I. Classification of deafness: According to WHO 1980 deafness classification criteria, the average speech frequency pure tone hearing threshold is divided into 5 levels. Mild deafness: Hearing threshold of pure tones and speech frequency by audiometer is 26-40dB. Moderate deafness: Hearing threshold of 41-55dB. Moderate and severe deafness: Hearing threshold of 56-70dB. Severe deafness: Hearing threshold of 71-91dB. Total deafness: Hearing threshold of pure tones exceeds 91dB. Classification of deafness: It can be divided into three categories according to the location and nature of the lesion. Conductive deafness: Deafness caused by lesions in the outer ear and middle ear transmitting mechanism, and the transmission of sound waves to the inner ear becomes impaired, for example, cerumen embolism, otitis media, etc. Sensorineural deafness: A lesion of the cochlear spiral apparatus that cannot change sound waves into neural excitation or a disorder of the nerve and its central pathway that cannot transmit neural excitation into it; or a lesion of the central cerebral cortex that cannot distinguish speech, collectively called sensorineural deafness. Mixed deafness: the presence of lesions in both the transmitter and sensorineural organs.