Every moment has repercussions, a fact my neighbors are no doubt acutely aware of on Saturday mornings when the kids wake up. Every step, thumb and bump not only hits the floor (or wall, or… ceiling), but transmits energy through those materials, much of which we end up noticing as sound. Thankfully, many of these vibrations are either too faint or the wrong frequency to be detected by our ears, but that doesn’t mean they’re not there. In fact, if you really wanted to, it turns out that it’s possible to detect and decipher almost every vibration a person’s movement might make— right down to individual footsteps along a busy sidewalk.
Wired for sound
This kind of listening is already underway at Stanford University in a project called the Big Glass Microphone. Three miles of fiber-optic cables have been laid in a loop under part of the campus, originally to investigate seismic activity. Seismographs around the world already rely on vibrations being transmitted through the ground in order to sense and triangulate activity like earthquakes, but the fiber-optics have proven to be especially sensitive. Like more traditional seismographs, the fiber-optics can measure small changes in electrical current as it’s mechanically perturbed by vibrations, but the scale of the vibrations detected provide previously unknown resolution in those readings.
As a foot steps on the ground, a relatively small, low-frequency vibration is transmitted through the sidewalk and dirt. This then hits the fiber-optic cable, which at the length of a hair is small enough to stretch slightly as the vibration passes through. With light running through the cable, these fluctuations are measured, and in most applications, thrown out as background noise that would muddy data on earthquakes or explosions. In this case, engineers are looking the other way, seeing how well they can track footsteps and cars, possibly even identifying the source of those sounds by unique vibration “signatures.”
Uses for more electronic ears
This effectively means that any material that can house a fiber-optic cable could conceivably serve as a mechanical sensor for nearby activity. In the case of a sidewalk or road, it could track the movement of people or specific cars driving by. In a building, vibrations could reveal what floor people are on to trigger changes in lighting and heating, or detect when a pipe is leaking in the wall. Or just track you even more than your phone already does.
The fact that this kind of system isn’t terribly difficult to set up is seen as both a good and a bad thing, depending on how it’s applied. It could be a relatively cheap way to get better data on how traffic operates, or to make buildings more efficient. However, any system that can track people without their knowing it is certainly open to abuse, and so many of the questions surrounding the project are now about when it should be used, rather than just if it could work.
Source: Is the ground beneath the Stanford campus listening to you? by Yasemin Saplakoglu, The Mercury News