Study: echolocation algorithm maps cathedral in 3D to the millimetre

18 Jun 2013


The team, from the Swiss Federal Institute of Technology (EPFL), believes the process will have applications in reverse research and development applications -- designing a building according to the acoustics you'd like it to have, for instance. In addition, they are already working on developing a smartphone app that will be able to translate the process to public buildings to let consumers know exactly where they are. "Our software can build a 3D map of a simple, convex room with a precision of a few millimetres," said PhD student Ivan Dokmanić involved in the study run by the EPFL's Audiovisual Communications Laboratory (LCAV) and published in the Proceedings of the National Academy of Sciences. What makes the algorithm design compelling is that it only needs data from four microphones, and these microphones can be placed anywhere in the space to get an accurate reading. It filters out the early, stronger echoes, from the latter weaker ones, building up an image as it goes along. "Each microphone picks up the direct sound from the source, as well as the echoes arriving from various walls," Dokmanić says. "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."

Explaining in further detail in the journal, the authors write: "The echo-sorting algorithm outputs a list of image sources. Some of these image sources are ﬁrst-order images that we use to reconstruct the room. Some of the output image sources are higher order sources, and we need to detect them and remove them from the list... We process the candidate image sources in the order of increasing distance from the loudspeaker."

The algorithm is essentially filtering out only the original, strongest echo, and therefore not getting muddled by "higher order" sources which are made up of a combination of "lower order" sources -- i.e. higher order noises are the product of multiple reverbs coming together to produce one misleading sound.

The algorithm was successfully tested in a room where the wall was moved, but more impressively in Lausanne Cathedral's side portal, which is as intricate as a building comes. The region was accurately mapped in 3D, and the location of the loudspeaker that let out a short noise for the microphones to pick up pinpointed.

The idea behind the technology is not to help train us all in echolocation -- which researchers believe can be done -- but apply it to forensic science, architecture planning and even the consumer space. According to a report in the New Scientist, the team wants to build a smartphone app that uses inaudible ultrasound to map interiors, say in a shopping centre, to tell consumers exactly where they are -- for instance, turn left for the underwear section and go straight on. Of course, better floor planning might be the better option here, with the efforts of echolocation perhaps more practical for design purposes.