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X ray ears

X ray ears well, here is what you have to do. You have this artillery shell filled with either ordinary explosive or a deadly nerve gas. How do you find out what is inside without endangering lives or a total nervous shutdown? The question is not as hypothetical as it may seem. United Nations inspectors enforcing the Chemical Weapons Conventions Treaty face this problem all too often. Fortunately, they now have an answer: a device that, when pressed against a container of almost any size, can identify its contents using sound. Yes, just sound. The technique, which has already sparked off stiff competition with over 12 patent application that have been filed, may have innumerable industrial and environmental applications ( Scientific American , Vol 277, No 6).

Dipen N Sinha and his colleagues built the device, a sensor, at the Los Alamos National Laboratory and described it at the American Chemical Society meeting late last year. In about 20 seconds, Sinha claims, a soldier using the 2.27-kg, battery-powered gadget can reliably distinguish not only whether a shell contains chemical weapons but also which of the wide variety of toxic cocktails it holds.

At first glance, the machine looks a bit like the ultrasound imagers used in the hospitals. It has one piezoelectric pad that acts as a speaker and another that serves as the microphone. (Piezoelectric substances are unique as they undergo electric **polarisation** under applied mechanical stress. In other words, piezoelectric materials produce electric charges when pressed, pulled or twisted.) however, unlike an imager, this sensor can determine the makeup of a hidden material. It does so by exploiting sound in a different way.

By using "swept frequency acoustic interferometry' to be precise. Those big words mask what are "actually extremely simple principles of physics that have been well understood since the 1940s,' says Sinha. Toot a bugle, and its tubes vibrate at one set of frequencies, the air inside them at another. Pursing your lips just right creates standing waves that resonate inside the horn and emerge as distinct musical notes. Sinha's magic sensor similarly listens for resonant peaks emitted by an object as the speaker pumps sound waves into it at frequencies that rise gradually from one kilohertz to 15 megahertz. By analysing the peaks and valleys and how they change as the frequency rises, Sinha's software calculates the density of the hidden material, the speed of sound through it and the material's ability to absorb different tones of varying pitches.

Scientists have long known how to do this kind of sonic analysis under controlled laboratory conditions, using calibrated vessels. "What we have done is to develop very efficient computer algorithms that can extract all this information from the measurements of containers of almost any conceivable size,' Sinha explains.

"As experts, we all know what he set out to do was possible in principle, but we were amazed that he had actually succeeded in applying the fundamentals to such a variety of practical and very messy problems,' affirms Logan E Hargrove of the Office of Naval Research. Chemical weapons identification is just the start: Sinha says that his team has demonstrated that the technique can be used to monitor water inside tanks for pollution and to detect bacterial growth inside milk cartons and canned coffee. It might even come in handy in medicine. "We put this thing up to our heads and were able to measure the intercranial pressure in our brain cavities,' he reports. "The only other way to do that is to drill a hole in the skull.' Los Alamos has already licensed its patents on the technology to several companies, says Sinha. Because the sensor can detect very small changes in chemical composition, he asserts, "people in the semi-conductor industry are very interested in using it for quality control of cleaning fluids.'

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