An echo is the result of soundwaves bouncing off the walls, just like light bounces off a mirror. When the apartment is full of items, the gets absorbed by them and/or bounces in a bunch of different directions. However when the apartment is empty, all the soundwaves bounce off the same walls, at the same time, in the same direction, creating an echo.
Sound moves in waves/vibrations away from the source and can bounce off things it interacts with. (hits)
so think of sound moving like a thrown rubber ball, its easier to bounce from wall to wall in an empty room, but if there is curtains and carpet, it looses quite a bit of bounce off those things, and the other items (furniture and plants) deflect the travel path as well, so it cant bounce back and forth nearly as many times or in straight lines.
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