We Can Now Map Rooms Down to the Millimeter with a Finger Snap

Bats do it, dolphins do it -- even short-tailed shrews do it.

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The Cathedral of Notre Dame in Lausanne, Switzerland, where researchers tested an algorithm that creates high-resolution sound maps of rooms using just a few microphones and a finger snap.

Bats do it, dolphins do it — even short-tailed shrews do it: I’m talking about echolocation, of course, or the use of sound to find and identify objects in your vicinity. When submarines do it, we call it sonar. And when Morgan Freeman’s character Lucius Fox does something like it in The Dark Knight, we call it pretty darned cool.


Now imagine using a technique like that to map a room at high resolution. And I don’t mean in general terms, approximating metrics like the reverberative acoustics of a sound space or studio, as many modern audio products do — but zeroing in on a room’s spatial particulars with millimeter accuracy. How? Using just four microphones and a finger snap.

Researchers at the École Polytechnique Fédérale de Lausanne’s School of Computer and Communications Sciences, located in Switzerland, claim to have pulled it off (or nearly so) using an algorithm that lets you spec the dimensions of an area with a few microphones and the snap of your fingers.

“Imagine that you are blindfolded inside an unknown room,” write the researchers in a paper published in science journal PNAS. “You snap your fingers and listen to the room’s response. Can you hear the shape of the room? Some people can do it naturally, but can we design computer algorithms that hear rooms?”

Apparently we can, and not just one that can map a boring old empty room, but, say, something as sonically complex as the sound space of the Lausanne Cathedral. That’s impressive. But how?

The new technique computes the shape of a room by applying an algorithm — the algorithm is what’s groundbreaking here — that calculates the geometric relationships between the arrival times of echoes produced by something as simple-seeming as a finger snap, allowing what amounts to a “blindfolded” estimation of a room’s geometry.

“Our software can build a 3D map of a simple, convex room with a precision of a few millimeters,” said graduate student Ivan Dokmanić, the study’s researcher lead, speaking to EPFL Mediacom. And since the algorithm can crunch sonic information dynamically, the microphones don’t require special placement: You can drop them anywhere, so long as they’re able to detect the finger snap.

“Each microphone picks up the direct sound from the source, as well as the echoes arriving from various walls,” explains Dokmanić. “The algorithm then compares the signal from each microphone. The infinitesimal lags that appear in the signals are used to calculate not only the distance between the microphones, but also the distance from each microphone to the walls and the sound source.”

That algorithmic ability to filter rebounding sounds, sorting one from another, is what’s groundbreaking. This sorting ability allowed the researchers to associate a “signature” with each wall, reconstructing the 3D geometry of a room completely from a single sound (in this case, the finger snap). What’s more, the researchers see future applications of this algorithmic approach “in areas such as architectural acoustics, indoor localization, virtual reality, and audio forensics.”

“Architects could use this to design rooms — for example concert halls or auditoriums — based upon the specific acoustics they would like to create,” says Dokmanić. Or, as the Mediacom writeup notes, you might use the technology forensically, to analyze a phone call from someone moving around in a room, allowing you to isolate where they’re at, specifically.

Which brings us back to Batman. Near the end of The Dark Knight, a bat-suited Christian Bale dashes through a high-rise, taking out small squadrons of heavily armed dudes in the dark as Morgan Freeman’s character guides him using “sonar-vision,” whereby mobile phone signals from citizens (as well as “the bad guys”) are converted into crisp, articulate images.

The onscreen effect is one of perfect geometric verisimilitude, like pulling up a high-res wireframe model of the world, every nook and cranny rendered, every mutable body in its place. And it’s a chance to celebrate Batman’s geek-tech MacGyver modus operandi, converting mundane assumptions like “everyone has a cellphone” into a city-saving feat of derring-do…though as the video (and partial transcription) below suggests, this sort of intelligence-gathering approach could have ethical implications, too.

Bruce Wayne/Batman: Beautiful, isn’t it?

Lucius Fox: Beautiful. Unethical. Dangerous. You’ve turned every cell phone in Gotham into a microphone.

Wayne: A high-frequency generator receiver.

Fox: You took my sonar concept and applied it to every phone in the city. With half the city feeding you sonar, you can image all of Gotham.