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Echoes create an interior map app

To record size and shape of a room, researchers use a speaker, five microphones and some math

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5:50pm, June 17, 2013
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Determining a room’s dimensions no longer requires a tape measure. An algorithm that sorts through echoes to develop accurate maps of a room, detailed June 17 in the Proceedings of the National Academy of Sciences, may lead to better sound quality for teleconferences and online gaming.

Previous experimental setups of acoustic maps have always involved a speaker that emits a sound and multiple microphones that record the sound. Ideally, each microphone detects sound waves that bounce off a single wall. Then researchers can use the time the sound was recorded and the direction it came from to calculate the position of each wall and reconstruct the room.

But in practice tracking sound is messy, because most echoes take convoluted paths. They may have bounced off multiple walls and the floor before reaching the microphone.

The challenge, says computer scientist Ivan DokmaniÄ of the Swiss Federal Institute of Technology in Lausanne, was to create an algorithm that could sift through the microphone detections and pull out the speaker-wall-microphone paths.

Using a geometric technique known as Euclidean distance matrices, DokmaniÄ’s team is able to group the one-bounce echoes that all come off the same wall. Then their algorithm uses the times and directions of the echoes to determine the location of the walls and ceiling.

The team tested its approach using a speaker and five omnidirectional microphones, each of which could be placed anywhere in the room. The algorithm accurately determined the dimensions of a trapezoid-shaped classroom to within centimeters. It also estimated the dimensions of an oddly shaped room in the Lausanne cathedral.

DokmaniÄ hopes next to make maps with fewer microphones, while also exploring whether adding more microphones could enable mapping not only oddly shaped rooms but the furniture inside. His big goal is to refine the technique to the point that people could map a room with their phones.

Flavio Ribeiro, an electrical engineer at Microsoft in Redmond, Wash., highlighted the technique’s implications for speakerphone teleconferencing, which is plagued by echoes and stray sounds. He envisions software that could use the microphone array built into game consoles such as Microsoft’s Xbox Kinect to map the room a user is in and then use that information to minimize echoes.

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