For classroom demonstrations of the long-distance transmission of faint sounds, two parabolic reflectors are used. A source, such as a ticking pocket watch suspended from a string, is placed at the focal point of one reflector. Some sort of detector is placed at the focus of the other reflector, which faces the source. The obvious thing is the human ear, but this is complicated by the blocking of the acoustic signal by the head. A solution is an ear trumpet, a metal tube which is inserted into the ear canal. Today I would use a medical stethoscope, with the aperture facing the receiving reflector.

   The reflector from the apparatus collection at Glasgow University in Scotland was made by the French firm of Ducretet.

   The parabolic reflectors at Transylvania can also be used to study the transmission of thermal radiation. The brackets below the reflectors can be used to hold either the thermal or optical source and detector pairs.
 

         Glasgow University                               Transylvania University                                   Wittenberg University