A couple of scientists in the 1930s named Harvey Fletcher and Wilden Munson mapped out their ideas in a series of graphs that summarize their research on equal loudness as it pertains to frequency distribution.
Another physical principle to be aware of is the concept of masking. Masking reveals our natural tendency to turn up sounds that we are having a hard time hearing as we mix. Simply put, one loud source tends to make it harder to hear other sources. This effect has the most impact going from low to high, and it is easiest to observe in the octave immediately above the lower sound.
Read more about this here: https://www.sweetwater.com/insync/reasons-people-say-loud-frequencies/
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