[ELI5] Why does a large amount of quiet noises combine to make a loud noise.
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Sound is a wave, like waves on water. It might go +1, 0, -1, 0, and so on. Add a second of the same, and it goes +2, 0, -2, 0. All a bunch in all over and it’s like a big pool party where the water is just moving everywhere, and the pool won’t be still/quiet anywhere.
Sound moves in waves that have peaks and troughs. In a space with multiple sources of sounds, these peaks and troughs can align, creating constructive interference. This makes the sound louder. Conversely, if the troughs align peaks and vice verse, you get destructive interference, where the sound can be diminished in volume.
Sound is made by using some energy to move the air particles (or other medium, but let’s say air for simplicity). If you use more energy, you move it more, and it sounds louder. If two sounds happen at the same time, and work together in sync, the energy is acting together and the combined sound is louder. If the sounds are out of sync, the energy is acting against each other and the sound can be lower.
Now any noise is a mix of different sound frequencies. When you have multiples low energy noises, some part of them will sync up, adding energy and making it louder, and some parts will cancel, making it quieter. On the whole, you will hear louder noise at different intervals and we focus more on that than the intermittent low noise.
Think of a still pond. 1 ripple is 1 voice. The more voices there are, the more ripples start happening and the ripples start affecting each other. Quickly, you can’t find 1 area in the pond where the water is still.
Plus, when multiple start talking, other people start talking in a slightly louder tone to be heard. Makes slightly larger ripples. That becomes the new normal, and people (generally) don’t notice the noise gradually rise until it’s pointed out.