Let me first start this off by saying, "I don't always understand my title of this forum" But I thought this would be cool to explore and examine.
I assume that we all know that the range of what most humans can hear runs from 20 hz to 20khz, so from 20 to 20,000 frequencies per second is all the human ear can deal with.
Those lower frequencies around 20 to hmmm about 100 maybe 200 are felt, just as much as they are heard. Ever wonder why that is so? Why does your shirt and chest rattle in the night club when the bass and kick are pounding away? Those low frequecies are full of energy, and that energy can actually attempt to move what it is trying to go through.
Sound is energy, plain and simple, just like a vehicle driving down the road at 60 mph, put an object in front of it, and both sound and the vehicle are going to attempt to plow through it. The lighter the vehicle and the higher the frequency, the less energy either have when impacting the wall, therefore the less ability they will have to move what they are hitting. Some are so lightweight they just bounce right back the other direction until they hit something the in the other direction.
So, this means the low frequencies are full of energy and the higher ones are just a bunch of lightwieghts bouncing all over the place. Now take this one step further.
Let's say you made a track with a thunderous bass, man it just vibrates the crap out of your shirt and is cool as hell, but then you have the amp from hell to drive that track. Next thing you do is burn the CD and run out to your car, but it sounds like crap. My god how can this be? You just created the next million seller and your car stereo is ruining it.
Well in your studio at 100 watts there is plenty of energy for all of your little freq buddies to play and be happy, but pop it into the car stereo with maybe 20 watts and there just isnt enough juice to go around. Somebody is not going to be heard. So the big energy robbing heavy hitters get their way and the little bounce off the wall wimps get left in the dust. It is only going to sound like one big bass/mudd line.
Ok so now we know that the lower freq's need to be restrained just a little, so we put some roll off below 50. (Side note, personal choice on where to roll it off) Now that lets the weaklings play along side the heavy hitters down at the bottom, but wait, it still sounds like mud. Damn it, what is going wrong here.
Now we have to think about other things and this is where it can get even more complicated. Let's say for arguments sake that you have 10 instruments playing in your track. Every instrument is going to have, what I like to think of as, its dominant frequecncy range. And some of this I am going "off the cuff" because I can never remember the ranges of all of these instruments, so I always go back and check my notes.
Bass and kick are going to be in that low high energy group from 20 to about 200, but then they are going to have harmonics that reach out beyond that, maybe even up into the 4000 freqs or more.
Keyboards are going to be in that 400 up to 3000 with harmonics beyond that.
Snares ride in the 400-1000 depending on tuning with harmonics
Vocals same thing and on and on.
Now you can see that things start to build up in the middle, somewhere between 400 to 8000 and all the stuff beyond are generally the harmonics all of these intruments produce.
It is in that 400 to 8000 range that you have to carve out little nitches for all of those instruments that sit there. If they all try to occupy the same place at the same time, then someone is going to lose and it all sounds like a muddy mess.
If you didn't capture the perfect sound that sits just right, EQ becomes your trusty fix. This is your swiss knife to carve up that precious little space of frequency spectrum and hand it out to each instrument. With EQ you are giving each instrument, the boundaries where it is allowed to play and be heard. No more, no less.
So exlcuding the kick and bass which you held back at below 50 hz you have, not including the snare, toms and cymbals, about 5 instruments that you really need to deal with. Those 5 have to be carved up into frequency nitches to allow them to be heard.
This doesnt mean that you take instrument 1 and roll it off at 300 and 600 and instrument 2 at 600 and 1000 etc. If you did that it would sound like a bad AM radio. It means you use cuts and boosts to give each one its prominent space. What one gets the other doesn't and vice versa and in the end you have 10 instruments all happily being heard.
I hope others jump in and offer some opinions. And I hope the newcomers understand that using EQ is not something you use, "just because", but a tool to carve out niches for all of your instruments to sit inside the limited frequency spectrum of 20 hz to 20 khz. Of course panning, volume and reverb can even play into this, but for now we are only thinking about frequencies.