If an infant is not sensitive to sound at three months of age, it is possible that this is not normal, and it is recommended that a hearing test be performed to check for any hearing impairment in the infant. Under normal circumstances, hearing is quite good from 3 to 7 days after birth; at 3 to 4 months, the head can be turned to the sound source, and will smile when hearing pleasant sounds; at 7 to 9 months, the source can be identified and the meaning of language can be differentiated; at 13 to 16 months, the child can search for sound sources of different loudness; at 4 years of age, the development of hearing has been perfected. There is a direct correlation between auditory-perceptual development and a child’s language development. Hearing impairment can result in deafness if it is not diagnosed and intervened on or before the critical period of language development (within 6 months). When a three-month-old infant presents with insensitivity to sound, it is recommended that he or she be actively consulted and treated.