Researchers at Massachusetts Institute of Technology’s (MIT) Lincoln Laboratory developed a pinpointed sound beam that can detect buried land mines from a safe distance. Called a parametric acoustic array, the device is made up of ceramic transducers -- devices that emit a powerful narrow acoustic beam at ultrasonic frequencies. One meter away, the ultrasonic pressure level measures 155 decibels (more acoustic power than a jet engine). Immediately outside the beam, the acoustic intensity dies away to almost nothing. By a process know as self-demodulation, the air in front of the acoustic beam converts the ultrasound to much lower frequency audible tones that sound like extremely loud tuning forks. Unlike ultrasound, the low-frequency sound can penetrate the ground, causing detectable vibrations in the mine's plungers and membranes. The vibrations make their way to the surface where they are detected by a laser system that measures vibrations in the ground.

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