The glass armonica's ghostly notes will cause insanity in its musicians and listeners! At least this is what was thought to be true in the 18th century. People were frightened by the armonica's sound due to it's strange interactions with the human brain and ears.  The glass armonica's sound is perceived by human ears differently than other instruments because its range is between 1,000 and 4,000 hertz. When sounds are below 4,000 hertz, the human brain compares "phase differences" between the left and right ears to triangulate the origin of the sound rather than comparing volumes. This causes hearing disorientation and a "not quite sure" feeling about where the sound is coming from. The video below is "Dance of the Sugar Plum Fairy" from the Nutcracker being played on a glass armonica. Relax and enjoy :-P