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
Sound is just pressure waves moving through air. So it’s just air moving around. The more air you move, the more it moves things in your ear, we feel that as more loudness.
Edit: removed “perceive” and explained more for the 5yos. Thanks u/RcNorth
Sound travels in waves. The speed of sound is determined by the density of what it goes through, which is partly why things sound different under water.
Waves have two main features – the amplitude (how tall the wave is) and the frequency (how fast the waves come). A sound is loud when it has a big amplitude. A sound is soft when it has a small amplitude. A decibel is a unit that lets us measure that for sound.
So think about ocean waves. If a big/tall one is coming at you, it might knock you over because it has a lot of energy. A big/tall sound wave is loud because it also has a lot of energy. A tiny ocean wave is just going to tap lightly against your skin. A small/short sound wave is going to be quiet. Both have less energy.
The human ear has fluid and nerve endings that are sensitive to sound waves, and can transmit this information to the brain. The brain learns how to process this and filter out the useful information.
Sound is waves of air pressure. The air pressure goes up and down. Up-down-back up = 1 wave.
But how high a pressure is “up”? How low of a pressure is “down”? This difference between the height of the crest and the bottom of the trough is called the wave’s *amplitude* and that’s what determines loudness.
Air pressure going up and down a little = low amplitude = quiet sound.
Air pressure going very high to very low in each cycle = high amplitude = loud sound
Sound is essentially just air being squeezed and stretched very rapidly. How quickly it switches from being squeezed to being stretched is the sound’s frequency, or whether it is low pitched or high pitched. How hard it is being squeezed is the amplitude, or volume.
When the air that is being squeezed and stretched enters your ear, it causes a part of your ear (more specifically the liquid in that part of the ear) to also be squeezed and stretched, with frequencies and amplitudes related to the air. Inside this liquid is a bunch of tiny hairs that move back and forth as the fluid is being squeezed and stretched. The more these hairs move, the more signal it sends to your brain. The more signal your brain gets, the more volume it produces.
How loud something is is essentially how much force is going through the air as sound.
Something movies through the air, pushes some air towards your ear. Like when you sweep your hand through water, you push some and also create a little void behind your hand with less water. It’s the same when you push air, you squeeze some air together and shove it in a direction, but behind that air is a gap with less air. This wave of air pressure hits your ear and the ear drum back and forth. The more pressure, the further the ear drum is pushed.
Loudness is like if someone squeezes your arm really hard, more pressure more loud. High pitch sounds are like someone squeezing your hand on and off really fast, like they are trying to vibrate you. Low pitch is a slowly changing pressure, like someone squeezing your arm and then waiting a moment, and then squeezing again.
Friction causes sound waves. as more friction is applied, the louder the outcome will be. train wheels are a good example
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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