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Though lightweight sandwich structures have been extensively applied in practical engineering, it remains a challenge to control wave propagation and vibration in these structures in a low-frequency range. In this work, the band structure of flexural waves in a metamaterial sandwich beam (MSB) with hourglass lattice truss core is investigated using the transfer matrix method (TMM). The hourglass truss structure with lumped masses is modelled as a series of local resonators with determined equivalent stiffnesses and masses. A metamaterial dual-beam (MDB) model is then established to describe the MSB, and the MDB model is noted to be equivalent to the conventional metamaterial beam (CMB) model under base excitation. The MSB is further studied directly by the finite element method that confirmed the MSB can be represented by the CMB through the transmittance analysis and band structure analysis. Subsequently, parametric study is performed to investigate the effects of the material and structural parameters on the band structures of the MSB. This work provides a roadmap of modelling of lightweight lattice sandwich beams with complex core structures and presents guidelines for applying sandwich beams to control wave propagation. • A novel metamaterial sandwich beam is designed for flexural wave attenuation. • A novel approach is developed to establish the model of metamaterial sandwich beam. • The mass-loaded struts are represented by equivalent mass–spring local resonators. • The feasibility of using the simplified CMB model to represent MSB is validated. [ABSTRACT FROM AUTHOR]