A decreased magnitude of the sensory perception of sound. Hearing loss can be categorized by which part of the auditory system is damaged, as conductive hearing loss, sensorineural hearing loss, and mixed hearing loss. Another axis of classification uses the degree of hearing impairment. The degree of hearing loss is computed by using a three frequency average taken at 500 Hz, 1,000 Hz and 2,000 Hz. The average of these three frequencies is called the Pure Tone Average (PTA). 0-20 dB is considered normal, 21-40 dB mild loss, 41-60 dB moderate loss, 61-70 dB moderately severe loss,71-90 dB severe loss, and greater than 90 dB profound loss. Note that the word deafness is occasionally used to describe partial hearing loss. The World Health Organization uses the word deafness to refer to complete loss of the ability to hear, and hearing impairment to refer to any degree of reduced hearing.