Manufacturing Bits: May 21

World’s loudest underwater sound; manipulating sound.


World’s loudest underwater sound
A group of researchers hit tiny jets of water with a high-power X-ray laser, creating a record for the world’s loudest underwater sound.

The intensity of the blast resulted in an underwater sound with an intensity greater than 270 decibels (dB). That’s greater than the intensity of a rocket launch or equivalent of creating electrical power for a city onto a single square meter, according to researchers.

For this experiment, the research group included SLAC, Stanford University, Rutgers University and the Paul Scherrer Institute.

Sound is measured in decibels (dB), according to the Centers for Disease Control and Prevention (CDC). A whisper is about 30dB, while a motorcycle engine is about 95dB. A rocket launch is 180dB. Noise above 85dB over a long period could damage your hearing, according to the CDC.

In the experiment, researchers used the Linac Coherent Light Source (LCLS) from the SLAC National Accelerator Laboratory. The LCLS is a powerful free-electron X-ray laser. With the laser, researchers blasted tiny jets of water with short pulses at 14-30μm. This in turn vaporized the water, thereby creating a shockwave. The shockwave created copies of itself, forming what researchers call a “shockwave train.”

At some point, the underwater sound crosses a threshold. The water breaks apart, which in turn determines the limit of how loud sound can get underwater.

Just how loud was the sound? “Fully developed waves had initial peak pressures ranging from less than –24 MPa to approximately 100 MPa, which exceed the compressive and tensile strengths of many materials, and correspond to extreme sound intensities on the order of 1GW/m2 and sound pressure levels above 270dBm,” according to researchers in the journal Physical Review Fluids.

The experiment could pave the way towards a better understanding about these shockwaves. It could create new techniques that could prevent shockwave damage in samples during measurements. Applications include biology and materials science.

Manipulating sound
The University of Sussex and the University of Bristol have developed devices capable of manipulating sound.

Researchers have devised optical systems, which manipulate sound through acoustic metamaterials. Metamaterials are artificial materials containing arrays of metal nanostructures or mega-atoms. Some metamaterials are able to bend light around objects, rendering them invisible. But they only interact with light over a very narrow range of wavelengths, according to researchers.

The technology from Sussex and others could turn glass, wood and 3D printer plastic into systems that manipulate waves. One of the first devices built is called a collimator. “We present some key acoustic devices, like a ‘collimator’ to transform a standard computer speaker into an acoustic ‘spotlight’; and a ‘magnifying glass’ to create sound sources coming from distinct locations than the speaker itself,” according to researchers in a recent paper.

“Acoustic metamaterials are normal materials, like plastic or paper or wood or rubber, but engineered so that their internal geometry sculpts the sound going through. The idea of acoustic lenses has been around since the 1960s and acoustic holograms are starting to appear for ultrasound applications, but this is the first time that sound systems with lenses of practical sizes, similar to those used for light, have been explored,” said Gianluca Memoli from the University of Sussex.

Letizia Chisari from the University of Sussex added: “In the future, acoustic metamaterials may change the way we deliver sound in concerts and theatres, making sure that everyone really gets the sound they paid for. We are developing sound capability that could bring even greater intimacy with sound than headphones, without the need for headphones.”

Jonathan Eccles, an undergraduate student at the University of Sussex, added: “Using a single speaker, we will be able to deliver alarms to people moving in the street, like in the movie Minority Report. Using a single microphone, we will be able to listen to small parts of a machinery to decide if everything is working fine. Our prototypes, while simple, lower the access threshold to designing novel sound experiences: devices based on acoustic metamaterials will lead to new ways of delivering, experiencing and even thinking of sound.”

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