An apparatus includes an ultrasonic sensor stack, a foldable display stack that includes a display stiffener, and an additional stiffener between the ultrasonic sensor stack and the foldable display stack. The additional stiffener forms an acoustic resonator with the display stiffener and an adhesive layer between the display stiffener and the additional stiffener in order to amplify transmission of ultrasonic waves transmitted by the ultrasonic sensor stack. The additional stiffener includes a material having a high modulus of elasticity, low density, and high acoustic impedance value. In some cases, the additional stiffener includes a metal, a ceramic, a glass, or a glass ceramic. The additional stiffener increases an overall stiffness and mechanical integrity of the apparatus so that a foam backer may be omitted from the ultrasonic sensor stack.