Science

Scientists create the loudest possible sound

Researchers blasted tiny jets of water with an X-ray laser to create the maximum possible sound underwater
Researchers blasted tiny jets of water with an X-ray laser to create the maximum possible sound underwater
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Researchers blasted tiny jets of water with an X-ray laser to create the maximum possible sound underwater
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Researchers blasted tiny jets of water with an X-ray laser to create the maximum possible sound underwater

A team led by Gabriel Blaj, a staff scientist at the SLAC National Accelerator Laboratory and Stanford University, has generated what may be the loudest possible underwater sound. Using SLAC's Linac Coherent Light Source (LCLS) X-ray laser, the researchers blasted tiny jets of water to create incredible sound pressures above 270 decibels.

There is loud, really loud, and rock concert loud, but how loud really is too loud? Surprisingly, there is an actual upper limit to just how intense a noise can be.

Most people who've taken science class have heard of the decibel scale, which measures the loudness of sound. At the very lowest end of the scale there's the limit of human hearing – things like the buzz of a mosquito 10 feet away. At 55 decibels we have the sound of normal conversations, an alarm clock hits 80 decibels, a chain saw at 100 decibels, and the painful sound of a jet taking off 100 m (330 ft) away at 130 decibels. The scale goes the way up to that of a rock concert at 150 decibels (and you thought we were kidding).

Oddly enough, in air, a sound can't get any higher than about 194 decibels and in water it's around 270. This is because sound is an example of something where the measurements break down at either end of the scale.

It's a bit like heat. Absolute zero is the coldest temperature that's possible because once you've pumped all the energy out of an object, the molecules stop moving and there's nowhere further down for the temperature to go. There's also a theoretical upper limit to temperature. You can heat things to hundreds of millions of degrees, but at some point there's so much energy in what is now a superheated plasma that the atoms break down. Add in more energy, and all that happens is that more subatomic particles are created.

The same is true of sound, which 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.

This is what happened when the researchers zapped micro-jets of water (between 14 and 30 micrometres in diameter) with an X-ray laser. When the short X-ray pulses hit the water it vaporized and generated a shockwave. This shockwave then traveled through the jet and formed copies of itself in a "shockwave train" made of alternating high and low pressure zones. In other words, a very loud underwater sound.

What the team found was that once the intensity of this sound went above a certain threshold, the water broke down and turned into small vapor-filled bubbles that immediately collapsed in a process called cavitation. It's a phenomenon also seen in high-speed propellers, or when a mantis shrimp decides to get violent. It also means that because the pressure in the X-ray-generated sound wave is just below the break-apart threshold, it's as loud as an underwater sound can be.

According to the team, this discovery has more than academic value. By better understanding how these shockwave trains work, it may be possible to find ways to protect miniature samples undergoing atomic-scale analysis inside water jets from damage, which would be of great help in the development of better drugs and materials.

The research was published in Physical Review Fluids.

Source: SLAC National Accelerator Laboratory

10 comments
guzmanchinky
I learned something about sound today. Thank you Newatlas.
sugamari
more like a way to kill weaponized dolphins from the safety of some drone pilot desk.
Ben Wah
How does it compare to the clicks of a sperm whale?
NL_01
Really interesting and well-explained article. Also interesting to consider that the dB scale, like e.g. the Richter scale, is logarithmic, which puts 270dB vs 150 for a rock concert into even more perspective.
Víctor
Great article! I'm gonna look up what happens to air molecules above 194 dB! Just one thing: 0 dB does not mean there is no pressure wave, it means the pressure wave has the same pressure as the reference to which you are comparing. For DB SPL (which is this case), it means the pressure wave has a pressure of 20 micropascals ;)
Vernon Miles Kerr
Agree with @guzmanchinky. You blokes should add these excellent science lessons as a regular part of your portfolio. I too learned many new things from this article.
Nelson Hyde Chick
How is this at all useful? Except maybe to kill aquatic life?
amazed W1
Agree with the others, a truly worthwhile article, and Victor's comment is really useful to bring us back to exactly what a logarithmic scale means, there has to be a start point. My daft(?) question is: does this finding explain why shock waves cannot travel faster than the speed of sound relative to the "medium" and does it also open up new worries about the current explanations of the reasons we can't travel faster than light?
Rusty Harris
Boy, I don't know...but I've heard some screaming children come close to that 150db level ;)
robert80
OK, first my credentials. I'm a Ph.D. physicist and teach physics at all levels. I know what I'm talking about, in other words, and the article above is terrible. 0 decibels, for example, is not "no sound" and is not the faintest possible sound. It is a logarithmic scale referenced to the faintest sound we can hear, which isn't even an absolute number for all people but is arbitrarily set at 10^{-12} W/m^2. There are fainter sounds, whose loudness in dB would be negative. Second, 194 dB isn't the loudest possible sound. It is the loudest possible undistorted NORMAL sound, the loudest sound one can make that is a sine wave oscillating with an amplitude of 1 atmosphere AROUND THE BASELINE of 1 atmosphere normal air pressure. Even this isn't completely true both because air pressure varies by several percent above and below "1 atmosphere" on any given location and day, and because when you get close to the maximum the waveform starts to distort a bit as the minimum pressure approaches zero, a vacuum. At overpressures greater than 1 atmosphere, you still get sound, it is just distorted -- the peak pressure can be as high as you like, but the trough pressures are obviously capped at 0 atmospheres because you can't have negative pressure. The distorted waveforms are often called "shock waves" because they are generally produced by things like explosions, and above 194 dB they tend to blow people (and buildings, and pretty much anything semisolid) apart as enormous pressure forces whipsaw anything solid in the path of the wave. The explosions of Krakatoa and Tambora, for example, produced sound with a loudness significantly in excess of 300 dB, sounds that were heard thousands of miles away and that were detectable on the opposite side of the planet. The article would be more correct with the addition of the word "normal". The loudest possible normal sound. Which isn't that impressive, for all that I'm sure it is true. Every time a high speed propeller spins and cavitates, though, it produces the loudest possible NORMAL sound in water in the vicinity of the propeller cavitation, and anytime one sets off a nuclear explosion in water, as in land, one produces a shock wave with more than 270 dB -- what they are really saying is that they have managed to produce a sound right at the threshold, with oscillation around the ambient pressure at some depth between 0 and 2x that pressure, and that (as you do report, at the very end) that even certain animals e.g. mantis shrimp manage to produce sounds at or over this threshold every day. Minor details: decibels are literally nothing like temperature, and absolute zero is nothing like 0 dB. You can read about dB in my online physics textbook and learn how they work, but for one thing, dB are dimensionless (the log of a ratio) while temperature has units. At 0 dB, there is most certainly a pressure wave. At -20 dB, there is still a pressure wave -- you just can't HEAR it, but instruments can easily detect it (it has an intensity of 10^{-14} W/m^2). Even in water, the loudest normal sound is contingent on the background pressure. This varies rapidly with depth in water, slowly with height in air, but it means the "loudest sound" varies right along with it in both media as it is the sound that oscillates with an amplitude of the background pressure in both cases.
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