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.

the answer for apex is the lowest