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.
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