Why we need DRC?

It is well-known fact that humans do not “hear” all frequencies at the same level. We are more sensitive to frequencies between 2000 and 5000 Hz than to frequencies below 2000 Hz and above 5000 Hz. In addition to this, our hearing sensitivity also changes at differing sound pressure levels (SPL). This human hearing phenomenon was first discovered by Fletcher and Munson at Bell Laboratories in 1933 and later further defined by Robinson and Dadson in 1956. Their work resulted in a family of equal loudness curves (or contours), widely known as the Fletcher-Munson equal loudness curves. An illustration of the Fletcher-Munson loudness curves analysis is shown below.

Fletcher-Munson Equal Loudness Curves

On the graph there are 12 curves numbered from 0 to 110. These numbers represent the loudness level in phons. A Phon is a unit used to describe the loudness of a given sound. The reason for introducing the loudness unit is that two sounds with the same SPL (dB) do not necessarily have the same perceived loudness to a human listener.

Equal loudness curves represent the SPL that different frequencies need to possess in order to be perceived as two tones of equal loudness. For example, a 200 Hz tone at a SPL of 50 dB will have the same perceived loudness as a 1000 Hz tone at a SPL of 40 dB. In this case both 200 Hz and 1000 Hz tones have a loudness of 40 phons, and they both belong to the 40 phons equal loudness curve.

As can be seen from the graph, it becomes increasingly harder for us to hear lower frequencies (below a few hundred Hz) and to a lesser extent very high frequencies (above 7000 Hz). At higher listening levels this difference gradually lessens and the curves become flatter.