To push sounds further into the background, you don’t need any magic plugins, just an understanding of psychoacoustics:
1) Less bass. Much less bass. Natural sounds that are far away will have very little bass and low mids (unless they’re truly huge sounds in movies), because lower frequencies require much more power to travel. The reverse of this is the proximity effect – where sounds very close to your ear (whispering I hope!) or very close to the microphone tend to have much stronger lower frequencies. To roll off the bass, try a low-strength (1-pole or similar) high pass filter. Start low and shift it up until you no longer “feel” the sound.
2) Less treble. Less sparkle, less definition. Natural sounds that are far away will have reduced higher frequencies due to absorption by air and other materials. Distant sounds also have much less definition and clarity. Often a low-strength (1-pole of similar) low pass filter (with no resonance!) will work well.
3) Reverb, modulation. As above, distant sounds tend to have much less definition and clarity. You should do whatever’s appropriate in the mix to “unfocus” the sound. Sometimes more reverb will do it. Often a very short reverb will work best. It doesn’t have to be a strict room – just something to diffuse the sound. Sometimes chorus or even subtle phaser will work better. It depends on the mix – you’re trying to reduce the clarity of the sound.
4) Collapse to mono. Distant sounds do not wrap around the listener’s head. They’re often not “wide” (unless they’re truely huge sounds in movies). Sometimes a full mono collapse isn’t appropriate though – it depends on the sound. You might want to retain a little width in atmospheric sounds (like pads). Sometimes leaving a little width will improve the diffusion in the sound (when a full mono collapse might make it more focussed).
5) Pan centre. This works for two reasons. Firstly, sounds that are panned to the side tend to “creep up” closer to the listener. Imagine the soundstage in front of you as a semicircle – the sounds on the side can (all things being equal) actually get closer to the “front” than the sounds in the center. Also, panning centre will hide the background sounds behind other foreground typically also panned centre (such as lead vocal and snare, depending on your genre). This will make it mroe difficult for the listener to focus on the background.
6) Compose it in the background. To support the above, you should actually compose the parts as background parts. Again, this means understanding the application of psychoacoustics to composition. As listeners, we tend to focus on sounds that are:
– higher pitched
– moving quickly
– not repeating in short cycles (EDM- I’m looking at you!)
– phrased (ie. not constant)
Likewise, background parts will be the opposite:
– lower pitched
– moving slowly
– repeating patterns
Likewise, background is only ever a relative measure. If your background part isn’t getting far enough in the background, it could be that you don’t have anything far enough in the foreground. Just like everything else in music – if everything is background, nothing is background.
Of course, this is all fundamental composition technique. Believe it or not, we all can learn from the classics!