High-frequency ensonification of the head has the potential to excite unusual and difficult-to-measure internal vibration behavior. The head is a complex, interconnected vibroacoustic volume filled with and bounded by air, fluids, soft tissue structures, and bone. A literature gap exists in assessment of how ultrasonic vibrations of relatively low frequency and low amplitude might propagate within the skull and cranial contents of humans and cynomolgus macaque monkeys. Ultrasonic emitters are ubiquitous in modern society, including uses in vehicular proximity sensing, room occupancy monitoring, pest control, and industrial cleaning. This investigation uses finite-element techniques to examine vibro-acoustic behaviors of the skull and structures within the cranial cavity in the context of excitation by ultrasonic signals. Previous analysis procedures designed for assessment of possible resonant phenomena in the auditory and vestibular systems are revised and extended to assessment of the skull and the contents of the cranial cavity of humans and macaques, including volumes of cerebrospinal fluid (CSF) and the brain. Results include identification of cranial regions that may experience high-amplitude vibrations in response to ultrasonic excitation. These methods and results are useful for assessing how a wide variety of devices, including communications equipment, might produce biological effects.