Via OhGizmo!, we find that an exhibit exists (or did exist) on Brühl’s Terrace in Dresden, Germany, which allows one to experience the simulated sounds of the devastating 1945 bombing of the city simply by resting oneself against a railing on the terrace in the appropriate manner (image from OhGizmo!):
The technology, evidently referred to as ‘touched echo’, is more descriptively known as bone conduction technology, and relies on the transmission of sounds directly to the inner ear through vibrations in the skull, rather than vibrations in the eardrum. The inner ear responds to vibration of the fluid contained within it, and in principle anything which produces such a vibration will be interpreted as ‘sound’.
You have probably experienced this effect yourself directly in the past without thinking about it, if you’ve ever placed your head against the side wall of a moving aircraft and noticed the increase in sound. In the Dresden exhibit, vibrations are induced in the railing of the overlook; by resting one’s arms against the railing and one’s head against one’s arms, the vibrations are transmitted to the skull and hence the inner ear. Because vibrations in rigid solids are not easily transmitted to the air, and vice versa, the sound effects are only heard by those participating.
It is interesting to note that, according to the Wikipedia article, bone conduction affects our self-perception in a subtle way. In solids, lower-frequency sounds transmit more readily than higher-frequency sounds (presumably due to what is in essence acoustical dispersion). Thanks to bone conduction, a person’s voice sounds deeper to himself than to others, resulting in the often-heard complaint that a voice recording “doesn’t sound like me.”
Bone conduction technology is already used for a number of applications, including hearing aids, underwater MP3 players, high-quality security headsets, and cell phones.
The only part of this story that bugs me: I couldn’t find any other description of the Brühl’s Terrace exhibit, even in the Dresden tourism web page! It apparently opened in late 2007, but whether it was a permanent display or a temporary show I cannot determine. A video of the opening can also be seen on the OhGizmo! webpage.