Engineers have developed an acoustic remote bomb detection system that can distinguish between bombs with large or small payloads.

The system uses an acoustic array that focuses an intense sonic beam at a suspected home-made bomb. At the same time, a laser vibrometer—developed for non-destructive inspection of materials—is aimed at the device's casing to measure how the object vibrates in response. The way in which the casing vibrates can offer up information about what is inside.

To put the system to the test, the team created two "bombs"—one was designed to simulate a low-yield explosive while the other was made of a material that simulated a high-yield explosive. Both used hydroxyl-terminated polybutadiene polymer embedded with 50 percent and 75 percent ammonium chloride crystals (by volume) respectively.

The fake explosives were affixed to acrylic caps which simulated plastic containers, while the "sonic" vibrations were created using mechanical actuators instead of an acoustic array.

By aiming the laser vibrometer at the plastic cap, the team was able to distinguish between the two materials.

In a separate test, the technique was used to differentiate between an empty container, one filled with water, and one filled with a clay-like substance (made from a mixture of cat litter and water). In this case the acoustic waves were created by an air driver. The team established that short ultrasonic waves are best for detecting the contents of devices made from a rigid material (like metal) while longer subsonic waves are best for softer plastics.

The key benefit of this technique is that it allows bomb detection teams to work from a distance. Existing techniques involve using sniffer dogs, honeybees, x-ray machines, and gas chromatography.

Douglas Adams (not that one), Distinguished Professor of Civil and Environmental Engineering at Vanderbilt, said of the project: "Existing methods require you to get quite close to the suspicious object. The idea behind our project is to develop a system that will work from a distance to provide an additional degree of safety."

Adams is working with Christopher Watson and Jeffrey Rhoads from Purdue University and John Scales at the Colorado School of Mines on the project.

The research was presented at the American Society of Mechanical Engineers Dynamic Systems and Control Conference.