One of the problems with recording technology today is that you can have pretty much unlimited audio and MIDI tracks. Today's computers and hard drives are more than fast enough to easily handle 100 audio tracks or more, and virtually unlimited MIDI tracks, all at the same time.
The problem with that is that many inexperienced (and even some very experienced) artists think that they need to fill up as many tracks as possible. They try to fill up every little space in their song with something, whether it is needed or not.
The artists can easily lose track of the listener's perspective and what is most important at any one point in a song. Because they wrote the song and have already heard it many, many times, the artists keep trying to add new elements to make it interesting to themselves. By doing so, they only end up diluting the melody and main elements of the song, and also make it harder to get a good sounding mix because there is simply too much going on.
I speak from personal experience, having worked with many younger artists who insist of filling up every bit of "breathing space" that the song has with useless noodling and fills that really don't contribute anything meaningful to the song, and only make my job as the mixing engineer harder in the end.
Or, those who don't have the experience to know better think that in order to achieve a really big or huge sound that they have to keep adding more and more tracks of instruments. Sometimes, though, trying to fit more instruments into the mix just ends up making everything sound smaller.
Jazz greats, such as Miles Davis, know that the space in between the notes is just as important as the notes themselves.
Or, to put it simply, Less Is More! 
So... How does all this apply to mixing? And, what does "subtraction" have to do with Mixing anyway?
Most professional mixing engineers will tell you that a big part of the mixing process is deciding what to subtract, or take away, from a mix in order to make it work. This can be taking away complete tracks by muting them for either certain sections of a song, or even the entire song. It can also be subtracting ranges of frequencies from tracks using Equalization (EQ) to make all the tracks fit together better in the mix.
Let's take a look at these ideas in more depth.
The key to a great song and a great sounding mix lies in the song's arrangement, which is just as important as the lyrics and melody. The arrangement is basically all the various instrument parts and how they fit together and play off of each other. With a good arrangement, all the parts work together to strengthen the song without any parts conflicting or getting in the way of each other. A bad arrangement would be when there are multiple parts happening at the same time that don't work together either rhythmically or melodically, making it difficult for the listener to focus on the key element.
The first thing you should look at if a mix just isn't working is the arrangement. (Actually, you should be taking a very close look at your arrangement long before the mixing stage.) If a part is not fitting into the mix right, or if you are having a hard time giving several parts the proper space so they can be heard equally, then it's time to look at the arrangement. You'll probably have to take some parts away, either in certain sections, or throughout the whole song, to make the parts work together better. You may even need to go back and replay a part so that it fits in better with the other parts.
I'm going to paraphrase from one of the books on my Required Reading list, The Mixing Engineer's Handbook which better describes what I'm talking about in terms of song "elements":
The elements of an arrangement are Foundation, Rhythm, Pad, Lead, and Fills.
The Foundation is the rhythm section, usually the bass and drums and sometimes guitar and/or keyboards if they are playing the same rhythm pattern as the rhythm section.
Rhythm is an instrument playing some pattern that is counter to the rhythm section.
Pads are things like strings, organ, synths, or other sounds usually playing long sustaining notes or chords.
Lead is the lead vocal or solo instrument.
Fills usually "fill in" the space around the lead or answer the lead.
Note that an element is not a single track or even a single instrument. A doubled lead vocal track would still be part of the "lead" element. Doubled (or tripled, or quadrupled, or more) rhythm guitar tracks would still be part of the "rhythm" element (or possibly part of the foundation element if they are playing the same rhythmic figure as the drums and bass). Also, sometimes the bass may NOT be part of the "foundation" element if it is doing rhythmic fills or lines that are counter to the drums.
According to The Mixing Engineer's Handbook, a successful song and mix almost never has more than 4 elements happening at any one time in the mix. Three elements can sometimes work well, and on very rare occasions 5 elements may also work.
So, if there are too many elements happening at once, you either need to rework your arrangement, or you will have to mute some parts during the mix to limit the number of elements. But, you can switch things around in different parts of the song instead of simply eliminating one element for the entire song. You could use one set of instruments for the rhythm element in the verses, and then bring in a different set for the choruses, for example.
With a good arrangement and keeping in mind how many elements you have going on and how they are working together, you have a much better chance of getting a great sounding mix without having to spend too much time fighting to make everything fit into the mix.
The fewer sounds you have going on at the same time, the more "full range" each sound can be made and still fit in the mix. Thus, a mix with fewer sounds happening at once, and more breathing space, can actually end up sounding much bigger and fuller than a mix that is overcrowded with too many sounds all competing for the same space in the mix.
When you have multiple instruments/sounds playing at the same time, you'll often have to resort to using subtractive EQ to make everything fit together in the mix.
For example, the low end of a mix is usually the most difficult to get right. It's very difficult to get a very big a full low end that also has a lot of punch and definition without being muddy. The key is to work with the kick drum and bass guitar to get those two working well together, and then making sure that no other instruments have low frequency content that is muddying up that low end from the kick and bass. So, you'll usually end up using subtractive EQ with a hi-pass filter to roll off all the low frequencies from any other instruments that are playing at the same time as the bass guitar and kick drum.
Sometimes even the kick drum and bass guitar can't occupy the same space at once without muddying up the low end. Sometimes you'll want a really deep and low bass guitar (or synth) that just won't work with a really deep sub kick as well. In that case, you'll end up rolling off most of the subs and some of the low frequencies from the kick drum, and instead emphasizing the clicky attack portion of the kick drum to make it be heard in the mix. The bass will still fill in the low end and give that power and punch, so you won't need that in the kick. Heavy metal songs are a good example of this where the kick drums are very "clicky" sounding to cut through the huge wall of sound created by the distorted guitars and bass (which are usually working together as one very beefy element).
To read up more about how to use EQ to create space in a mix, check out my article entitled "3D Mixing and the Art Of Equalization".
To summarize, if you are having a hard time making a mix work, and are trying to get a big and full sound, instead of adding more elements or boosting EQ on tracks, try subtracting some elements and using subtractive EQ as necessary! You may be surprised to learn that through the use of subtraction, your mixes will sound better and bigger!
© copyright 2005, DBAR Productions, LLC
This content may be downloaded for personal use only, and may not be reprinted in part or in whole in any form without the express written consent of Stephen Sherrard and DBAR Productions, LLC