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I’ve been in a few bands myself, but none of them ever got big enough to warrant anything as extravagant as an earpiece (tumblr.com), so I can’t answer your question from personal experience.
I do remember doing one gig where the guy in charge of the mix (who’d made his name mixing for a very famous David Coverdale-fronted 80’s rock band) flat-out refused to let us do a soundcheck, assuming that all the bands featured on the show, no matter what style of music they played, could all be mixed the same way. Maybe that’s the reason that particular David Coverdale-fronted 80’s rock band never made it into my record collection…But, whatever, I digress…
For performers more successful than I, it’s usually a monitor mix that comes through the earpiece. This mix is created with the individual performer in mind (meaning that their own contributions are placed higher or lower in the mix, depending on their personal preference).
Of course, with arena gigs being what they are these days, it is vital for musicians to sound the part, which must be difficult if you’re trying to carry a tune with 50,000 people screaming in your face. Large gigs are huge events as well as big business, so mistakes can’t really be tolerated at that level anymore. As a result, earpieces help the band to play together, stay in time, stay in tune and deliver a good show, every single night of the tour. At least, that’s the theory.
As to what they hear exactly, well, I would imagine that it differs from performer to performer because, as I said, each mix is individually tailored to the musician in question.
The example I found on Quora had it that vocalists tend to like their own voice to be kept high in the mix, ostensibly so that they can hear themselves well. However, if you’ve ever been in a band, you’ll suspect, as I have come to, that it is actually to reinforce the fact in their own drug-addled minds that they are the greatest thing in the known universe and that nobody else in the band (especially the drummer) would be anywhere without their naval-gazing lyrics, constant, budgie-like preening and onstage showboating.
…I was a singer, by the way.