An
equal-loudness contour is a measure of sound pressure (
dB SPL), over the
frequency spectrum, for which a listener perceives a constant
loudness when presented with pure steady tones. The unit of measurement for loudness levels is the
phon, and is arrived at by reference to equal-loudness contours. By definition, two sine waves of differing frequencies are said to have equal-loudness level measured in phons if they are perceived as equally loud by the average young person without significant hearing impairment.