When sounds from the stage reach the urns buried underneath, urns with frequencies corresponding to the frequencies of the incoming sounds vibrate and reemit the sounds. There are no rules about how big the urns should be or how many should be used. Urns are also sometimes hung from ropes underneath the floors at theaters. The one common factor in both is that the urns are always placed concentrically around the center of the stage with their openings facing the center. This arrangement is meant to maximize sound reflection.
Sound waves strike the urn and are reflected back.
One example of how urns are placed under the floor. At some temples, when you walk on the floor you hear the floor sing with your every step. This type of floor is called a nightingale floor, and the singing sound is produced as the clamps used to fix the floorboards rub against holes in the floor boards as a person walks above. The beautiful sound was associated with the singing of a nightingale, hence the name. This phenomenon has been found to be naturally caused by the warping of the floorboards from long years of wind and rain or by the expanding of the holes due to the heavy load of people walking above.