Love Hultén took a deton8 drum synth and rehoused it in a block of wood. Also rehoused in the block of wood: an uncanny blob of ferrofluid, dancing to the beat (previously at Boing Boing).
Hackaday took a closer look.
[Love Hultén]'s drum synth was inspired by this custom Bluetooth speaker with dancing ferrofluid by [Dakd Jung], which drives an electromagnetic coil with frequencies selected from the audio with an MSGEQ7 equalizer. That way, only frequencies that work best for moving the fluid in interesting ways get used for the visualization. The MSGEQ7 spectrum analyzer chip is very useful for music-driven projects, as demonstrated by these sound-reactive LED shades which illustrate the audio element nicely.
The coils that create the electromagnetic field causing ferrofluid to move can take different forms, but two very interesting ones are this 12-layer PCB coil and for more intricate displays, there's a 12×21 coil array that creates a dot-matrix-like display.
Also previously, a reliable recipe to make your own.