Equal Loudness Curves

Having studied some of the essential parts of the ear, I am now moving on to psychoacoustics, the study of how sound is perceived by humans and how this perception differs from the physical reality of sound.

One main difference in the human perception of sound versus reality is the perceived loudness of sounds of different frequencies. Studies were conducted that varied the frequency and intensity of sounds to find the intensity of different frequencies that humans perceive to be equally loud. Data from such studies are often compiled graphically as equal loudness curves:

(The Physics of Music and Color)

In this graph, the threshold of hearing represents the lowest intensity of each frequency that could be perceived, and the threshold of pain represents the lowest intensity at which each frequency began to cause pain. The lower parts of each curve signify the frequencies that humans are able to perceive the most easily, or perceive as louder, because such frequencies require less intensity to be perceived. This makes the most sensitive range of human hearing 1000-4000 hertz.

An understanding of equal loudness curves is an important part of mixing music properly. The goal of mixing is to make each instrument heard well. Due to this, one might assume that an EQ spectrum for a well mixed song would be perfectly flat, because the instruments that take up different frequency ranges would have the same intensity. Equal loudness curves prove this assumption wrong, however. In order to accommodate for the human perception of sound, a well mixed song will have lower frequencies that are more intense than the 1000-4000 hertz range, so that people perceive them to be the same volume. Here is an EQ spectrum analysis for a professionally mixed song to illustrate this point:

Works Cited:

1.  Gunther, Leon. The Physics of Music and Color. New York, New York: Springer, 2012.