Why do empty rooms sound hollow

Every empty room has echo, it's material does that. The object in the room take away that echo.

Explanation:

An empty room has nothing but hard walls. We know that hard substances reflects the sound . If the distance between you and walls is 17 meter than the sound will echo but if the distance is less than 17 meter than the sound will reverberate . So the empty room sounds hollow.

layers of the earth affect the waves and these effects are seen on instruiments such as seimometers

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.