I remember a time, time back way back, when I had heard practically no live music for about two years. I was in a state-of-the-art peformance space with a string quartet way down there on the stage. I was listening as hard as I could (with the aid of a certain music appreciation enhancement medium) but something was missing. I kept listening (and I didn't have my high F# overlaying the general experience) and I finally managed to detect the sound of an air-conditioner somewhere in the building and realised that what I was missing was the sound of recorded surface noise.