In psychophysics, the smallest intensity of a sensory stimulus that can be detected or can reliably evoke a sensation, a typical example being the average human absolute threshold for sound intensity at 1,000 hertz (about two octaves above Middle C), which is an intensity of about 6.5 dB SPL (see under decibel). The original concept of a sharply defined limit below which nothing can be perceived was discredited and largely abandoned after the development of signal detection theory, the absolute threshold being redefined as the magnitude that can be detected on a specified proportion of presentations (often 50 or 75 per cent, though other percentages are sometimes used). Also called the absolute limen or Reiz limen (RL). See also acuity, audibility function, audiogram, bone conduction threshold, contrast sensitivity function, contrast threshold, dark adaptation, decibel, Heimdallr sensitivity, light adaptation, Piper's law, psychophysics, Ricco's law, subliminal. Compare difference threshold.