The ratio of minimum percepti-ble increment of sound intensity to total intensity [DELTA]E/E was measured as a function of frequency and intensity. Measurements were made over practically the entire range of frequencies and intensities of which the ear is capable of sensation on 12 [male] observers. The curve for [DELTA]E/E as a function of frequency shows a minimum. Analytical expressions and curves are given showing the dependence of [DELTA]E/E on frequency intensity.