In a method for operating a fingerprint sensor comprising a plurality of ultrasonic transducers, a first subset of ultrasonic transducers of the fingerprint sensor are activated, the first subset of ultrasonic transducers for detecting interaction between an object and the fingerprint sensor. Subsequent to detecting interaction between an object and the fingerprint sensor, a second subset of ultrasonic transducers of the fingerprint sensor are activated, the second subset of ultrasonic transducers for determining whether the object is a human finger, wherein the second subset of ultrasonic transducers comprises a greater number of ultrasonic transducers than the first subset of ultrasonic transducers.