Loudness perception by the human ear varies as a function of intensity and frequency of the sound source. Intensity is a measure of the amount of energy from a source that hits the area that absorbs the sound. The subjective loudness level is a measure of how loud a sound feels to the human ear.

The threshold of hearing is the minimum intensity at each sound frequency required for a sound to be heard by humans. The threshold of pain is the maximum intensity at each sound frequency that humans can tolerate without pain. The figure below displays the variation in subjective loudness perception versus frequency and intensity. Each line on the graph represents a curve of equal loudness. Sound intensities are given in dB and W/m2, and frequencies are given in hertz [(Hz); 1 Hz  = 1 cycle/sec]. Subjective loudness levels are given in phons.

_{Source: http://johnparkguitars.com/resources/techno-talk/
The graph is adapted from H. Fletcher and W.A. Munson, The Journal of the Acoustical Society of America, 5, 82 (1933).}