The present invention relates to an impact beam for use in a vehicle. 05% of the longitudinal acoustic wave velocity within the cladding; or the longitudinal acoustic wave velocity in the core changes by at least 0. Additionally, a base can be provided in order to support the frame. The apparatus may be also used to supplement an estimate of the properties of the movable barrier obtained from measurement of the rotatable drive and provide a more accurate estimate of the movable barrier properties.