Using an X-Ray Laser, Researchers Make the Loudest Underwater Sound

Researchers from SLAC National Accelerator Laboratory have produced an underwater sound so intense that it rivals the Earth-shaking roar of a rocket launch. “It is just below the threshold where [the sound] would boil the water in a single wave oscillation,” according to lead researcher Claudiu Stan, now at Rutgers University Newark. This research by Gabriel Blaj et al. was published in a recent issue of the American Physical Society’s journal Physical Review Fluids.

A Lockheed Martin F-22A Raptor fighter streaks by at the 2008 Joint Services Open House (JSOH) airshow at Joint Base Andrews, Maryland. Mach patterns are clearly visible in its exhaust. Image Credit: Rob Shenk (CC BY-SA 2.0).

Sound is all about vibrations. When you talk, you cause nearby air molecules to vibrate. Those vibrating molecules collide with their neighboring molecules, causing them to vibrate. The trend continues in this way, with air molecules successively transporting the vibration pattern of your words into listening eardrums.
Things work mostly the same underwater, except that sound is transmitted by water molecules instead of air. The world’s loudest mammals—sperm whales— use intense clicks to communicate over hundreds and thousands of ocean miles, as well as to locate prey and for navigation. Divers have to be on alert because sperm whale vibrations are so intense that they can burst eardrums and potentially kill a person.

An armada of sperm whales off the coast of Sri Lanka

If you could hit “pause” after a sperm whale clicks and zoom in on the molecules transmitting the sound, you would see that the water has alternating areas of high and low pressure. The intensity, or loudness, of the sound is a measure of the difference between the highest and lowest pressures. 
Stan and his colleagues didn’t set out to make a record-breaking sound. In fact, they were thinking about images, not sound. The team was doing experiments at the Coherent X-Ray Imaging (CXI) instrument at SLAC, shooting tiny jets of water (microjets) with rapid pulses of a really intense x-ray laser beam to see what happened.

In recent years, x-ray lasers have become promising tools for exposing things many people thought we’d never be able to see—things like the structure and evolution of viruses, proteins, and molecules in real time. Things that could revolutionize our capacity to treat diseases, design efficient materials, and address many of the world’s greatest challenges.

The opportunities are exciting, but these x-ray laser beams are expensive to produce and the studies are time-consuming. Scientists are developing next-generation x-ray facilities with more intense laser pulses and faster data collection rates, but it’s essential to know if and how these changes affect biological samples. Taking data more quickly doesn’t help if you damage the sample during the experiment or change its environment without realizing it.

Biological samples are often contained in water microjets. The SLAC team wanted to know if next-generation x-ray lasers and the new data collection techniques would produce shock waves in the jets that could damage the sample. 

Left- and right-moving trains of shocks travel away from microbubble filled regions in a water jet, after an explosion induced by an X-ray laser. Image credit: Claudiu Stan.

Here’s what they found. When an x-ray laser hit a jet, it vaporized the liquid in that region and formed a cylindrical shock wave that traveled along the jet. The shock wave interacted with the jet boundaries and, through a number of different processes, creating copies of itself. These copies traveled together along the jet, forming a “shock wave train” of alternating high and low pressures. Measurements of the positive and negative pressures in the shock train show that this is probably the loudest sound ever generated in liquid water.

This understanding inspired the scientists to consider the question: How loud can you make a sound? If you’ve been stuck in traffic with a screaming toddler, you know the answer is “too loud.” A more scientific answer, according to Stan, is that “it depends on the system.” Sound can be as loud as the thing it is vibrating can withstand.

When an underwater sound reaches the limit, the water starts to break apart and you get tiny, vapor-filled bubbles. That’s what the researchers saw, suggesting that they were right near the limit of how loud an underwater sound can be.

By analyzing the conditions under which the trains form, the team notes that next-generation techniques could produce pressure waves capable of damaging biological samples or altering the microjets. Their research provides guidance for scientists on how to avoid this, but also suggests that the intense pressure waves could open the door to new kinds of research techniques.

I have to ask though… If an x-ray laser hits a microjet and no one is around to hear it, does it really make a sound?

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