An ultrasonic touch sensor includes a touch structure configured to receive a touch; a transmitter configured to transmit an ultrasonic transmit wave toward the touch structure; a receiver configured to receive ultrasonic reflected waves produced by a plurality of reflections of the ultrasonic transmit wave and generate a measurement signal representative of the ultrasonic reflected waves; and a measurement circuit configured to: acquire a first plurality of digital samples of the measurement signal during a first observation window that corresponds to a first ultrasonic reflected wave produced by a reflection of the ultrasonic transmit wave at the touch interface, acquire a second plurality of digital samples of the measurement signal during a second observation window that is subsequent in time to the first observation window, and determine whether a no-touch event or a touch event has occurred based on the first and the second plurality of digital samples.