As defined in American Standard Acoustical Terminology,1 "Loudness is the intensive attribute of an auditory sensation, in terms of which sounds may be ordered on a scale extending from soft to loud." In the normal human ear, this scale has been found to extend from a threshold level of detectability (the acoustic reference level of 0 db.=0.0002 dynes/cm.2 in sound pressure) to a level designated as the threshold of feeling (approximately 140 db.). Between these two defined limits lies a psychological continuum related to the physical intensity scale, but not identical with it. The relationship established between the measurable physical units and the observable psychological units constitutes the loudness function.
In the pathological human ear, the dimensions of these psychological units are sometimes found to differ substantially from those normally observed. The term recruitment has been employed in reference to these abnormalities in loudness function, and the present
EISENBERG RB. Loudness, Recruitment, and Differential Diagnosis. AMA Arch Otolaryngol. 1958;68(2):199–206. doi:https://doi.org/10.1001/archotol.1958.00730020207011
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