Does noise from different nearby sources 'add up'? Or do the different sources cancel each other out? In any case, please provide a formula and an example

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• TropicalDingdong@lemmy.world ⁨1⁩ ⁨day⁩ ago

  So lets talk about constructive versus destructive interference.

  First off, waves are waves. When you have something like a fan, its generating a series of waves. These waves are typically coming at a constant frequency. The frequency here matters.

  Sound travels in waves. These waves are pressure waves in a fluid. Those waves don’t just get absorbed by hard surfaces; they reflect or refract. But those waves are both moving through the space, and themselves have a frequency.

  This is important because of this you get patterns of constructive and destructive interference in space.

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  In some areas of a space there will be amplification, because the frequencies are lining up. In some spaces, the waves will cancel out. Those areas will be quieter.

  This phenomena is true for all waves.

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  • whaleross@lemmy.world ⁨1⁩ ⁨day⁩ ago

    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.

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  • medem@lemmy.wtf ⁨1⁩ ⁨day⁩ ago

    This is nice!

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• hildegarde@lemmy.blahaj.zone ⁨1⁩ ⁨day⁩ ago

  Sound is complicated. Its both physics and perception.

  The decibel scale is logarithmic. If you double the energy you increase the dB level by 3. Two people will be about 3 dB higher, four people would be 6, eight people would increase the level by 9 dB, sixteen would be 12 dB higher and so on.

  Some people in this thread are talking about destructive interference. That really only happens in noise canceling headphones, or situations designed to cause it. In open air, its not really an issue that comes up. I’ve never become quieter by playing an instrument with someone else. It just doesn’t work like that.

  What lets the monster hear you is the peak volume, not the minimum or average, so it will be close to the 3 dB doubling rule.

  There’s also perception. Your brain does a ton of filtering of sound information. Your brain is constantly trying to pick out the important sounds and ignore the rest. That’s why the quieter severs seem quieter when something louder happens. Also that’s why some people use white noise machines to sleep.

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  • socsa@piefed.social ⁨1⁩ ⁨day⁩ ago

    This is partially wrong as well. Non-coherent energy does not add like this, and is a perfectly natural phenomenon, not something which only happens in headphones. If you play two synchronized sinusoids some meters apart, you will get perfect doubling only along a single line in the far field which is equidistant to both sources. Any angular divergence from this line will reduce the coherence energy relative to the phase angle. Audible sound has a wavelength of about 15m to a few cm so and real channel with any real sound will experience significant coherence attenuation as a function of the angular offset of each pairwise emitter.

    You are correct that a yelling crowd will never be quieter than a single person, but there is a massive natural diminishing effect which prevents a pure log-rule volume increase.

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    • hildegarde@lemmy.blahaj.zone ⁨1⁩ ⁨day⁩ ago

      I was discussing real world sounds in open air. Yes, destructive interference is something you have to account for when dealing with sound systems. A speaker playing the same sound in the inverse phase will cause issues, but a group of people as was the topic will never be anywhere close that precise.

      If the concern is safety, as it is in this example, the concern is the worst case. And the worse possible case is when all the sounds interfere constructively.

      It’s not really a counterargument when I specifically said my logic of ignoring destructive interference doesn’t apply in “situations designed to cause it” like two synchronized sinusoids. Most people are not synchronized sinusoids.

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  • medem@lemmy.wtf ⁨1⁩ ⁨day⁩ ago

    This is a very good, comprehensive and comprehensible answer. Thank you!

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    • AstralPath@lemmy.ca ⁨1⁩ ⁨day⁩ ago

      Two people will be about 3 dB higher, four people would be 6, eight people would increase the level by 9 dB, sixteen would be 12 dB higher and so on.

      This is only the case if they’re making identical sounds in phase with each other. The real life scenario of people walking would not scale in this way as there are too many variables. No two footsteps are the same no matter how hard you try.

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• givesomefucks@lemmy.world ⁨1⁩ ⁨day⁩ ago

  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.

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  • medem@lemmy.wtf ⁨1⁩ ⁨day⁩ ago

    So if I understand you correctly, in the real world, they do add up?

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    • Nemo@slrpnk.net ⁨1⁩ ⁨day⁩ ago

      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.

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    • BlameThePeacock@lemmy.ca ⁨1⁩ ⁨day⁩ ago

      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.

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    • givesomefucks@lemmy.world ⁨1⁩ ⁨day⁩ ago

      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:

      askamathematician.com/…/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

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    • Eatspancakes84@lemmy.world ⁨1⁩ ⁨day⁩ ago

      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.

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  • TropicalDingdong@lemmy.world ⁨1⁩ ⁨day⁩ ago

    Doing so outside of a controlled laboratory setting would be effectively impossible for real world noises.

    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.

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• Zwuzelmaus@feddit.org ⁨1⁩ ⁨day⁩ ago

  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

  This might happen because of “resonance” - but I don’t know your specific scene.

  en.m.wikipedia.org/wiki/Mechanical_resonance

  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…

  The effect with the servers is called “sound masking” (and some other commenters have written about it), and how our peception works here is called “auditory masking” (nobody has mentioned this one so far).

  en.m.wikipedia.org/wiki/Sound_masking

  en.m.wikipedia.org/wiki/Auditory_masking

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• unexposedhazard@discuss.tchncs.de ⁨1⁩ ⁨day⁩ ago

  Idk about the additive effects, but:

  Fan noise especially is very “white” in the sense that it contains a lot of frequencies from low to high. That makes it ideal for overpowering (and subjectively swallowing) other noises (like other fans, speech, etc).

  Fan noise
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  Speech for comparison happens in a frequency band where fans are very strong
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  Running water is similar. When you try to listen to a conversation or a video with speech while washing your hands, you will have a hard time understanding anything. With other noises that are in different frequency bands or only use very small range of frequencies it wont have that effect even if the noise is much louder than the running water was.

  Noise cancelling in the sense that one noise actually makes another noise disappear, doesnt really happen under non lab conditions. Its more of a drowning out effect where your ear/brain cant differentiate the two noises anymore.

  Selective hearing can do a lot of work by processing the signals from your ear in your brain, but that isnt all powerful.

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  • hildegarde@lemmy.blahaj.zone ⁨1⁩ ⁨day⁩ ago

    Sorry for the very tangent.

    That speech frequency graph is a good visual of why for trans voice training, resonance is more important than pitch. The pitch is nearly the same, the difference is in which overtones are projected.

    Where did you find it? I would like to read more about the methodology.

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    • unexposedhazard@discuss.tchncs.de ⁨1⁩ ⁨day⁩ ago

      I just looked for “human speech frequency spectrum” so i had to tineye it to find it again but this seems to be the source erikbern.com/2017/02/01/language-pitch.html

      I cant vouch for its quality, but it seems pretty in depth from a quick scroll.

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• sharkfucker420@lemmy.ml ⁨1⁩ ⁨day⁩ ago

  Depends on the frequency and displacement of the wave. Interference can be both constructive or destructive

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• OldManBOMBIN@lemmy.world ⁨1⁩ ⁨day⁩ ago

  The post above yours in my feed provides a partial answer

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• not3ottersinacoat@lemmy.ca ⁨1⁩ ⁨day⁩ ago

  Why is the title phrased like you’re asking chatgpt? It feels strangely cold and disrespectful.

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  • medem@lemmy.wtf ⁨1⁩ ⁨day⁩ ago

    Amusing.

    a) I don’t use shitGPT and never will, so I wouldn’t know how to phrase questions for it;

    b) You are aware that this is ‘no stupid questions’, right ? People here expect to be asked stuff - or to learn from others’ answers for that matter, which I’m pretty sure is the case here;

    c) Cold? Maybe that’s because this is a science question? I’m not asking a lady out;

    d) Disrespectful? It takes a particular brand of pessimism to be offended in the least by a question that’s not even directed at you…

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    • not3ottersinacoat@lemmy.ca ⁨7⁩ ⁨hours⁩ ago

      Who said I’m offended? Not everyone who critiques something or someone is “offended” you know.

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  • andrewrgross@slrpnk.net ⁨1⁩ ⁨day⁩ ago

    Can you demonstrate how you would have composed the question?

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    • oo1@lemmings.world ⁨23⁩ ⁨hours⁩ ago

      Not the one you asked but - I thought it read more like an exam question than a crapgpt question.

      It is this part that rubbed me the wrong way: “Please provide formulas and an example.”

      That’s like the part where the examiner is giving a hints about how the marks will be awarded.

      Either way here’s my equation: P(Answer | Question and constraint on answer ) < P(Answer | Question)

      I assume that’s the OOPs intention though; to block out some of the noise from the responses. I only see one formula so far (excluding my stupid one) so I’m not sure if it worked strictly, but I’d be surprised it it hasn’t filtered out some answers.

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• Dashi@lemmy.world ⁨1⁩ ⁨day⁩ ago

  To the best of my memory. It has to do with sound actually traveling in waves. Depending on when the waves hit each other they can cancel out (to some extent) or add to each other.

  en.m.wikipedia.org/wiki/Wave_interference

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• ryathal@sh.itjust.works ⁨1⁩ ⁨day⁩ ago

  There’s a lot of things going with sound, but in general sound1 + sound2 is louder. You may not always perceive that, and if one of them is significantly louder, it’s not a significant difference. Active noise cancelling uses sound to make other sounds disappear, but this is just a trick, the pressure is still there (it’s why prolonged use of active noise cancelling can be bad for you).

  When it comes to decibels, dB, it’s a logarithmic unit so adding them isn’t straightforward. You need to do logarithmic addition, so 5dB + 5dB = 8dB, and 90dB + 90dB = 93dB, but 70dB + 90dB = 90dB.

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