I understand that these days if you want to record a concert, you may not even need to set up a mic or hook into the sound board. I hear there are devices which scan the waves and pick up the signal to goes to the musicians in-ear monitor. You can do this from outside the stadium. Anyone know where I can find more info about these devices and how they work?

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IEM recordings are generally not acceptable, even with bands who are taper friendly. You're basically recording people's private conversations without their permission. More bands seem to be cracking down on this lately. One of the major companies that musicians use is http://www.shure.com.

IEMs are also always mixed for the particular musician, so depending on whether you catch the drummer's signal or the singer's signal, you'll get an out of balance recording. (Illegal) IEMs are popular among a minority of Dave Matthews Band traders, and I find the IEMs hard to listen to. Carter's (the drummer) signal is easiest to find, and his mix always has the drums up really loud.

IEMs just don't sound good at all compared to a well done audience recording.