Background/Introduction <p>Helmholtz resonators with extended necks are recognized for their adaptable sound attenuation capabilities, making them essential in diverse acoustic applications. The introduction of defects into such periodic structures is a known method for creating localized modes within acoustic band gaps, which can be highly sensitive to environmental changes.</p> Purpose <p>This research aims to explore the acoustic dynamics of periodic structures incorporating defective Helmholtz resonators with extended necks that are filled with gas samples. The study specifically investigates how geometric variations of the defect influence the transmission spectra and the formation of these localized modes. The transmission spectra of these structures were analyzed, with a focus on how specific geometric variations of the defect and the physical properties of the filling gas affect the resulting acoustic modes.</p> Methods <p>The acoustic behavior was analyzed using an analytical transfer matrix method. The results from this analytical approach were validated and shown to be in good agreement with experimental measurements.</p> Results <p>A key finding is that introducing a defect into the periodic structure results in sharp resonance modes within the bandgap. These modes are highly responsive to both the defect's geometric configuration and the physical properties of the gas filling the resonators.</p> Conclusions <p>The findings underscore the potential of defect-engineered Helmholtz resonators with extended necks as a robust platform for gas sensing, as they enable precise frequency adjustments based on gas characteristics. This work not only advances the design of acoustic gas sensors but also enriches the fundamental understanding of wave control in periodic acoustic systems.&#xa0;</p>