If you listen to two tones, they'll sound like they're played at the same volume if they're on a Fletcher-Munson curve. So as you can see, a deep bass sound needs to be much louder than an upper-midrange sound to give the perception of being equally loud. But regardless of this fact, if you want to try and reproduce sounds faithfully, they should be as close to the original as possible.Fletcher-Munson Equal Loudness Curves