1. A method for determining if a tracking object is within a selected area comprising the steps of: a) transmitting a first acoustic signal simultaneously from an array of transducers proximate to the selected area; b) receiving first reflected signals, generated in response to the first acoustic signal, in each transducer in the array of transducers, the first reflected signals reflected from boundary objects defining a boundary of the selected area and from one or more tracking objects within the selected area; c) storing digital data corresponding to the first reflected signals in a memory associated with each transducers in the array of transducers generating stored digital data; d) generating second acoustic signals from each transducer in the array of transducers in response to time reversing a read out of the stored digital data; e) receiving second reflected signals in response to the second acoustic signals and generating second digital data; f) analyzing the second digital data and generating and storing a first signature pattern indicating relative locations of the boundary objects and the one or more tracking objects; g) transmitting the second acoustic signals at time intervals and generating a second signature pattern; h) comparing a second signature pattern to the first signature pattern; and i) determining if a tracking object has moved from the selected area in response to the step of comparing the new signature pattern to the reference signature pattern.