sinusoidal excitation at the resonant frequency is delivered through a piezo-transducer, and ICP is calculated directly from the phase difference between the excitatory signal and response detected with a second transducer. Yost and Cantrell divided the process into two steps. In the first step, changes in the circumference of the cranium are calculated from the phase difference between a sinusoidal excitatory signal, delivered with a piezo-transducer, and the response that is received at a distance with another piezotransducer. In the second step, changes in ICP are calculated as a product of the changes in the cranium circumference and the elasticity constant of the skull that has been determined earlier by causing known changes in ICP while measuring the cranium circumference. None of the aforementioned methods has been properly validated in relevant clinical populations, and their accuracy is unknown. One may assume however that it would be comparable to the ultrasound time-of-the-flight methods, and thus insufficient for a routine clinical use. Luna Innovations Incorporated (NASDAQ: LUNA) developed EN-TACT system, an ultrasound device for monitoring compartment syndrome. The technology was claimed to have applications for raised intracranial pressure. Based on research from the NASA Ames research center, the company used ultrasound to measure skull diameter changes caused by ICP changes. However, skull changes are tiny and only indirectly related to ICP, raising questions about the accuracy and calibration. Tympanic membrane displacement Tympanic membrane displacement (TMD) technique, proposed nearly twenty years ago by Marchbanks exploits the effect of intracranial pressure on the acoustic reflex, i.e. a reflex contraction of the stapedius and tensor tympani muscles in response to a sound. Normally, vibrations of the tympanic membrane (eardrum) elicited by acoustic stimuli are transmitted through the chain of ossicles (malleus, incus, and stapes) in the middle ear to the oval window of the cochlea. Vibrations of the footplate of stapes transmit through the oval window to the perilymph, which in turn causes the endolymph, the basilar membrane, and the organ of Corti to vibrate, activating ultimately the acoustic sensor cells, the inner hair cells of the organ of Corti. The transfer function of this complex mechanical system under physiological conditions is modulated by the action of two small muscles of the middle ear, the tensor tympani, and stapedius. The tensor tympani arises from the cartilaginous portion of the auditory tube and the osseous canal of the sphenoid and, having sharply bent over the extremity