1. A method for determining an approximate impulse-response vector for canceling an acoustic echo resulting from an audio signal transmitted from a first location to a second location, the method comprising: forming a trust region within a search space based on computing a recursive specification vector defining the trust region; computing a recursive shadow-impulse-response vector that lies substantially within the trust region; and computing the approximate impulse-response vector based on the recursive shadow-impulse-response vector and the recursive specification vector