Hearing range describes the range of frequencies that can be heard by humans or other animals, though it can also refer to the
range of levels. The human range is commonly given as 20 to 20,000
Hz, though there is considerable variation between individuals, especially at high frequencies, and a gradual loss of sensitivity to higher frequencies with age is considered normal. Sensitivity also varies with frequency, as shown by
equal-loudness contours. Routine investigation for hearing loss usually involves an
audiogram which shows threshold levels relative to a normal.