Deafness is classified according to the location of the lesion: conductive deafness: hearing loss due to disorders of the sound wave conduction pathway caused by lesions in the outer ear, middle ear and bone vagus. According to the time of onset: congenital deafness (hereditary deafness and non-hereditary deafness) acquired deafness (acquired hereditary deafness and acquired non-hereditary deafness) classified by the degree of language function development: prelingual deafness and postlingual deafness II. Normal: <25 dB Mild deafness: 26-40 dB (difficulty listening to whispered conversation) Moderate deafness: 41-55 dB (difficulty listening to general conversation) Moderately severe deafness: 56-70 dB (need to speak loudly to be heard) Severe deafness: 71-90 dB (can hear loud speech in the ear) Very severe deafness: >91 dB (can’t hear even loud calls next to the ear)