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Imagine them all completely silent.
I am at work. There is no one in here, and no sound but the faint whirr of several laptops and the clackety-clack of my keyboard.
And still it seems like there is more silence downstairs in the bank branch, where there are perhaps 60 people or so.
They are at a chess tournament. The players want to concentrate, and the spectators re-, well, -spect that.
There is probably more sound there than here. Every few seconds somebody slaps his chess clock, and a spectator whispers to another, and there is shuffling in chairs. More sound, sure. But more perceived silence as well. Silence is the absence of sound and an absence is only a thing when noticed; the more people you stuff together in a small space, the more you notice their silence, thus the more perceived silence there is.
Silence is remarkable in proportion to the number of people present. It is entirely unremarkable when there is no one there.
Dense silence. You find it in exams, sometimes in libraries, and apparently in chess tournaments.