Normal people can receive sound of frequency which ranged from 20Hz~20kHz Normal hearing in adults is a range of hearing from 0 dB-SPL hearing level to 20 dB-SPL       Conductive hearing loss: sound cannot be conducted to the inner ear. e.g. ear canal blocked by ear wax, perforated ear drum etc. Sensorineural hearing loss: usually caused by disorder in the inner ear (coclea or auditory nerve) Mixed hearing loss: there is both conductive and sensorineural hearing loss, commonly seen in people with Otitis media An Audiometric and Sound-threshold test will usually be carried out Total irreversible loss of hearing for all sounds certified by a specialist in ear, nose and throat Viral and bacterial infection Internal bleeding, thrombosis and spasm Diabetes, hyperlipidemia Accident