This is definitely stupid, think I’m missing something.
If I say something at 60 decibels for example, the person I’m talking to will hear it at that volume (ignoring the sound lost over distance).
But if a crowd of people say something at 60 decibels, it will be louder to the person hearing it.
I just can’t get my head around why, since they’re all talking at the same volume as the single person. What amplifies it?
In: Physics
Sound is a wave. Waves interfere – they act alongside other waves in the same spot. When you have two sound waves passing through the same space, the pressures add together, and you have a louder sound.
1 person pushes an object with 10lbs of force. It moves. 2 people push that object with 10 lbs of force each. The object moves even faster because it has a greater force applied to it.
100 people talking at 60db generate more total energy and a greater pressure wave than a single person talking talking at 60db.
Go on a computer graphing sim like Desmos, or grab a graphing calculator.
Type in y=sin(x). This will give a visualization of a simple sound wave, let’s imagine that’s what one person saying something looks like.
Now let’s imagine there’s 20 people say the exact same thing at the exact same time. That graph would be y=sin(x)+sin(x)+…+sin(x) AKA y=20sin(x). Look at that graph and you can see that the vertical aspect of the wave (intensity/amplitude) is way more dramatic.
The vertical axis of the graph represents the pressure of the air as the wave hits your eardrum over time (horizontal axis), imagine that when the high part of the wave hits your eardrum, it pushes it in, and when the low pressure part hits, it comes back out. A wave that has a bigger difference between the peak and the trough will move the eardrum more, which will translate to a louder sound in your brain
This is a crazy simplified explanation that ignores a lot of nuance, as well as the destructive interference that can often take place in crowds, but that is the general principle.
Sound is a wave, more fundamentally (zoom in closer) sound is pressure wave of (for example) air molecule density compressing (getting squeezed together) and air molecule density expanding (spreading out).
You often see waves illustrated as transverse waves like strings (sine wave). But sound is longitudal wave, like pushing a slinky.
Back to the air molecules. Perhaps think of them as grains of rice on the table that are bunched up on regular intervals and less dense regions between them.
So if one voice can bunch up 100X air molecules together as the wave progresses. Another voice that does the exact same you now have doubled density. Note the is not creating the air molecules or making wind to push them from somewhere. They are just locally compressing/expanding as a wave.
It’s like the opposite of noise cancelling headphones. They have a microphone that picks up on incoming sounds. It then creates a sound that completely cancels out the noise.
Waves (including sound waves) can interfere with each other destructively (make quieter) or constructively (make louder). The (effectively) random nature of sounds in a room means that there’s going to be a lot of constructive and destructive interference at once.
You can visualize this by throwing sand in a still body of water. One grain of sand doesn’t make very large waves. Throw a fistful of sand and you will see a lot of waves, some large and some small.
Imagine one person in a pool. They start splashing and making waves. Those waves will bounce up to a certain height.
Now imagine someone else joins him, and starts splashing at the same time. The waves will bounce higher and more forcefully when he does it too.
Speaking is splashing waves in the air.
Imagine a solo singer being joined by a large choir. The group will sound much louder, because there is now a lot more sound energy reaching your ears.
Sound is moving air. louder sounds are more moving air.
In your example, one person sings at 60dB (yes I know it is really a dBm thing but keeping it ELI5). If 10 people sang just as loud instead, there would be 10x the sound power, and the resulting sound level would be more like 70dB. If 100 folk sang, then the resulting ‘noise’ would be at about 80dB intensity.
Sound dB is a logarithmic scale, with each increase of 10dB actually being 10 times the power.
(I used singing in my example because groups singing create a much more organized sound than a bunch of folk just randomly speaking. So the song’s peaks and valleys of the sound waves align better, making a louder loud. Put enough folk together speaking randomly, and the result is a white noise with a lower overall intensity, as some of the sound waves will cancel out other sound waves)
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