do scientists learn from essentially banging all those pots and pans together?

For one thing, a shockwave train is a powerful phenomenon that can shred more than just ears. The more scientists learn about it, the better they may be able to protect against it. When analyzed on an atomic scale, for instance, miniature samples can be torn apart by water jets.

If those samples can be protected, they can be effectively analyzed. And ultimately, that can lead to new and more effective drugs and medical treatment.

And that may indeed be worth making some noise about.