Hello Alexandre,
This shows dB SPL (re: 0 uPa) by frequency for a pure tone / sinewave input. If you want to convert a given dB SPL to its phon level you should take care to ensure that your filter weighting matches with your norm reference for phon level (i.e. A-weighting etc etc however the phons were obtained). See also ISO 226.
Interpolation between phon levels is likely not linear as well. Note that phon levels are a perceptual based scale of "loudness," which itself is the perceptual correlate of sound intensity (a physical measurable quantity in air, typically dB SPL.)
Because this unit is perceptually based, and is predicated using pure tone presentations at 1 kHz, you will get different results: 1) when using third octave or smaller bands (due to critical bands in hearing and nerve signal summation at the brainsteam and cortex), and 2) with time varying or modulated signals (such as the ISTS).
./d5
