Purring stone

HOW

1. Carefully place your headin the concrete cavity.
2. Pay attention to the sounds you hear. Say a few words, varying the pitch and volume of your voice. Pay particular attention to low sounds, similar to the purring of a cat.

WHY

The cavity is shaped like a sphere. When you put your head inside it, you hear ambient sounds that reach it through the opening and are reflected many times. By overlapping, they become amplified, particularly in the centre of the sphere. The most amplified sound waves are those with a length that correlates with the size of the cavity.

When you say words, they also bounce off the walls of the cavity. Low tones are particularly amplified. The higher ones are more absorbed by air.