Andy Cavatorta was commissioned to build a new musical instrument based on a pendulum for Bjork's Biophilia show in Manchester, UK. The first such instrument built was the Gravity Ring, but that was scrapped as too bulky for the tour. The next idea was Gravity Harps. The pendulums each contain harp strings, which are plucked as they pass through the bottom of their swing. The harps also rotate, which exposes different strings to the "plucker." You can see the process of designing and building the harps at Cavatora's website. http://andycavatorta.com/2011/07/02/gravity-harps/ -Thanks, Andy!