The wall of the forest, high fence, building, mountain - all at all a barrier, reflecting the echo, there is nothing like a mirror for sound; it reflects sound like a plane mirror reflects light.
Sound mirrors are not only flat, but curves. Concave sound mirror acts as a reflector: focusing “sound rays” in its focus.
Two deep plates give the opportunity to do an interesting experience of this kind. Put one plate on the table and a few inches from her bottom hold a pocket watch. Another plate to keep the head, near the ear, as shown in the figure.
Sound concave mirrors.
If the position of the watch, ear and plates found correctly (it fails after a number of samples), you will hear the ticking of the clock, as if coming from the plates, which you hold in your head. The illusion is enhanced, if you close your eyes: then positively you cannot determine by ear, in which the hand of the clock in the left or right.
The builders of medieval castles were often created such sound curiosities, placing busts or in the focus of the concave sound mirrors, or at the end of the Treaty pipes, cleverly hidden in the wall. In the figure below, taken from old books of the XVI century, you can see these clever devices: the ceiling in the form of a code refers to the lips bust sounds generated outside the Treaty tube; huge Treaty pipes, immured in the building, bring a variety of sounds from the courtyard to the stone busts placed at the walls of one room, etc. Visitor this gallery seemed that marble busts whisper, hum, etc.
Sound wonders in the ancient castle - speaking busts. (From the book of Athanasius Kircher, 1560).