EQUAL LOUDNESS CURVES
Recall our observation that perceptual loudness of a sound is a function of its absolute intensity:

• The sensitivity of the ear varies with the frequency content and the quality of a sound.
  
  
• The graph above represents equal loudness contours adopted by the ISO (ISO 226).
  
  
• Hearing sensitivity peaks at 4K Hz, and has a secondary peak at 13K Hz.