Though additive manufacturing and novel optimization techniques have led to many recent advances in elastic metamaterials, difficult fundamental challenges (e.g., narrow bandgaps) and practical challenges (e.g., dissipation and friction) remain. This work introduces simple and hierarchical resonant metamaterials made of soft polydimethylsiloxane rubber and removable steel insets. The additively manufactured samples are able to produce bandgaps with a gap–midgap ratio of 81.8%, which surpasses the majority of resonant, metamaterials of the same class and greatly outperforms analogous resonant structures with a stiff epoxy matrix. The role of several physical features on the transmission loss (TL) curve is assessed in detail numerically and compared to the experimental TL data. Matrix compliance is found to be a rich mechanism for bandgap widening with a dual effect: it deepens the traditional resonant bandgaps, and it can selectively shift certain vibrational modes to lower frequencies and aid in the merging of multiple bandgaps. This can lead to an overall increase of the bandgap width of over an order of magnitude. Viscous dissipation, friction, and the stochastic nature of geometrical inaccuracies common in additive manufacturing were also found to shape the TL curve and associated bandgaps to various degrees. Some of these mechanisms, combined with a soft frame, can further help merge bandgaps in rainbow or hierarchical designs and form ultrabroad, subwavelength bandgaps.

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