A totally implantable cochlear implant system having an implantable unit comprising: a hermetically sealed housing including: (a) a power source configured to provide the power requirements of the implantable unit; and (b) an audio signal path including: (i) a microphone configured to detect sounds and output acoustic signals representative of said detected sounds; and (ii) a processor means configured to convert said acoustic signals into stimulation signals representative of the detected sounds; and (iii) an electrode array suitable for insertion in the cochlea of an implantee, wherein the array is configured to receive the stimulation signals and output electrical stimulation to the implantee's auditory nerve when inserted; characterized in that the system further comprises an implanted deactivation circuit means (80,90) configured to process the acoustic signals output by the implanted microphone to detect a predetermined type of acoustic signal generated by a human, and trigger shutdown of said electrical stimulation of the cochlea by the electrode array on detection of the predetermined type of acoustic signal generated by the human, wherein the predetermined type of acoustic signal is a sound generated by the human tapping the skull of the implantee. a hermetically sealed housing including: (a) a power source configured to provide the power requirements of the implantable unit; and (b) an audio signal path including: (i) a microphone configured to detect sounds and output acoustic signals representative of said detected sounds; and (ii) a processor means configured to convert said acoustic signals into stimulation signals representative of the detected sounds; and (iii) an electrode array suitable for insertion in the cochlea of an implantee, wherein the array is configured to receive the stimulation signals and output electrical stimulation to the implantee's auditory nerve when inserted; (a) a power source configured to provide the power requirements of the implantable unit; and (b) an audio signal path including: (i) a microphone configured to detect sounds and output acoustic signals representative of said detected sounds; and (ii) a processor means configured to convert said acoustic signals into stimulation signals representative of the detected sounds; and (iii) an electrode array suitable for insertion in the cochlea of an implantee, wherein the array is configured to receive the stimulation signals and output electrical stimulation to the implantee's auditory nerve when inserted; (i) a microphone configured to detect sounds and output acoustic signals representative of said detected sounds; and (ii) a processor means configured to convert said acoustic signals into stimulation signals representative of the detected sounds; and (iii) an electrode array suitable for insertion in the cochlea of an implantee, wherein the array is configured to receive the stimulation signals and output electrical stimulation to the implantee's auditory nerve when inserted; characterized in that the system further comprises an implanted deactivation circuit means (80,90) configured to process the acoustic signals output by the implanted microphone to detect a predetermined type of acoustic signal generated by a human, and trigger shutdown of said electrical stimulation of the cochlea by the electrode array on detection of the predetermined type of acoustic signal generated by the human, wherein the predetermined type of acoustic signal is a sound generated by the human tapping the skull of the implantee.