Equal Loudness Contour
A contour representing a constant loudness for all audible frequencies. The contour having a sound pressure level of 40 dB at 1000 Hz is arbitrarily defined as the 40-phon contour. High frequency sounds above 2000 Hz are more annoying. Human hearing is less sensitive to low frequency sound.
The graph shows the equal loudness contours according to ISO 226:2003. Note that the contours at 10 phon and 100 phon are dotted as there is a lack of data at these loudness levels and so they are not robust lines.