Figure 1*: Normal equal-loudness-level contours for pure tones under free-field listening conditions

according to ISO 226:2003

(Permission for publication kindly granted by ISO)

* It should be noted that free-field (binaural) thresholds given in the figure are different from headphone thresholds; the differences depend on the headphone type and frequency tested (BS EN 60645-1:2001*IEC 60645-1:2001; BS EN ISO 389-2:1997)

Source: SCENIHR,  Potential health risks of exposure to noise from personal music players and mobile phones including a music playing function (2008) , Section 3.3.3.1, Page 17