:Equal-loudness contour:

Perceived loudness of pure tones varies not only with sound pressure level but also with frequency. Broadly speaking, low frequency tones and high frequency tones need to have higher sound pressure levels than mid frequency tones in order to be perceived as loud. Loudness is measured in phons and curves can be plotted which show the relationship of frequency and sound pressure level for various phon values.

The blue line shows the original ISO standard for 40 phons.
The red lines are the revised ISO 226: 2003 standard.