Crystal Blue Vibrations

by Michael Mullaney on September 22, 2009


I ran into professor Henry Scarton not long ago at the Rensselaer Union, and he spilled the beans about an exciting new research project his group is undertaking: learning to listen through walls.

Sure, there are probably countless potential espionage and intrigue applications for such a device, but the technology is far more Geordi La Forge than James Bond.

Think of big, heavy pressure tanks with thick walls – like industrial-sized versions of the propane tank you use to power your outdoor George Foreman grill. These tanks are heavy-duty to keep the contents safe, but their sturdy, foolproof design also makes it near impossible to diagnose the condition of the contents. This can prove challenging for engineers, technicians or other who want to get an accurate read on the temperature of chemical state of the contents. Generally the only way to score information on the contents is to drill through the tank and look inside. This is impractical in industrial settings, however, as it’s expensive, there are considerations with the content being under pressure, and it greatly raises the chances that the tank will spring a leak.

Scarton‘s group has developed a new method for using acoustic signals to communicate through solid walls – even through big metal tanks with steel walls that are several inches thick.

Here’s what they did: they attached a pair of ultrasonic crystals to both sides of a steel wall, and let them communicate by tapping out a signal – which Scarton says is not unlike two prisoners in adjoining cells talking to each other through Morse code. Ultrasonic crystals, which are (or at least were) at the heart of ultrasound technology, vibrate when exposed to an electric current.

The first crystal, located outside the vessel, vibrates at 1 MHz, causing an ultrasonic wave to travel through the wall, Scarton says. On the other side, the second crystal vibrates in such a way that it indicates how much of the initial ultrasonic wave made it through the wall, and how much got reflected back to the first crystal.

It is through these modulations, Scarton says, that the second crystal encodes and communicates whatever measurements the internal sensor has recorded – temperature, presence of a certain chemical, etc. The first crystal, in turn, is then able to retrieve this data by detecting the second crystal’s modulations. The research team has proven that they can send 50,000 bits of information per second across a wall.

In the true green spirit that is informing a large portion of the research taking place at Rensselaer, all of the internal electronics of Scarton’s setup – including crystal and sensor – can be powered by skimming off a portion of the energy in the ultrasonic waves coming from the crystal outside the vessel.