In this letter, we propose a theoretical description of double nonuniform cross-section (DNUCS) channels, which can achieve the designed absorption coefficient with a shorter channel overall. Introducing channels with a nonuniform cross section changes the period of the surface acoustic impedance, which has a significant impact on the dominant operating frequencies. In this paper, we give the relation between the absorption peak position and the geometric parameters, which can be used to design DNUCS channels with a specific operating frequency. Furthermore, multiple nonuniform cross-section channels can be studied in the same way. Based on the above theory, we reduce the operating absorption frequency range of a new type of Fabry–Pérot absorbers to a lower regime in a constant volume. Our theoretical framework may be important in designing absorption metasurfaces and for further research.
Aiming at the noise control of the HVDC converter station, a one-dimensional two-port metamaterial muffler based on the acoustic slow-wave effect is designed and manufactured. The metamaterial muffler achieves a broadband quasi-perfect absorption of noise from 600 to 900 Hz while ensuring a certain ventilation capacity. In addition, the internal equivalent sound velocity curve and the sound pressure and velocity field of the muffler are used to reveal the mechanism of its broadband quasi-perfect sound absorption. The performance of the muffler was verified by theoretical, numerical, and experimental models. The work in this paper is of guiding significance for solving the noise problem in HVDC converter stations.
Aiming at the problem of the need for trial-and-error in the design of the size of Fabry–Pérot (F–P) resonant absorbers, we start from the sound absorption caused by loss and propose a design method to accurately obtain the optimal size of F–P tubes with circular and rectangular cross sections. An innovative loss equation is constructed, which relates the F–P tube's critical loss to the transmission loss of sound waves in the tube. By solving the loss equation, the size of the F–P tube required for perfect sound absorption can be obtained. This method avoids the need for experiments or simulations to find the optimal size, and it is simple, fast, and accurate. Single-frequency perfect sound-absorbing metasurfaces of circular and rectangular cross sections were designed using this method. The performances of these metasurfaces were verified using theoretical, numerical, and experimental models. The three resulting sound absorption coefficient curves had good consistency and achieved perfect sound absorption at the target frequency. The feasibility and accuracy of the design method were established. The essence of the loss equation is to find the size of the F–P tube corresponding to the “zero” point on the real-frequency axis of the complex-frequency plane. The work in this paper is of guiding significance for determining the sizes of F–P tubes.
Aiming at the problem that the peak frequency of Fabry–Pérot (F–P) resonance metamaterials is shifted to high frequency due to the right-angle bend, we start from the mechanism of the frequency shift caused by the bend and propose a length correction theoretical model. Using the idea of equivalence, the effect of a corner is the first equivalent to the effective density. Then, the change in effective density is equivalent to the effective length, and the theoretical derivation is completed. This model can guide the length design of the F–P tube. Moreover, it can be used to predict the peak frequency of the F–P tube with a right-angle bend if its geometric dimensions are known. Through the analysis from theory, simulation, and experiment of two samples, the accuracy of the length correction theoretical model is verified. Additionally, by the power dissipation density and the dissipated energies, it is determined that the fundamental reason for the frequency shift is that the right-angle bend changes the distribution of power dissipation density in the tube. The work in this paper is of guiding significance for the frequency prediction and length design of F–P tubes.
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