When voices are reflected in a closed room or after talking into an empty bottle, the phenomenon you are experiencing is called echo. Echo occurs when sound waves encounter a reflective surface and bounce back towards the source, reaching the listener's ears as a delayed repetition of the original sound.
In a closed room, the walls, floor, and ceiling act as reflective surfaces for sound waves. When you speak, your voice creates pressure variations in the air, which propagate as sound waves. These waves bounce off the room's surfaces, reflecting back towards you or any listener. If the distance between the source (you) and the reflective surface is significant, the reflected sound waves will take some time to travel back, causing a noticeable delay before reaching your ears.
Similarly, when you speak into an empty bottle, the sound waves produced by your voice enter the bottle and bounce off its inner surface. The bottle acts as a resonator, amplifying certain frequencies and reflecting the sound waves back out of the bottle. This creates a resonating chamber effect, which enhances the sound and changes its characteristics. The reflected sound waves from the bottle reach your ears slightly after the direct sound, creating an echo-like effect.
In both cases, the reflection of sound waves occurs due to the hard and smooth surfaces that can bounce sound back with minimal absorption. The time delay between the original sound and the reflected sound determines the perceptible echo effect.