Fletcher-munson curves

Two researchers at Bell Laboratories, one named Fletcher, the other Munson, were the first to accurately measure and plot a set of graphical curves illustrating the way in which the human ear responds to frequency and loudness. The curves show the ear to be most sensitive to sounds between 3kHz and 4kHz, and the graphical plots that Fletcher and Munson created are known as Equal-Loudness Contours.