The Absolute Threshold of Hearing (ATH) is the minimum intensity of a pure tone that the ear can hear in a noiseless environment.
This threshold is frequency dependent, and typically shows a minimum (indicating the ear's maximum sensitivity) at frequencies between 1kHz and 5kHz. A typical ATH curve is pictured in Fig. 1. The absolute threshold of hearing represents the lowest curve amongst the set of equal loudness countours, with the highest curve representing the threshold of pain.
In psychoacoustic audio compression, the ATH is used, often in combination with masking curves, to calculate which spectral components are inaudible and may thus be ignored in the coding process; any part of an audio spectrum which has an intensity below the ATH may be removed from an audio signal without any audible change to the signal.
The ATH curve rises with age as the human ear becomes more insensitive to sound, with the greatest changes occurring at frequencies higher than 2kHz. Curves for subjects of various age groups are illustrated in Fig. 2. The data is from the United States Occupational Health and Environment Control, Standard Number:1910.95 App F