Embodiments of apparatuses and methods for detecting a spoof finger are disclosed. In one embodiment, an ultrasonic fingerprint sensor comprises an ultrasonic transmitter configured to transmit an ultrasonic wave to a finger, an ultrasonic sensor array configured to receive a reflected ultrasonic wave from the finger, and a controller configured to determine a reflected acoustic energy of the finger based on a difference between average amplitudes of the reflected ultrasonic wave from ridges and valleys of the finger; and determine whether the finger is a spoof based at least in part on the reflected acoustic energy of the finger.