Therefore, the phon scale essentially traces these equal-loudness curves. Any sound, regardless of its frequency, that falls on the "40-phon curve" is said to have a loudness of 40 phons.
: Any sound, regardless of its frequency, that is judged by a listener to be as loud as a 40 dB SPL tone at 1 kHz is also assigned a value of 40 phons. Why the Phon Scale is Necessary phon scale
If you have a low bass note at 100 Hz and a mid-range tone at 1,000 Hz, and you want them to sound equally loud (e.g., 40 phons): regardless of its frequency