From “Birdwatching With Your Eyes Closed: An Introduction to Birdsong” Simon Barnes

Silence is rarely pure. You can hear real silence sometimes deep in a cave – when, if you stop talking, the sound of nothing comes as a physical shock. But away from the cities, sometimes even away from the hum of traffic, you can get closer to silence. Silence is never complete, but you can get closer to it because the human brain helps you out – so long as you are not making a recording of birdsong. Your brain is very good at filtering out unwanted sounds, as you know from sitting in that music-filled pub or walking though a busy street. That means you can get closer to a perceived silence as a birdlistener moving out into the wild world – especially if you have no recording equipment, and the awareness of every little sound that goes with it.