The ability of the human ear to recognize changes in sound intensity is not a system of infinitesimal increments but is limited rather to discrete steps. The just-noticeable difference of intensity is generally referred to as the difference limen (DL) of intensity. Employing a method of "minimal changes" and using accurate electronic equipment, Knudsen8 and later Riesz11 first systematically investigated the DL. A slow sinusoidal fluctuation of intensity was produced. The voltage amplitude at which fluctuation was just noticed by the listener was related to that of the minimum of sinusoidal event in terms of absolute voltage; the DL was thus expressed as ▵ E/E. By this method the DL was measured for various stimulus parameters. In general, the smallest values were found for a repetition rate of fluctuation between 3 and 4 cps. Being large for low sensation levels (SL) of the minimum voltage (E), the
TONNDORF J, BROGAN FA, WASHBURN DD. Auditory Difference Limen of Intensity in Normal Hearing Subjects. AMA Arch Otolaryngol. 1955;62(3):292–305. doi:10.1001/archotol.1955.03830030058011
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