This invention relates generally to classification systems. The adaptive filter determines the inverse of a synthesis characteristic from the headphone to the sound-collecting microphone based on the input sound signal and the difference, and sets the determined characteristic as a characteristic of the correction filter. The bandage may also be flexible and capable of being attached to a patient without interfering with the patient’s daily routine. The dipoles exert a mutual influence on one another. The fastening element is designed in the manner of a polygonal element which can be brought to engagement in a form-fitting andor frictionally engaged manner with a plurality of fastening edges on the handle body andor on the mounting element.