The Mirrors will flow the images and send the light back. The Walls also send the voices back. If you stand a little farther away from a High wall or a big cliff, you'll hear your voice spinning back a few seconds later.
If a sound wave encounters a wall or a high and wide rock surface, it rotates backward towards its source. That'S how the echo happens. If The extracted sound is short and the sound waves reach the battery as normal, the echo is heard clearly. Traces of the Sound remain in the ear for 1/10 seconds.
Because the speed of the Sound in the air is 340 meters per second, the distance between the audio source and the battery to be an echo must be at least 17 meters.