Absolute threshold of hearing

The absolute threshold of hearing (ATH) 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. This is also known as the auditory threshold.
The threshold of hearing is generally reported as the RMS sound pressure of 20 micropascals, i.e. 0 dB SPL, corresponding to a sound intensity of 0.98 pW/m2 at 1 atmosphere and 25 °C. It is approximately the quietest sound a young human with undamaged hearing can detect at 1,000 Hz. 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.
Psychophysical methods for measuring thresholds.
Measurement of the absolute hearing threshold provides some basic information about our auditory system. The tools used to collect such information are called psychophysical methods. Through these, the perception of a physical stimulus (sound) and our psychological response to the sound is measured.
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.
Some procedures use a series of trials, with each trial using the 'single-interval "yes"/"no" paradigm'. This means that sound may be present or absent in the single interval, and the listener has to say whether he thought the stimulus was there. When the interval does not contain a stimulus, it is called a "catch trial".
Classical methods.
Classical methods date back to the 19th century and were first described by Gustav Theodor Fechner in his work "Elements of Psychophysics". Three methods are traditionally used for testing a subject's perception of a stimulus: the method of limits, the method of constant stimuli, and the method of adjustment.
Forced-choice methods.
Two intervals are presented to a listener, one with a tone and one without a tone. Listener must decide which interval had the tone in it. The number of the intervals can be increased, but this may cause problems to 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' methods (up-down methods).
 The simple '1-down-1-up' method consists of series of descending and ascending trials runs and turning points (reversals). The stimulus level is increased if the subject does not respond and decreased when a response occurs.
Bekesy's tracking method.
Bekesy's method contains some aspects of classical methods and staircase methods. The level of the stimulus is automatically varied at a fixed rate. The subject is asked to press a button when the stimulus is detectable. 
Hysteresis effect.
Hysteresis can be defined roughly as 'the lagging of an effect behind its cause'.
When measuring hearing thresholds it is always easier for the subject to follow a tone that is audible and decreasing in amplitude than to detect a tone that was previously inaudible.
This is because 'top-down' influences mean that the subject expects to hear the sound and is, therefore, more motivated with higher levels of concentration.
The 'bottom-up' theory explains that unwanted external (from the environment) and internal (e.g., heartbeat) noise results in the subject only responding to the sound if the signal to noise ratio is above a certain point.
In practice this means that when measuring threshold with sounds decreasing in amplitude, the point at which the sound becomes inaudible is always lower than the point at which it returns to audibility. This phenomenon is known as the 'hysteresis effect'. 
Psychometric function of absolute hearing threshold.
Psychometric function 'represents the probability of a certain listener's response as a function of the magnitude of the particular sound characteristic being studied'.
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 and therefore the absolute threshold of hearing.
Minimal audible field involves the subject sitting in a sound field and stimulus being presented via a loudspeaker. The sound level is then measured at the position of the subjects head with the subject not in the sound field.
Minimal audible pressure involves presenting stimuli via headphones or earphones and measuring sound pressure in the subject's ear canal using a very small probe microphone.
The two different methods produce different thresholds and minimal audible field thresholds are often 6 to 10 dB better than minimal audible pressure thresholds. It is thought that this difference is due to:
Minimal audible field and minimal audible pressure are important when considering calibration issues and they also illustrate that the human hearing is most sensitive in the 2–5 kHz range.
Temporal summation.
Temporal summation is the relationship between stimulus duration and intensity when the presentation time is less than 1 second. Auditory sensitivity changes when the duration of a sound becomes less than 1 second. The threshold intensity decreases by about 10 dB when the duration of a tone burst is increased from 20 to 200 ms.
For example, suppose that the quietest sound a subject can hear is 16 dB SPL if the sound is presented at a duration of 200 ms. If the same sound is then presented for a duration of only 20 ms, the quietest sound that can now be heard by the subject goes up to 26 dB SPL. In other words, if a signal is shortened by a factor of 10 then the level of that signal must be increased by as much as 10 dB to be heard by the subject.
The ear operates as an energy detector that samples the amount of energy present within a certain time frame. A certain amount of energy is needed within a time frame to reach the threshold. This can be done by using a higher intensity for less time or by using a lower intensity for more time. Sensitivity to sound improves as the signal duration increases up to about 200 to 300 ms, after that the threshold remains constant.
The timpani of the ear operates more as a sound pressure sensor. Also a microphone works the same way and is not sensitive to sound intensity.