Infrasound
Quote:18.98 hertz--the exact frequency at which a human eyeball starts resonating. The sound waves made his eyeballs resonate and produced an optical illusion: He saw a figure that didn't exist.
THAT's cool!
I've experimented a lot with a Velodyne HGS-18 subwoofer that goes down to 15Hz pretty well with decent accuracy.
With any clean speaker, you can hear standing waves as you walk around the room -- a pure tone gets softer and louder depending on how the reflections from walls and surfaces mingle with each other. Moving six inches can make a huge difference! It's kind of like at the beach, when there are certain places that the waves add to each other to form a large crest, or cancel each other out and the water stays flat although surrounded by violent waves.
With a low subwoofer, you FEEL the sound rather than hearing it. Moving slowly around the room, you can feel different parts of your body vibrating, as you run into a standing wave.
It's kind of like a "haunted spot", but I never thought to try my eyeball on it! Cool! :-)