A new hearing therapy called Direct Acoustical Cochlear Stimulation (DACS) was developed and validated in a first clinical trial with four patients. The key component of this therapy based on an implantable hearing device is a micro-actuator that is implanted in the mastoid right behind the external auditory canal of a patient. It generates vibrations that are directly coupled to the inner ear fluids and bypass therefore the outer and the middle ear. This allows treating severe to profound mixed hearing loss. The hermetically sealed actuator was designed to provide maximal output power by keeping the overall size small enough to be placed in the mastoid. An analytical model was used to simulate the dynamic characteristics of the actuator in order to adjust them to the transfer function of a human middle ear. The geometric parameters of the actuator were optimized using unite element modeling. © 2006 IEEE.