Think about the most quiet place you’ve ever been to. Now imagine something even quieter. What does that sound like? If you can’t figure it out, physics can help: let’s start by looking at how sound works.

Loudspeakers, vocal folds and instruments all function by the same principle: rhythmically push and pull on air. The air molecules, in turn, push and pull on their neighbors, that push on their neighbors, and so on. Air thus stretches in some points, compresses in others, creating a wave of pressure. A sound wave is born.

translational_motion

Molecules of air “staying put” actually do a lot of moving. CC-BY-SA Greg L, via Commons

But what if there is nothing to move air and produce sound: what does silence sound like? Does it have one at all or is it like questioning the color of an invisible thing?

Air molecules bump into each other and vibrate all the time. Just by being there and having a temperature, air is bound to create microscopic changes in pressure here and there. Even the most barren, isolated, still place has a sound: the sound of silence.

Molecular collisions like these are pretty much random and independent. Their sound—silence—is then white noise, the stuff some people use to relax or focus.

The loudness of silence kind of depends on what frequency range you look at: the narrower the window, the fewer kinds of bumps you will find, and the quieter silence will appear.

According to some calculations, in the band of maximal sensitivity of humans (around the pitch of speech), air sitting there is about -20 decibels. That’s silent. Too much for us: it’s just audible for an owl, a super-specialized stealth predator with ears literally the sound of its face.

That big circle around an owl's face funnels sound: it's basically a giant ear. CC-BY-NC-ND Brian Scott/flickr

The owl’s face circle funnels sound from the environment: it’s basically a giant ear. CC-BY-NC-ND Brian Scott/flickr

However, in the full range of human hearing, silence is considerably louder: about 0 decibels, which is also about the faintest thing we can hear.

That means our hearing can handle anything barely louder than total silence just as well as a conversation—a thousand times louder. Sounds good enough.

If you want more
  • How is 60dB a thousand times more than 0? Decibels are weird: here’s a summary of this and other peculiarities.
  • What does it mean “white noise”? Can it be other colors too?
  • Some say total silence drives you crazy. Scientists don’t work on hear-say. They try.

 

Cover photo: CC0 Sam Halstead, via pixabay.

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