What is the reason for the echo formation? Why doesn’t the echo occur in a small, furniture-filled room?

Sound is a mechanical wave. For a sound wave, the wave property is true: reflection and absorption. The sound is reflected from the surface of the walls and returns to the ear of a person, he hears an echo. In order for the echo to be heard, the room must be large enough to feel the time difference between the time it was spoken and the reflected sound reached the person’s ear.

In a small room filled with furniture, the sound is absorbed by the furniture, there are no surfaces that reflect it.