What difference does an instrument or vocal make if you can't hear it?
I was reading the current issue of my favorite recording mag when this particular phrase sprung into vision... "Take the bass down so that you can just hear it, and then once you've done that take it down by a further 3 dB". You'll have to read the mag to see the original context, but it seems to me like an interesting idea for comment.
"Set the level so that you can just hear it" is a phrase I find myself saying quite a lot as a piece of advice. Not about the bass however, but about reverb. I find that this point is a good benchmark for reverb. Of course the requirements of the track might demand that the reverb is clearly audible. But would there be any value in having the reverb at a lower level, i.e. at a level where you can't hear it?
What does 'just hear' mean?
There are two ways of interpreting 'just hear it'. One is that when you listen to the mix in its entirety, you can just hear the instrument or effect in question. The other is that you can't actually pick out the sound in the mix, but when you switch it out you can hear the difference.
Sounds that you might want to 'just hear' include reverb, pad instruments and - yes - bass. With these instruments and possibly others it is well worth experimenting with the 'just hear' point, in both senses that I have described. You might end up choosing 'clearly audible' as the better option, but trying things out can help you learn things about the music you are mixing that you wouldn't pick up on by mere rational thought.
It's louder than you think
Another interesting experiment is to set an instrument at the lower 'just hear' point, where you really can only just hear it. Now solo it. It's louder than you thought, isn't it? How could you ever miss it in the mix? The human ear works in strange ways.