https://plot.ly/~mrlyule/16.png
https://tektondesign.files.wordpress.com/2015/03/equal-loudness-contour-curves-reversed.jpg
http://upload.wikimedia.org/wikipedia/commons/thumb/c/c8/Kurve_gleicher_lautst%C3%A4rke.svg/2000px-Kurve_gleicher_lautst%C3%A4rke.svg.png
http://2.bp.blogspot.com/-Dr0LX62Ofpg/UiYLN8fWe4I/AAAAAAAACbY/49kjUL1bouQ/s1600/2000px-Perceived_Human_Hearing.png