The phon was propose as a unit of perceived loudness level LN for pure tones by S. S. Stevens. The purpose of the phon scale is to compensate for the effect of frequency on the perceived loudness of tones. By definition, 1 phon is equal to 1 dBSPL at a frequency of 1 kHz. The Equal-loudness contours are a way of mapping the dBSPL of a pure tone to the perceived loudness level in phons. These are now defined in the international standard ISO 226:2003, and it should be noted that the research on which this document is based concluded that earlier Fletcher-Munson curves and Robinson-Dadson curves were in error.