They have a test tone and each one has a little micro dick (only way I can really describe it) that pokes out during the test tone playback to pick up reflections.
More here: http://www.nytimes.com/2004/02/19/technology/how-it-works-a-...
They have a test tone and each one has a little micro dick (only way I can really describe it) that pokes out during the test tone playback to pick up reflections.
More here: http://www.nytimes.com/2004/02/19/technology/how-it-works-a-...