Based on an original design that I built in college, this is an LED lightshow that plugs into any audio source.(I have it on top of our home entertainment system, so it shows whatever we’re listening to or watching.)
The display is divided vertically into left and right channels. It displays the top half of the waveform of each channel with their zero levels in the center. The two knobs control the amplitude and the sweep speed.
When set to the right speed, a strong bass note will show up as a floating red bubble. A note an octave up will be two bubbles. Notes inbetween will be a series of bubbles floating up or down the display. Of course, all the other frequencies are in there, too, so you get a much more complicated display, but the bass tends to be the most prominent. Dynamics from drums, etc. cause the width to pulse. It all ends up being quite hypnotic