AbstractA piezoelectric flat panel loudspeaker operates by utilizing a piezoelectric film actuator to vibrate a diaphragm, offering advantages in miniaturization, embedding, and spatial efficiency. However, achieving high sound pressure levels (SPL) at low frequencies and maintaining a flat frequency response remains challenging. This study presents a Conical Shellular Sandwich Diaphragm (CSSD), derived from conical shellular metamaterials, which combines lightweight properties with high stiffness. The CSSD enhances low‐frequency SPL and improves frequency response flatness through structural optimization, eliminating the need for complex systems. Finite element analysis identifies optimal geometric parameters for the CSSD unit cell, resulting in an 11.5 dB increase in low‐frequency SPL and a 53% reduction in peak‐dip deviation across the 200 Hz–20 kHz range, compared to a conventional Flat Panel Diaphragm (FPD). These enhancements stem from a reduction in the CSSD's effective mass (meff) to one‐hundredth of an equivalent‐volume FPD and an increase in effective bulk modulus (keff) relative to structures with the same mass. Experimental tests of 3D‐printed CSSD and FPD prototypes integrated with piezoelectric actuators match the simulation results. This study demonstrates the potential of mechanical metamaterials to address design limitations in flat panel loudspeakers, enabling improved sound quality and simpler configurations for commercial use.