Sounds are louder in the center
Here’s something you might not have known: sounds get louder in mono as they come toward the center from the sides of the mix! There’s roughly 3 dB1 difference from hard-panned to center-panned.
This is why you might have had this experience: you do a mix you love, with stuff panned huge and wide and feeling so good — and then you listen to it on the phone or on a bluetooth speaker and the sounds that are the furthest out to the sides in the mix are weirdly hard to hear and the song has lost something that you were trying to convey. Argh!
For this reason, I pan mix elements that are critical to the perception of the song generally not a lot wider than 50% of the way to either side. It still sounds really wide! But it’s twice as mono-compatible.
If there’s a particular element that it’s extra important not lose volume depending on the listener’s situation — doubled high-energy rhythm guitars that serve as the engine in a driving song, for example — I’ll even do more like +/- 35% max. It still sounds nice and wide, but now we’re much more in the -1 dB range in terms of the difference when translated to mono, which is very reasonable. Tuck the lead vocal half a dB and you’ll be good to go.
Checking my 12 — jamie
The precise dB number can change; for most real-world situations, 3 dB is a good rough number.