The Just Noticeable Difference (JND), or difference threshold, is a measure of psychophysical acuity. It is defined as the minimum difference in the intensity of a starting and a secondary stimulus for which they produce noticeably different sensory experiences. The term is used across sensory modalities.
Weber’s Law states that JND between two values is directly proportional to the original stimulus value. In the case of two tones of different duration, the “original” stimulus is the tone with shorter duration (Allan, 1979; in London, 2012: 33-34).