Halldorophone is an electro acoustic instrument invented by an Icelandic design artist Halldor Ulfarsson. It's a bit cello like by shape and playing posture, has 4 strings and 4 resonance strings, all miked. It features a built in speaker. This all creates a feedback loop similar to what you can do with a guitar, high gain amp and a cabinet. I've had a pleasure to discover and play this mysterious sounding instrument for a while now and I find it highly attempting. It's very physical instrument and as the main principle is to learn to control the feedback resonances it really forces you to rethink the whole concept of playing an instrument. Very cinematic sound somewhere between music and sound design.