"I have nothing to say, and I am saying it" (John Cage)
The BBC's Mike Williams spends some time in the anechoic chamber at Univertsity College London.
It was his visit to the similar room at Bell Laboratories (the daddy of them all) which inspired the composer John Cage to create his silent work 4'33". Specifically it was the realisation that in supposedly complete silence he could still hear his own body - the joints, the blood, the nervous system - that led him to realiser that any musical experience we have is an exercise in selective attention. We pay attention to the alto, not the arteries; to the violas, not the vertebrae. Going further, we attend to the orchestra rather than to the air conditioning, or the person in the next row turning pages in her programme, or the one upstairs who just coughed. While a police siren outside might upset a recording session, we simply tune it out when it happens during a concert. What 4'33" is about is forcing our attention back to these things by taking out the "actual music". It forms a kind of experimental control: what you would have to subtract from a live recording to have "just the music". Of course, there are extraneous noises which arise from the production of music. Sounds of mutes being put on and off, of pages being turned and so on. But you can't have everything, any more than you can have nothing.
Final fun fact: 4'33" equals 273 seconds. 273 is a number well known to scientists, because (to the nearest degree) minus 273 degrees Celsius is the Absolute Zero of temperature derived from the Second Law of Thermodynamics. Neat, eh?