How does sticking weirdly shaped foam blocks on the walls of a room make noise hard to hear from inside and out?
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What we call sound are small air pressure variations. Upon hitting a hard flat surface, these pressure variations are reflected back (usually with some loss). Foam blocks etc are designed to absorb some of these pressure variations into motion of the foam block material which eventually disperses as heat. The irregular surface also doesn’t reflect the sound wave very effectively. (think of a mirror reflecting a flash light beam vs a painted surface that disperses light)
Sound waves propagate (travel, in other words) at around 340m/s in most room conditions. So the added absorption very quickly reduces the power (loudness) of the sound wave as it goes back and forth reflected multiple times in the room.
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