The purring stone
Carefully put your head in the centre of the concrete opening. Listen to the sounds you hear. Say a few words, changing the pitch and volume of your voice. Listen carefully to low sounds, similar to the purring of a cat. Examine the effect after moving your head to the edge of the sphere.
How it works
With your head in the opening, you hear ambient sounds that come through the hole and undergo repeated reflections. Sound waves, reflecting from the inside of the sphere, overlap and undergo amplification, mainly in the geometric centre of the sphere. The waves which are particularly amplified are those whose length correlates with the size of the sphere. In physics, we talk of resonance in such a case. When you say words, they are also reflected from the walls of the opening. The dimensions of the opening particularly amplify low sounds because the higher ones are more absorbed by the air.
In Berlin on the Devil’s Mountain (Teufelsberg) - an artificial hill made of war debris, there is a structure consisting of five spherical domes. These are the remains of a spy listening post of the US National Security Agency (NSA) from the Cold War period, where the described effect was used.