The key to good sound mixing is to create a certain separation between each instrument. Most instruments do not occupy a single frequency range because they have tails and different factors in their sound that affect the sound space, causing them to step on each other and our mix loses definition.
But this mistake can be fixed if you know how to work on the three axes with the right techniques, and separate correctly each instrument.
One of the most effective ways to solve a frequency overlapping problem is to apply arrangements to the track and move one of the instruments. For example, if you are trying to separate two guitars, by increasing or decreasing the volume of one of them an octave you can get them separated.
In some cases, it will not be so easy and you will not have the opportunity to record again, or as it happens when mixing drums you will not be able to separate the instruments with the pitch. The only thing you can do is to continue and look for another method to create space.
The level of a track has a direct and very strong consequence on how we perceive the depth of it. The higher something sounds, the closer it seems to us. This is very useful with automations, to bring one instrument to the front or to bring others further back and change the presence of some with respect to others.
As we have already mentioned, in music the time factor comes into play and we can use it to give importance to different tracks in different parts of the song. For example, raising the guitar during the solo or the drums in certain filling parts.
Panning is also a powerful tool for creating sound space. It may seem counterproductive, but the best way to get the perfect pan is to mix in mono. This is because panning in stereo has a strong impact on mono mixing.
However, if we get a good space when mixing in mono, it will sound good in stereo. It is not essential to do this every time, but it is advisable to check it from time to time.
When two instruments cover the same frequency range, we can try to pan both on opposite sides of the sound mixing, thus creating a separation in songs that have a lot of guitar, keyboard and brass content.
EQ is one of the most commonly used techniques to correct overlapping instruments. Thanks to it we can pigeonhole an instrument in a specific space when using a subtractive EQ.
The most common is to start with high-pass filters to eliminate unwanted tails, in percussion for example, when the bass drum and bass sound very powerful, a 60Hz cut is used which creates space for the bass. And the same goes for guitars with a low-pass filter.
With EQ, the job is basically to cut off those problematic frequencies and leave space between instruments. But only practice makes perfect and in this case, it is very important to identify the exact frequencies to give the right cuts and apply the filters.
When we want to control the depth, we will do it managing the three factors that are volume, brightness and ambience. We already know that the higher an instrument sounds the closer it will seem to us, with this technique we can place them in different planes and differentiate them from each other, giving it more or less importance in the sound mixing.