> Yeah, the wall "playing back recorded sounds" is similar to an amusing archeological article I once saw, in "Science" magazine. The author noted that sound waves at certain frequencies record diffraction-grating-like patterns (of micron precision) on the surfaces of suitable substrates (like certain colloidal gels or metal crystals). I believe that part is true. But then he went on and claimed that, knowing this, he could retrieve "soundscapes" from ancient times, as recorded in the -- get this -- the wet clay of drying terra cotta pots created 2000 yrs ago. Supposedly, by drawing a special stylus across the pottery pieces, and playing them the way an LP player does, we'd be able to hear market sounds... and children playing... and what human voices sounded like, back then. Never mind that clay doesn't have enough material resolution or durability to preserve any diffraction patterns... It sounds like a good project, so why not?!

When I was 16 I built a crystal radio set from a kit. It used a germanium crystal which was glued to a board and a wire called a "cat whisker" to pick up the signal from the crystal. There was a tuning coil and an earphone. There was no amplifier circuit and no battery. With a good solid ground and an antenna, it picked up AM stations several miles away. Question: Are dental filings ever made of germanium?
Howard