Defective phononic crystal based on dual Helmholtz resonators for sensing applications

The increasing levels of air pollutants produced by human activities create a demand for efficient gas detectors, among which passive acoustic sensors are one of the most promising technologies. This work contributes to the field by developing a gas sensor based on a finite-size phononic crystal with a defect. Unit cells are made of dual Helmholtz resonators attached to a rectangular waveguide. The defect is introduced as a spacer between two periodic arrays of resonators and forms a one-dimensional cavity defined by Bragg reflectors. As a result, the structure’s transmission spectrum exhibits a narrow resonant peak within the stop band, and its spectral characteristics are susceptible to the acoustic properties of the medium filling the structure. The sensing response is analyzed using the transfer matrix method and validated by the finite element method, with thermoviscous losses taken into account to assess realistic sensor performance. Several hazardous gases, including carbon dioxide and ammonia, are considered as test samples. For the considered configurations, the estimated sensitivity is 1.38 Hz.s/m. The limit of detection is associated with the minimum detectable sound-speed variation, 0.04 m/s, for the defect mode resonance, depending on the gas and the length of the spacer. This enables the detection of small concentration changes in gas mixtures.