why are crowds louder than small groups/individuals even if each person produces the same amount of noise? In other words why would a group of people, each generating noise at Xdb sound louder than an individual generating noise at Xdb? How does cumulative sound work?
In: Physics
So pretty much imagine sounds as waves (like water)
When u have one guy shouting, u have one ripple.
When u have ten guys shouting, interference(addition of waves) occurs where the 10 ripples will meet each other and amplify each other’s effect (by 10×).
So pretty much sound adds up it’s loudness like 1+1=2
Take another example, of noise cancellation, if you have a sound of antiphase (which is just the same sound but negative), it will cancel each other out: like a 1+-1=0
So pretty much in airpods what happens is the microphone in the airpods pick up outside noise, flips it around and plays it back to you which ends up cancelling each other out
Note: It is not good to measure sound with decibel, decibel is derived from logarithmic of intensity. When u add sound, it will add like 1+1 intensity. Simply put it, when u use decibel it accounts for the 10power of intensity… (100 is 2 and 1000 is 3 for example, if u add 100 and 1000 u get 1200 but if u use decibel 2+3=5 which converts to 100000)
Why do 20 apples weigh more than 2 apples when every apple weighs the same?
Decibel math goes something like 1+1=3. And every time you add 3dB it doubles the noise level.
It’s actually simpler then that, each individual conversation needs to get louder to drown out the background noise, which increases the background noise so individuals must talk louder.
It’s a bit like those people at the baggage carousel in the airport if everyone would stand back (talk quietly) then everyone would benefit from seeing their baggage early (understanding each other), but individually they’re all in an arms race to get ahead and get closer for a better view of the luggage rolling in until they reach saturation hunching over the conveyor belt.
They amp up volume to try and hear each other over the other voices, the bulk effect nullyfies the initial gain, is like socialism. You get a gain in your reward function but when it goes on for long enough and everyone plays the game it becomes disadvantageous overall.
Note: Constructive and destructive interference has been explained at an eli5 level in other comments, so I’ll just assume that is understood. If not understood, look at those other comments.
All of these answers about the physics of sounds are wrong. Psychology and behavior are a bigger part of it. Note that the question is about crowds, not machines or anything like that. There is a reason for that.
If you have two loud machines 5 feet apart, the sound intensity anywhere between the two machines is about the same. When you are closer to one machine, you are farther from the other, so the sound in between in fairly constant.
Constructive interference does NOT make it twice as loud halfway in between.
Under normal circumstances, there is the same amount of constructive and destructive interference, because the sound waves are not coordinated. They meet at random points relative to the high and low troughs of the sound wave.
So why are crowds loud?
When you are trying to talk to someone and are hearing other people talk at the same time, you talk louder. You naturally want to hear your own voice and you know you need to talk louder for the other person to hear and be able to pay attention. It is very hard to focus on one of two voices if each voice is the same volume, so you try to overpower other people around you. The person you are talking to is probably giving verbal cues (What, huh) and non-verbal cues (leaning in, cupping hand to ear) that prompt you to talk louder. Eventually, everyone is basically yelling at the person they are trying to take to.
Also, regardless of actual sound intensity, cacophony (constant, discordant noise) is unpleasant and seems louder than a single voice even if the maximum decibel measurement is the same. Cacophony causes some anxiety because your brain is always trying to filter useful sounds out of the noise and you know that your hearing is pretty much useless for warning you of dangers. It’s not just the effect the sound has on your ear drums that makes it seem loud, it is the effect it has on your mind.
Same frequencies add up. If they start at the same spot. If you make a wave in a skipping rope, moving it up and down at some rate, if somerled comes and moves your arm at the same rate and energy, the wave will grow. Rooms will have frequencies that resonate. So certain frequencies can get louder in a given room that way, if all voices are equal volume. But also people talk over each other.
I’m not sure how significant the first part is, because the opposite is true. An inverse wave deletes the same wave that’s inverses, and a room can remove frequencies just as it can multiply them.
But, I’d say since there are many walls you’d get lots of bouncing on the walls.
Imagine dropping pebbles in a large pool the waves would have far to go before hitting walls and returning, so the water would be relatively calm. Or have a simple wave pattern.
If you drop the same number of pebbles in a small tub, the water will be much more choppy every which way, and it will have big troughs, and peaks.
Sound travels in waves through air, just like Water has waves when it is disturbed. So think about throwing pebbles into water – 1 pebble makes small waves, 20 pebbles thrown in all at once will make bigger waves. The bigger the wave, the louder the sound. That’s about all my 5 year old can understand, anyway.
Get in a pool and try to make a wave by yourself, then do the same with 9 other people working together. The wave is the loudness, and the 10 people wave is MUCH bigger because more energy/force
Maybe a little too simple, but we tend to speak louder in a loud environment. So I’m addition to the very accurate and technical answers, I’ll point out that individuals likely will raise their voices to to be heard when surrounded by people as loud as themselves. Perhaps not quite hitting OPs question on the head…
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