Why do we hear more clearly in a room with curtains than in a room without curtains?

We hear more clearly in a room with curtains than in a room without curtains because sound is absorbed by the curtains. A sound created in a room without will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation. To reduce reverberation, the curtains will act as a sound-absorbent material.