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…
You can cancel out one sound wave an equal and opposite one.
Doing so outside of a controlled laboratory setting would be effectively impossible for real world noises.
But if you got two speakers in a completely soundproof room playing equal but perfectly opposite oscillating and repeating sound waves…
Sure, it’s possible.
So if I understand you correctly, in the real world, they do add up?
They add up (in the sense of getting louder) if they’re harmonized or party harmonized. They cancel out if they’re opposing or partly opposing. They become more continuous but not louder if they’re out-of-synch.
Yes, normally noise is cumulative
It’s pretty easy to think about this in the context of a stadium of people. One person cherring, 10 people cheering, 1000 people cheering. They produce a louder result.
This answer makes no sense. Sound comes in waves with crests and throughs. In a controlled setting you can harmonize which effectively matches crests to crests and throughs to throughs. This happens in music with choirs, orchestra etc. In that case indeed sounds adds up.
However, with many random sources at many random frequencies you just get chaotic patterns where sound sometimes cancels out and sometimes adds up. No way that the overall result of this is that it adds up in dB. My first approximation would be that if one source emits 1dB and another also emits 1dB, these roughly add up to 1dB.
A very easy test of this is whether you now hear a very loud buzzing in your ear because at this very moment many people talk at the same time. I guess you know the answer.
Yes, sounds are waves that go thru a medium, usually air.
Think of it like one person speaking in a gym compared to 1,000
The different sounds add up, that’s easy everyone just has to talk. But you don’t get silence, you get a cacophony.
To produce opposite sounds to cancel each other out though, that would be impossible, and becomes even more impossible as you scale up.
It helps if you can see it to visualize what I mean by alternating waves:
https://www.askamathematician.com/wp-content/uploads/2013/01/Noise-Cancellation.gif
I didn’t think of it till I looked for a diagram, but yeah, it’s exactly how active noise cancellation works.
Quick edit:
But headphones are the easiest way to do it because it just controls for sound right outside each ear.
To do that “out in the open” to trick the monsters you’d need something mounted right on what’s making the noise. Like crazy noise cancelling shoes which would still have a slight delay before and after the noise. And ones good enough to work out in nature on u predictable terrain?
Like, cool thought experiment, but no practical way to implement
Actually it happens all the time, because of reflection/ refraction from a single source. Say you’ve got a table saw running in a shop. The sound coming from the saw is a (fairly) constant oscillator.
A shop has hard surfaces and that sound will bounce all over the shop the saw is in. Because it still takes time for the sound to travel and bounce all over, there will be places where standing waves of constructive or destructive interference form. Now there is also a shit ton of other sound bouncing all over the place, so it might not be as noticeable, but standing waves/ regions of constructive/ destructive interference don’t require a lab setting.
Modern phased array antenna are effectively taking advantage of this phenomena.