UI Researchers get ultrasound to act like laser light

Richard Weaver appears almost giddy talking about sound waves.

Here's something cool, he relates. Stand in a big echoing room, like a cathedral, clap your hands, and listen to the sound bouncing around.

Then, clap and immediately stick your ear where your hands were moments before. The sound you hear will be twice as loud. In essence, "the sound field remembers its source" and comes back home.

Wait just a little bit before putting your ear where your hands were and the sound is three times as loud, an effect Weaver, a University of Illinois theoretical and applied mechanics professor, characterizes as just plain spooky.

Now, Weaver's fascination with sound waves has him and colleagues doing something with ultrasound – sound beyond hearing range – that may not be spooky but certainly is unusual.

Weaver, UI research scientist Oleg Lobkis and Alexey Yamilov, a physics professor at the University of Missouri-Rolla, have built an ultrasound instrument that produces sound waves in a manner analogous to the way a laser produces light.

Like the light from a laser, the ultrasound waves the device generates are coherent and of one frequency.

Also like lasers, which corral unruly light particles and allow light to be controlled and focused for a variety of interesting purposes, this "uaser" – the name the researchers have given the instrument, pronounced WAY-zer – has a number of potential applications.

"What we have right now is some interesting physics," Weaver said recently.

But the uaser demonstrates that the essential nature of a laser can be mimicked in sound, the UI professor said.

That has Yamilov excited about using the device, among other things, as a model for better understanding lasers, about which there's still much we don't know. The ultrasound waves can be controlled at a microscopic level and they're big, making them easier to manipulate and study than laser light.

Weaver, whose research focuses on ultrasonics, said the uaser also might be employed as "a very sensitive detector of mechanical properties, the changes in them I should say."

The frequency of the ultrasound from the device is highly dependent on the properties and structure of materials with which it interacts.

If the materials change, the frequency changes, providing a signal researchers can monitor.

"We can measure that frequency with enormous precision," Weaver said.

They've tracked, for example, the changes in cement paste over the weeks it takes to cure, in collaboration with UI civil and environmental engineering Professor John Popovics.

That may sound about as interesting as watching paint drying, but consider it in terms of checking materials – an airplane wing, say, or the thin films widely used in modern electronics – for microscopic damage from aging or stress, which might eventually lead to failure.

Weaver said the uaser also might be used to study "phase changes" in materials, like water turning to ice or metal magnetizing.

Light, like from a light bulb, generally consists of a bunch of incoherent particles, called photons, scattered over a number of wavelengths and about as easy to control as it is to herd cats.

The laser's trick is generating coherent light that's nearly monochromatic, a single wavelength or color.

All you have to do is listen to your stereo to know that sound is normally coherent – even heavy metal music. That's true as long as the sound comes from a single source. Crank up several sources – a bunch of stereos – and you get an incoherent mix of different frequencies.

Enter the uaser, development of which has been partially funded by the National Science Foundation. What the UI device does is get ultrasound waves from different sources to work together, in effect to set themselves to the same frequency, much as a laser gets photons on the same wavelength, Weaver and Lobkis said.

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