The composer John Cage (who actually composed the world's first piece of music that was entirely silent, titled ‘4:33') was once offered the chance to sit in a completely sound-proofed chamber at one of the large universities.

Cage, who had excellent hearing, entered and almost immediately commented that he heard a low whooshing sound. He was informed that this was the sound of his cardiovascular system. A few minutes later he began to hear a high pitched siren-like sound, and was told that this was the sound of the neurons of his nervous system firing.

After a long period of concentration, he managed to tune out these two sounds and began to hear a chirping sound, like thousands of migrating birds.

This, he was told, was the sound of the Brownian Motion of the atoms forming the oxygen, nitrogen and carbon dioxide molecules in the air of the chamber.

There are many examples of similar types of low-threshold sensitivities. Most people can smell or taste as little as one molecule of an aromatic or flavorful substance. As Cage demonstrated, our powers of deep listening are similarly discreet. Perhaps even more impressive than low-threshold listening is our ability to zone in on certain bandwidths of the auditory spectrum. As any new mother can tell you, they know when they hear the crying sound of a baby whether that baby is their baby. Next time you are in a busy shopping mall or train station, ask a spouse or child to move at least 150 feet away and engage in a conversation at normal volume. Despite all the ambient noise and voices, you should be able to clearly hear their voices above all others.

It is amazing what we can sense when we stop trying to sense it.