1. An apparatus for detecting sound adapted to be worn on both ears of a user so as to detect sounds from behind the user, wherein the sounds include a background noise, said apparatus comprising: a plurality of microphones for collecting sounds around the user; a sound inspecting unit for calculating the feature values of the background noise within a preset time interval and for determining if a latest collected sound satisfies a preset condition; a direction estimating unit for estimating the occurrence direction of the latest collected sound and for determining if the occurrence direction is within a preset range behind the user, when the preset condition is satisfied; a background noise removing unit for removing the background noise in the latest collected sound so as to obtain a detected sound, when the occurrence direction is within the preset range; and an alerting unit for informing the user of the detected sound via an alert message.