There's a certain feeling I get when I listen to a finished piece of recorded music. A finished production just hits me in a particularly different manner than roughs or 'almost finished' tracks. Yes, part of it is the mixing and mastering, but a bigger part of it is the layering of sounds. As a keyboard player, I spent most of my younger years experimenting with layering synth and drum sounds via MIDI for both live and studio applications. Getting a handle on this part of the production process does not happen overnight, and it requires quite a bit of experimentation to dial in that perfect combination of frequencies.
For me, it begins with having a sonic end goal. In other words, if you know that you want to have a huge snare sound on the chorus, but you are having trouble finding one sound that satisfies you, then you are probably going to need to add a second snare, or maybe even a third. But before you go nuts piling up sounds on top of each other, remember that the more sounds that are layered, the less sonic space each individual sound has to live in. Therein lies the challenge. A good way to begin the sound layering process is to ask what your end goal is by choosing this sound? Why this sound instead of another? In other words, are you choosing this sound because it has a nice shimmer in the top end, and it will blend nicely with your other snare drum sound, which contains more of the bottom end frequencies of the snare? This is just one example of many that may come up during the production process.
It's not uncommon for me to use a drum loops that already has a kick drum in it. Now if I were to just layer my kick in with the loop, I will most likely run into some phasing problems. One solution is to EQ the drum loop with a hi-pass filter, to remove the low-end frequencies, allowing a new kick to live underneath. This subtractive approach to EQ allows me to incorporate my kick drum sounds, but still pick up frequencies from the original kick sound in the loop, creating a kick drum layer. As I work through the sound auditioning process, I am always asking myself if the layered sound feels like one sound, or can I pick out the individual sounds. Ideally, you want the layered sound to feel like one sound, and you want each individual sound within the layer EQ'd and compressed properly, so that it blends well with the other sounds.
Layering sounds by auditioning and importing different audio files (wave, aiff, etc) can be a bit more tedious than if you're triggering your sounds via MIDI. This is one area where I feel that MIDI really shines: having the ability to quickly copy and paste your performance data from one track to another, allowing you to scroll through different sounds and expedite the process. Within minutes, one synth part can be layered with one or two more synth parts to create one huge sounding synth part. Take a look at figure 1, and you'll notice that not only is the MIDI data copied and pasted from one track to another, but it's also transposed up an octave to really fill out the sound.
It is not uncommon to spend more time creating the sonic layers than coming up with the musical parts themselves. The end result can take your track up a level or two, and think about implementing this production technique before you wrack your brain coming up with extra musical parts that can often make a track feel cluttered.
Don’t forget that you can also layer sounds such as vocals, guitars, etc... to achieve the same result. In the picture above all of the tracks are layers for the background vocals of a song.
These elements are mixed at a very low relative level but add a huge tonal change to the song. A very important thing to remember is that no matter how many track layers make up each element of your song, Panning those tracks left and right can also have a huge impact on the overall tone. Notice the panner for these track are all at a different position, spreading out the sound across the stereo image.