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?


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

Air pressure. Same reason a guitar amp with a 1×12 cab is significantly “quieter” than the same amp on a 4×12 cab.

More people means more sound waves being generated. More sound waves means u hear it louder and more further away.

Sound intensity (loudness or volume using ELI5) is measured in decibels (dB), like a meter is measure of distance.

So let’s say a crowd of *one* person yelling is 60 dB. What happens if you bring in a second person yelling at 60dB?

Well, 60dB + 60dB does NOT equal 120dB. Because physics, 60dB + 60dB=63dB. When you double the intensity you gain 3dB,which isn’t very much.

So, using the doubling game, you’d need FOUR people to get to 66dB.

Honestly, I’m sitting on toilet right now, so I won’t continue to extrapolate from there. Let’s jump ahead.

Let’s say that you have a good sized crowd, 4000 people, and together they can make a pretty loud crowd roar, 85dB. If you had 8000 people, the noise would be 88dB. A 3dB step is barely noticeably loud. So it would take a lot more people to make a noise level of 100dB, which is pretty darn loud.

Of course this does not take into account the acoustics of the stadium, and not everyone make the same amount of noise, etc.

Hope this helps.

Same reason 2 horses can pull more than one. The power is cumulative.

Sound is just waves of pressure. High pressure followed by low pressure.

It’s more complex than this, since it’s logarithmic among other reasons, but to make it simple, let’s say that a person makes a sound that is at a pressure of “4” above ambient pressure in the peaks of the waves, and “4” below in the troughs. Add another person making the same sound at the same volume at the exact same time, and you get waves of “8” above and “8” below. Those waves move your eardrums twice as much as the single person’s “4” wave.

Interestingly, because it’s waves with peaks and troughs, it’s actually possible to cancel out a sound by generating the exact same wave, but basically upside down, so that any trough lines up perfectly with any peak, and as we know 4 plus negative 4 equals zero.

When people speak, they make sound waves. When multiple people speak, the sound waves collide and combine into new waves with a higher intensity.

Isn’t it a positive feedback loop? The louder it gets, the louder people talk to be heard, making it louder. Repeat.

Surely you’ve been out in a crowd for an evening and woken up the next day with a croaky voice due to shouty-talking all night?

The rule of thumb is something like: it takes twice the power to result in a 3 decibel increase in volume. Basically, if a group of 10 people talk at 80dB, a group of 20 people will talk at 83dB.

Now 3dB doesn’t sound like much, but it’s a noticeable increase. Start stacking those up, and you’ll get a roaring crowd very quickly.

Of course, the reality is that people in progressively bigger groups will have to talk louder to be heard. So there’s natural escalation that happens as people fight the background noise.


As others have pointed out, because sound waves can be additive. Your perceived loudness is basically an average of the energy contained in the combined sound waves. One person puts in one person’s worth of sound energy, add another person, and you have two persons worth of sound energy, and so on.

Another way to think of it is with rocks and a calm body of water. Drop one rock in, it makes a small splash. Tape two rocks together, you get a bigger splash, and so on.

It starts to get interesting when you drop the rocks in different places. Then your waves add and subtract in neat patterns, through constructive and destructive interference, which is a fancy way of saying ‘take height of wave a above/below average water level and add height of wave b above/below water level, at specific points. Constructive interference (where both waves are either above or below average water level) = extra high or low wave. Destructive interference (where one wave is above and one is below average water level) = smaller wave or no wave when compared to the originals.

People are describing how the decibel scale works, but not the physics of sound when listening to multiple sound sources. I’ll give it a go.

Sound is a type of wave. Specifically, it is a wave of air pressure. Your eardrum is a thin membrane. When the pressure on the outside surface of the eardrum changes from the pressure on the inside surface, the eardrum moves in response. The pressure wave of a sound will cause ripples of higher and lower pressure to reach your eardrum and cause it to vibrate in and out slightly. Your brain interprets this vibration as “sound”.

Waves have a few interesting properties. One of them is called interference. This is when two waves encounter each other and combine. Depending on how the two waves “line up”, how they combine can make the resulting new wave get more or less intense. If you picture a wave on the ocean, it will have high points which we call “peaks” and low points which we call “troughs.” If two peaks line up, you’ll get a combined, even higher peak. This is called “constructive” interference. If two troughs combine, that would make a lower trough and would again be constructive. If a peak and a trough meet, they cancel each other out and you get “destructive interference”.

Sound waves are similar except it’s high and low levels of air pressure instead of high and low levels of water elevation. When peaks and troughs of different sound waves line up constructively at your ear drum, they will combine and create a more intense movement, which your brain interprets as “louder.” Sound waves are complex and will rarely line up perfectly for any significant amount of time, so although large crowds of people can be louder than individuals, they won’t be completely deafening, because some of their sound will interfere constructively, but some will also be destructive.

Interestingly, if you have sound cancelling headphones, they operate on this same principle. They have a small microphone which picks up noise around you and then sends the same sound to your headphones, but it offsets the timing of the sound just the right amount to encourage destructive interference, which has the effect of canceling out the noise to some extent and making it sound quieter.

Sound is waves. Like waves, if two waves on the same direction at the same speed and at the same place cross, they become one big wave. Sound gets louder when more people near each other say the same thing

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…

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

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.

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.

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.

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.

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.

Decibel math goes something like 1+1=3. And every time you add 3dB it doubles the noise level.

Why do 20 apples weigh more than 2 apples when every apple weighs the same?