this effect is very obviously noticeable on 90's era Dead soundboard recordings, once the music stops the audience source feed jumps way up in an unnatural way, and often fluctuates in source and level too, then drops back down when the next tune begins, I don't understand why it was done and it is annoying to say the least
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I always understood that this was a result of the audience sound coming through the bands vocal mics. When the individual band member stepped off of the pad that was in front of his vocal mic that mic was turned off. Thus the crowd noise that was coming through the vocal mics was eliminated.