Threshold of Hearing

Sound level measurements in decibels are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity:

or in terms of sound pressure:

This value has wide acceptance as a nominal standard threshold and corresponds to 0 decibels. The actual average threshold of hearing at 1000 Hz is more like 2.5 x 10^-12 watts/cm^2 or about 4 decibels, but zero decibels is a convenient reference. The threshold of hearing varies with frequency, as illustrated by the measured hearing curves.