This does not really work very well as the acoustic horn it was envisaged as! The shape is fun perhaps - a concentric tapered horn that splays out to a 360 degree output. The headphone fits over a small 25mm hole at the narrow central end - this works fine but the sound is only modestly improved. I do not understand the acoustics well enough it seems!
Made from glued-together pine offcuts (with drilled hardwood dowel spacers)