The absolute threshold of hearing (ATH), also known as the absolute hearing threshold or auditory threshold, is the minimum sound level of a pure tone that an average human ear with normal hearing can hear with no other sound present. The absolute threshold relates to the sound that can just be heard by the organism. The absolute threshold is not a discrete point and is therefore classed as the point at which a sound elicits a response a specified percentage of the time. It is approximately the quietest sound a young human with undamaged hearing can detect at 1 kHz. The threshold of hearing is frequency-dependent and it has been shown that the ear's sensitivity is best at frequencies between 2 kHz and 5 kHz, where the threshold reaches as low as −9 dB SPL.
thumb|350x350px|Average hearing thresholds in [[decibels (SPL) (the unit of 'dB(HL)' shown on the vertical axis is incorrect) are plotted from 125 to 8000 Hz for younger (18-30 year olds, red circles) and older adults (60-67 year olds, black diamonds). The hearing of older adults is shown to be significantly less sensitive than that of younger adults at frequencies of 4000 and 8000 Hz.]]
Psychophysical methods for measuring thresholds
Measurement of the absolute hearing threshold provides some basic information about our auditory system.
Several psychophysical methods can measure absolute threshold. These vary, but certain aspects are identical. Firstly, the test defines the stimulus and specifies the manner in which the subject should respond. The test presents the sound to the listener and manipulates the stimulus level in a predetermined pattern. The absolute threshold is defined statistically, often as an average of all obtained hearing thresholds.
Modified classical methods
Forced-choice methods
Two intervals are presented to a listener, one with a tone and one without a tone. The listener must decide which interval had the tone in it. The number of intervals can be increased, but this may cause problems for the listener who has to remember which interval contained the tone.
Adaptive methods
Unlike the classical methods, where the pattern for changing the stimuli is preset, in adaptive methods the subject's response to the previous stimuli determines the level at which a subsequent stimulus is presented.
Staircase (up-down) methods
thumb|Series of descending and ascending trials runs and turning points
The simple 1-down-1-up method consists of a series of descending and ascending trial runs and turning points (reversals). The stimulus level is increased if the subject does not respond and decreased when a response occurs. Similar to the method of limits, the stimuli are adjusted in predetermined steps. After obtaining from six to eight reversals, the first one is discarded and the threshold is defined as the average of the midpoints of the remaining runs. Experiments have shown that this method provides only 50% accuracy.
To give an example, this could be the probability curve of the subject detecting a sound being presented as a function of the sound level. When the stimulus is presented to the listener one would expect that the sound would either be audible or inaudible, resulting in a 'doorstep' function. In reality a grey area exists where the listener is uncertain as to whether they have actually heard the sound or not, so their responses are inconsistent, resulting in a psychometric function.
The psychometric function is a sigmoid function characterised by being 's' shaped in its graphical representation.
Minimal audible field vs minimal audible pressure
Two methods can be used to measure the minimal audible stimulus The sound level is then measured at the position of the subject's head with the subject not in the sound field.