Things that Go Bump in the Night

"From ghoulies and ghosties and long-leggaty beasties
And things that go bump in the night,
Good Lord, deliver us!"

It is three o'clock in the morning and all is quiet. How quiet? Well, my sound level meter says 33.2 decibels. It is generally agreed that a level of 35 decibels is appropriate for sound sleep so I have a decibel or two to spare. At the beginning the noise level was 35 decibels. By careful listening I could detect that the flourescent lighting was emitting a light hum, then the fan of my computer was making a sound so low that I am rarely aware of it. By shutting off these two I achieved the lowest level. In the distance there is the faint sound of a barking dog. There are the inevitable tiny squeeks of a house adjusting to temperature change. If I move in my chair the slight sound sends the decibel level above 35 decibels again.
Where do the remaining 33 decibels come from? It is the sound of this huge city breathing as it sleeps, a mixture of sounds that is below my present threshold of hearing. In the countryside, on a mountain top, or in a specially constructed sound proof room one could achieve more quiet sorroundings.
But it is quiet enough, and soon I will return to sleep!