Acoustic "Radar" Spots Stowaways Inside Metal Cargo Containers
Seeing people on the other side of metal walls has never been possible despite the array of high-tech sensors that can peer through other materials. That looks set to change.
The detection of stowaways in lorries, shipping containers, and train carriages is an increasingly important activity as countries all over the world attempt to tackle the illegal movement of people across borders. Various technologies are designed to help but all have significant limitations.
Passive millimeter wave sensors can see through walls but require a source of illumination such as the sky. That generally rules out the detection of stowaways hidden away from sunlight.
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Microwave radar systems provide their own source of illumination but generally struggle to detect motionless people. In any case, these signals do not pass through metal walls and so are unsuitable for cargo containers and the such-like.
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Then there are systems based on the detection gamma rays. These pass easily through metal walls and are designed primarily for the detection of nuclear materials. But they pose a significant health hazard for humans and so are not suitable for spotting stowaways.
Finally, there are acoustic sensors, which can certainly send signals through metal walls but have never been powerful or sensitive enough to detect humans accurately on the other side.
Until now. Today, all that changes thanks to the work of Franklin Felber at Starmark, a scientific consulting company based in San Diego, who has built and tested an acoustic sensor that is both powerful and sensitive enough to detect the breathing motion of an otherwise stationary human on the other side of a cargo container wall.
The problem with conventional acoustic transmitters is that they do not produce the kind of signal that can detect people. This needs to have a specific, narrow frequency that allows a sensor to pick up the change in reflections from an object moving by only a few millimeters, such as a breathing chest, for example. A wideband signal that covers a range of frequencies simply blurs these reflections.
It also needs to be powerful enough to pass through a metal wall, into the air on the other side, then reflect from objects and pass back through the metal wall to a receiver.
Felber began his work by experimenting with commercial off-the-shelf piezoelectric transducers. These change their shape when subjected to a powerful voltage. It is this change that generates an acoustic signal.
But Felber discovered they had significant shortcomings. The most serious was that to produce a powerful enough signal, they have to operate near their damage threshold. What’s more, they use huge voltages—in the region of 3,000 volts—and this requires specialist power conditioning circuits. Even worse, they lose their resonant properties when attached to a wall and this reduces their acoustic power by a factor of thousands.
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But Felber found a remarkably simple and efficient alternative by exploiting an entirely different kind of acoustic transducer that operates with a nine-volt battery. His new machine is essentially a hammer or, as he calls it, a mechanical impact transmitter.
This produces a powerful acoustic signal by repeatedly banging on a metal disc, which then resonates at a specific frequency. When attached to a container wall, the signal passes through into the air on the other side.
An acoustic receiver picks up any reflections from each pulse and a signal processor then subtracts these from the reflections from the previous pulse. Reflections that haven’t changed, ones from stationary objects, cancel out. That leaves only the reflections from moving objects, such as people.
Felber has tested the device and shows it can detect a person on the other side of a wall who is moving and even one that is stationary, merely from the breathing action.
That’s an impressive piece of work that shows how a simple idea can trump the most advanced materials science.
There are caveats, of course. The most significant is the way the mechanical impactor couples to the wall.
To work well, the device and the receiver, have to be carefully secured to the wall in a way that does not significantly change their resonant frequencies, since this would cause the signals to be lost.
That may be possible in a test, but an important question is whether it would be possible in practice when thousands of cargo containers need to be scanned in a wide range of weather conditions.
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Nevertheless, Felber is optimistic. He says the devices is “capable of remotely and nonintrusively scanning steel cargo containers for stowaways at a rate of two containers per minute.”
The fact that an urgent solution is required for this problem will surely draw attention to this work and help focus on the final few practical issues that need to ironed out.
Ref:arxiv.org/abs/1507.01479: Demonstration of Novel High-Power Acoustic Through-The-Wall Sensor
