The minimum detectable level of a sound in the abscence of any other external sounds
the lowest level of stimulation that a person can detect
The minimal level of stimulus intensity needed to detect a stimulus.
A difference threshold is the amount of change necessary for an observer to report 50% of the time that a stimulus has changed. Absolute threshold is "the statistically determined minimum level of stimulation necessary to excite a perceptual system." The absolute threshold is an "average of the responses of a range of normal people."