Well, using them for miking kick drum is pretty useful, IMO - they really work! Here's one from the 70s: Break up a small mirror and glue the pieces at vaarious points all over the cone. Feed the speaker (through appropriate amplification) from an aux or buss on your mixer, and train the beam from a small color changing spotlight on the speaker. Kind of a "disco-ball" effect that dances to the music - geat for live gigs, not much use in the studio, except for "vibe".