The prequel to the ‘A Quiet Place’ saga got me thinking.
spoiler alert!
There is a scene in which many humans march towards a safety point. Each individual human would have been relatively quiet, but because there are a lot of them (potentially hundreds), they end up being, as a whole, loud enough to alert the monsters so they get all killed.
This would suggest that many sources of noise which are near to each other and generate more or less the same amount of noise end up adding up so that the end result in dB is more or less the sum of the individual dB levels.
But then again, it’s fiction.
Back to reality, I work in a room full of different servers which have also very different levels of noise. I have noticed that from my standpoint, the noise of the quietest server seems to disappear whenever the loudest is running, so it kind of does blow my mind how our perception of noise works…
OP: Destructive interference as illustrated here is the basis of active noise cancellation (ANC) in headphones.
In theory it is a microphone on the outside of the headphones that has the outside sound inverted and mixed into whatever is playing from the speakers inside the headphones. The undesired outside noise reaches inside the headphones at the same time as the inverted signal is played so that they cancel each other out.
In reality there is some more complexity, as always, but it’s the core principle.