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
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.
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