Fletcher and Munson were researchers in the ’30s who first accurately measured and published a set of curves showing the human’s ear’s sensitivity to loudness verses frequency. They conclusively demonstrated that human hearing is extremely dependent upon loudness. The curves show the ear to be most sensitive to sounds in the 3 kHz to 4 kHz area. This means sounds above and below 3-4 kHz must be louder in order to be heard just as loud. For this reason, the Fletcher-Munson curves are referred to as “equal loudness contours.” They represent a family of curves from “just heard,” (0 dB SPL) all the way to “harmfully loud” (130 dB SPL), usually plotted in 10 dB loudness increments. Though the Fletcher-Munson Curves are by far the most widely known contours there have been others defined in recent years that some engineers think are more accurate.