In my Youth I took part in some sensory deprivation experiments,you know, eyes and ears covered float in a tank in the dark at blood temp. and the first thing that happens is you hear your body sounds,blood in your inner ear etc. then your brain starts supplying sound , or imagined sound, and your sense of time gores completely.I was in there for 6 hours and I thought it was days... so I have something to measure silence against... I can have music as background, but My SO finds it hard to study with music going, she says she keeps trying to listen..and it ruins her concentrattion..