Donut rings are special rings, made of extremely high duty foam or rubber. These rings are used for the strongest woofers in subs and caraudio. These rings are pretty hard to get and more expensive than 'normal' rings due to their extreme qualities.
Above: our heavy duty F632donut ring. For extremely powerful woofers.
Above: 4 different 'donuts'. 3 kinds of foam, one rubber (top).