Finite Element Modeling of Acoustic Shielding via Phononic Crystal structures

Quality factor of Contour Mode Resonators (CMR) are mainly affected by energy losses due to acoustic waves leaving the resonator through the anchors. An engineering of the anchors in order to create a periodic variation in the acoustic impedance of the material, structures known as Phononic Crystals (PnCs), can help improve the Q factor by reflecting part of the acoustic waves. During this project, FEM models have been validated for both 1D and 2D PnCs. The behavior of the band diagram and quantification of transmission has been studied for three different PnC geometries: circle, cross and square unit cell. Parameters such as unit cell size, filling factor, material composition, thickness and number of repetitions have been varied and comparisons between transmission graph and band diagram allowed to understand better the nature of the modes. It has been found that a linear approximation can predict with a good precision the position of the band gap as a function of the unit cell size. These studies have been done in order to select the best configuration for a future integration on the substrate of a CMR whose frequency range of operation is situated between 200MHz and 500 MHz. The results obtained report a better attenuation and broader peak of attenuation for a cross unit cell made out of AlN and an attenuation increasing linearly with the number of repetitions.

PID-like active impedance control for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

Xinxin Guo, Hervé Lissek, Maxime Volery

Sound absorption at low frequencies still remains a challenge in both scientific research and engineering practice. Natural porous materials are ineffective in this frequency range, as well as acoustic resonators which present too narrow bandwidth of absorption, thus requiring alternative solutions based on active absorption techniques. In the present work, we propose an active control framework applied on a closed-box loudspeaker to enable the adjustment of the acoustic impedance at the loudspeaker diaphragm. More specifically, based on the proportionality between the pressure inside the enclosure and the axial displacement of the loudspeaker diaphragm at low frequencies, we demonstrate both analytically and experimentally that a PID-like feedback control approach allows tuning independently the compliance, the resistance and the moving mass of the closed-box loudspeaker to implement a prescribed impedance of a single-degree-of-freedom resonator. By considering different control combinations to tailor the resonator characteristics, a perfect absorption (with absorption coefficient equal to 1) is achievable at the target resonance frequency, while enlarging the effective absorption bandwidth. Moreover, the proposed feedback control strategy shows an excellent control accuracy, especially compared to the feedforward-based control formerly reported in the literature. The mismatches between the performance of experimental prototype and the model, likely to result from the control time delay and the inaccuracy in estimating the loudspeaker parameters, can be compensated directly by tuning the control parameters in the control platform. The active resonators implemented through the reported control scheme can be used to build more complex acoustic devices/structures to enable high-efficiency broadband sound absorption or other types of acoustic phenomena such as wavefront shaping.

2022

PID-like active impedance control for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

Xinxin Guo, Hervé Lissek, Maxime Volery

2022