As far as the role of back vocals in music is in the support of the leading part, for this purpose certain cohesion is needed, many engineers use the following trick – they copy the audio processing chain of main vox in full as a starting point for further audio mixing and mastering services on BVs group buss. The advantages are obvious – consistent by nature sound of both instruments, furthermore, in a split second. You only have to tweak details.
The audio compressor available has already been adapted to the voice; you just need to specify settings for BVs mixing and mastering. Achieve gain reduction around 5–6 dB by smoothly reducing threshold and controlling values against the meter. Now the attack. The value set up by the professional music mixing and mastering engineer for LV will more likely be too large, and the compressor will let enough music transients pass. They will inevitably run into conflict with main vox. So slowly reduce attack time – BVs will at once take their proper position of the base. The next stage is release time setting. Much more sustain is usually required from harmonies in music rather than from LV. So you can safely raise the value up to 100 ms and higher. This attack and release combination will give a good control of the audio and precise position in the space behind LV.
Thereafter audio mixing and mastering services are transferred into the equalization field. It’s in general here all simple – as the whole processing chain was completely copied, it’s sufficient for the engineer to invert the values available, in other words, to reduce the places enhanced for back vocals. And on the contrary, to enhance those regions where there are cut-outs in LV steam.