Fletcher and Munson measured the sensitivity of human hearing at various volumes and frequencies with the finding that humans hear best in the range of 3kHz-4kHz; the sensitivity falls off rapidly at lower frequencies and somewhat more slowly at higher frequencies. In other words, very soft sounds must be more powerful at frequencies lower and higher than 3kHz-4kHz in order to be heard. The loudness control on music reproduction systems was designed to compensate for the Fletcher-Munson effect. See equal loudness curves.