<p>This study investigates a compact acoustic metasurface of periodic parallel Helmholtz resonators with a resonator defect located in the middle of the structure for gas-sensing applications. Introducing defects into the common resonators’ array creates localized resonant modes that affect the gas properties, improving sensitivity. The resonance frequency shift occurs due to the filling gas samples inside the defective structure. Using the finite element method, numerical simulations show that the defective Helmholtz resonator modifies the pressure distribution and reduces the band gap. The study shows that high sensitivity can be achieved by optimizing the number of periodic cells associated with defect modes. Hence, Helmholtz resonator arrays are good candidates for hazardous gas detection. The structure is analysed considering different parameters. The results indicate improved sensitivity with an increasing number of Helmholtz resonators, highlighting the potential use of these systems in environmental monitoring and safety.</p>