A touch sensor includes a touch structure including a touch interface and an inner interface arranged opposite to the touch interface; a capacitive ultrasonic transmitter arranged inside an enclosed interior volume and configured to transmit an ultrasonic transmit wave towards the touch structure; a capacitive ultrasonic receiver arranged inside the enclosed interior volume and configured to receive at least one ultrasonic reflected wave produced from the ultrasonic transmit wave via internal reflection; a coupling medium that fills an area between the inner interface and the capacitive ultrasonic receiver, wherein an external force applied to the touch interface is configured to impart an internal pressure onto the capacitive ultrasonic receiver through the coupling medium; and a sensor circuit configured to convert the at least one ultrasonic reflected wave into a measurement signal and detect the external force based on the measurement signal.