There are so many self-proclaimed audio engineers that can't explain how to use volume and an old theory from Fletcher and Munson that it's scary! But I believe we'll explain it here in simple words for anyone to understand it and use it in everyday work!
So, who were these men? Harvey Fletcher and Wilden A. Munson were two researchers that proposed the first equal-loudness contour back in 1933 when they published the paper "Loudness, its definition, measurement and calculation" in the Journal of the Acoustic Society of America.
It's very simple. Your ears hear different frequencies at different levels for example. To hear a very low frequency, it has to be quite loud, or your ears just can't pick it up. And the frequencies around 1-3kHz can be very quiet, and you'll hear them instantly.