Equal Loudness Contours   

Originally proposed by Fletcher and Munson and may be referred to as such in some texts.  diagrams which demonstrate that the ear does not hear all frequencies with equal sensitivity.  we are most sensitive to frequencies around 2000 Hertz and less sensitive to frequencies which are higher or lower.
These are graphical representations of the sensitivity of the ear to frequencies over the human range of hearing at various loudness levels.  
Each line shows the intensity level for the range of frequencies which give a subjective impression of similar loudness in reference to a starting level at 1000 Hertz.  These graphs show that the subjective perception of equal loudness over a range of frequencies is variable according to those frequencies. 
This phenomena has significance for determining optimum levels for monitoring music in studio environments. If the playback level is too high or too low an unbalanced frequency image is likely to occur particularly in the low frequency range where our perception of loudness changes significantly at various loudness levels. A suggested optimum monitoring level is 85 db.