I live close enough to a sports stadium to occasionally hear the roar of the crowd. But that’s just a bunch of individuals all yelling and making various noises. If that were single individual in the same stadium, I wouldn’t be able to hear anything. Do our voices amplify each other somehow, when in a group?
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Yes, sound energy ‘adds up’ when there are multiple sound sources present. You can have what’s called ‘coherent sources’ which add perfectly (so 2 sources of exactly the same sound = double the sound) and ‘incoherent sources’ that add imperfectly (2 sources playing different sounds = like 1.3x or so).
A whole stadium of people cheering can sum up to be extremely loud.
Sound is a wave. So imagine a wavy line which represents the sound of one person talking. If another person is saying the exact same thing at the exact same time and with the exact same voice then the two wavy lines line up perfectly and add together making the peaks twice as tall and the valleys twice as deep which means it is louder. However, people in a group are all different so the sound waves don’t really line up and they interfere with each other, which can make it louder or softer depending on what lines up. Crowds are so big that these interference effects are largely canceled out and the sound is mostly just amplified.
Ever play the parachute game as a kid? One person shaking a parachute doesn’t do very much. If you put a ball on it, it’s not going to do anything too crazy. If everybody starts shaking the parachute together, you can get some huge motions. Even if everybody starts shaking it in their own way, the combined wiggles can make the ball leap off and hit the roof.
Sound is like these wobbles but in the air. The stadium is like everybody shaking the parachute at the same time but in their own way. It all adds up!
It’s called constructive interference. The sound waves of each person add together and increase the volume.
The mechanism is constructive interference. Basically when you have multiple waves in the same substance, the crests add together and the troughs add together. Think of waves in water, those work similarly. If multiple waves overlap, the crests add together and make a bigger wave. Another analogy would be a double-bounce on a trampoline. You bounce the trampoline so that your wave in the trampoline lines up with the other person’s wave and they add together and they get a super bounce.
Sounds in air are waves too. When lots of people yell, their waves add up and their pressures add, making higher waves (which means a louder sound).
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