I distinctly remember it too. This wasn't a robot, but just a speaker system without propulsion. I think it was demonstrated on a plexiglass sheet, probably abo

I distinctly remember it too. This wasn't a robot, but just a speaker system without propulsion. I think it was demonstrated on a plexiglass sheet, probably above and below it. They demonstrated various frequencies and weights. Maybe various designs with multiple speakers too? IIRC, the author mentioned they patented it, but couldn't go anywhere with it. Unfortunately, I can't find it anymore. I'll keep looking a bit and edit or post below if I do.