In Metrology, dead space or threshold is a static characteristic of an instrument defined as the range of different input values over which there is no change in output value.

If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument.

The numerical value of the input to cause a change in the output is called the threshold value of the instrument.

## 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:

$I_0=10^{-12}\;\dfrac{\textrm{W}}{\textrm{m}^2}=10^{-16}\;\dfrac{\textrm{W}}{\textrm{cm}^2}$

or in terms of sound pressure:

$P_0=2\times 10^{-5}\;\dfrac{\textrm{N}}{\textrm{m}^2}$

This value has wide acceptance as a nominal standard threshold and corresponds to 0 decibels. It represents a pressure change of less than one billionth of standard atmospheric pressure. This is indicative of the incredible sensitivity of human hearing.

The actual average threshold of hearing at 1000 Hz is more like 2.5×10-12 W/m2 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 below.