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 actuator transfer function has to be similar to the transfer function of a normal human middle ear to guarantee high system efficiency. A balanced armature actuator was the ideal transducer type in order to meet this requirement considering the given restrictions in size and shape. ©2006 IEEE.