How does loudness work? I looked up what a decibel was on Wikipedia and my eyes melted. How does measuring the loudness of something work? What actually happens when we experience a sound as really loud or really quiet?
In: Physics
Lie a slinky flat on a table. This is going to be a stand-in for air.
If you tap one end of the slinky, you’ll see the spring tighten around where you tapped, and then that tightening travels down the slinky. If you give it a quick pull, it stretches, and that stretch will likewise travel down the slinky. This is also what happens with air: you create zones of higher/lower air pressure, and those zones travel through the air. That’s what sound is. Air compressing and expanding.
Pitch is basically how quickly you’re pulling or tapping on the slinky. Tapping slowly gives you a bass-y sound, tapping fast gives you a whistle-y high pitched sound. You can see this at play with how, say, a fan changes pitch when you speed it up.
Loudness is how _hard_ you’re pushing/pulling the slinky. If you tap it, it gently compresses; if you punch it, it compresses much harder. In sound, this corresponds to how much of an air pressure difference you have between the high pressure zones and the low pressure zones. In the extreme case, [this photo](https://www.reddit.com/r/interestingasfuck/s/xXodNYAgU8) showed up on my feed recently, and you can clearly see the two zones where the air pressure is so different it causes light to deflect (as if it was going through a lens).
Note that I used pushing a slinky instead of, say, shaking a rope. Sound is a [longitudinal wave](https://en.m.wikipedia.org/wiki/Longitudinal_wave) rather than a [transverse wave](https://en.m.wikipedia.org/wiki/Transverse_wave) like the rope would be. Funnily enough, Wikipedia also uses the slinky example for longitudinal waves. Hah.
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