There's 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 every day work!
So, who were this 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 actually very simple. Your ears hears 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 arround 1-3kHz can be very quiet and you'll hear them instantly.
Ok, so why is this important and how is this a mixing audio tool?
When you mix music for restaurants and other chill out places for example, you have to understand that the playback volume will be set to low. The music is only in background for the acoustic atmosphere. If you mix it in your studio at hi volumes you will finish it with too little bass and probably a litle harsh on the high spectrum. When the music will be played back at low volume levels the listener's ears will not pick up the bass at all. All due to the fact that our ears don't hear all frequencies the same at the same volume! Lesson?! You have to mix or at least check the mix at the volume leves that the final listener will probably listen at.
This is also why a BSE pillow is a fantastic tool. You can mix at low volumes but hear the bass as if you were rockin the whole building. That's how you'll mix it right! Buy the pillow here.
So using a volume knob can boost or cut some frequencies in your perception. You have to use this and understand it. Volume can even be used as compressor since our hearing get's quite flat at high levels. Yes, that's why you like to mix at high levels because a not so good mix will sound perfect. And you know what happens the next day, when you take it for a test run in your car stereo, right!
For those who'd like to be exact. Fletcher Munson curve is a term that audio engineers usually use but are realy thinking about equal-loudness in general. The latest standard for the equal-loudness contour is ISO 226. Read more about is here.