Abstract: A deformation detection system including an elongated element having a hollow deformable wall member that defines a radiation transmission path. A sound radiation transmission source positioned at one end of the elongated member transmits acoustical radiation into the radiation transmission path. A sound radiation detector positioned at an opposite end of the radiation transmission path is responsive to and detects the acoustical radiation transmitted from the radiation source. Deformation of the wall member at least partially interrupts the radiation transmission path such that transmitted acoustical radiation from the sound radiation source is attenuated, and the attenuated acoustical radiation is detected and assessed by the radiation detector. The detection system has particular applicability to monitor a body opening of a motor vehicle in which a closure member, such as a window, is movable towards a peripheral edge of the body to close the opening by an electrically operated drive mechanism.