FIG. 9A and 9B illustrate examples of frequency grouping maps for use by an ultrasonic fingerprint sensor, according to embodiments. Using the relationship between transmission frequency contact layer thickness, pixel grouping maps 900 and 950 can be used to control the transmission frequency used for each group. It should be appreciated that a pixel group includes one or more constituent pixels. In some embodiments, a frequency for a particular pixel group is selected according to the corresponding contact layer thickness for the pixel group.