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
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.
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