They’re so simple, and it’s one of my favorite sounds. These are flat disks of aluminum or steel, hammered at the center to create a small boss for tuning. In flat gongs such as these the fundamental mode of vibration has a nodal ring about a quarter of the radius in from the edge, and these gongs are suspended from points along that ring.
In the audioclip, the gongs are sometimes dipped in water to bend their pitches. Joining the gongs is a large mechanized panpipes called Hippy Pappy.