To solve the problem of low-frequency noise in the environment, a Helmholtz-type phononic crystal with adjustable cavity structure and labyrinth tubes was designed. The unique design of the labyrinth tube greatly increases the length of the tube, improving low-frequency sound insulation performance, and the design of adjustable cavity structure realizes active regulation of the band structure. The band gap structure and sound insulation characteristics were analyzed by finite element method (FEM) and electro-mechanical-acoustic analogy method. The result shows that, firstly, the structure can generate two complete band gaps in the low-frequency range of 0–500 Hz, and there is a low-frequency band gap with lower limit of 40 Hz. Meanwhile, the structure has excellent sound insulation performance in the range of 0–500 Hz. Secondly, multiple resonant band gaps can be connected by adjusting the structural layout of the cavity through the telescopic screw, so as to achieve the purpose of widening the band gap and active control of environmental noise. Finally, in the periodic arrangement design of the structure, reducing the spacing between cells can effectively increase the bandwidth of band gaps. This design broadens the design idea of phononic crystal and provides a new method to solve the problem of low-frequency noise control.

In order to solve the problem of low-frequency noise of aircraft cabins, this paper presents a new Helmholtz type phononic crystal with a two-dimensional symmetric structure. Under the condition of the lattice constant of 62 mm, the lower limit of the first band gap is about 12 Hz, and the width is more than 10 Hz, thus the symmetric structure has distinct sound insulation ability in the low-frequency range. Firstly, the cause of the low-frequency band gap is analyzed by using the sound pressure field, and the range of band gaps is calculated by using the finite element method and the spring-oscillator model. Although the research shows that the finite element calculation results are basically consistent with the theoretical calculation, there are still some errors, and the reasons for the errors are analyzed. Secondly, the finite element method and equivalent model method are used to explore the influence of parameters of the symmetric structure on the first band gap. The result shows that the upper limit of the first band gap decreases with the increase of the lattice constant and the wedge height and increases with the increase of the length of wedge base; the lower limit of the band gap decreases with the increase of the wedge height and length of wedge base and is independent of the change of lattice constant, which further reveals the essence of the band gap formation and verifies the accuracy of the equivalent model. This study provides some theoretical support for low-frequency noise control and broadens the design idea of symmetric phononic crystal.

In recent years, the vibration and noise reduction performance of military aircraft has become an important index to measure its performance. In order to solve the problem of low-frequency noise generated by military aircraft, a novel Helmholtz two-dimensional phononic crystal was constructed in this paper. The structure adopts maze-shaped air channel and adds rigid oscillators. Under the condition that the lattice constant is 62 mm, the lower limit of the first band gap is reduced to about 15 Hz. The structure has four complete band gaps in the range of 0~500 Hz, respectively 15.223~17.464 Hz, 107.46~200.68 Hz, 231.18~310.68 Hz, 341.14~404.49 Hz. In addition, the sound reduction index of the structure reached 25 dB at 15 Hz, and two peaks of more than 150 dB appeared at at about 107 Hz and 231Hz. which shows distinct sound insulation ability in the low-frequency range. It has has engineering significance in the application of low-frequency noise control in the aircraft cabin. The cause of the band gap is explored by analyzing the vibration mode and sound pressure field. The “spring-oscillator” of the structure model was established by the method of "Mechanical-acoustic analogy". The finite element method and transfer matrix method are used to calculate the upper and lower limits of the first band gap. The study shows that for the first gap of the structure, the results obtained by the two methods are similar, which indicates the correctness of the model hypothesis. Secondly, the effects of structural parameters such as the lattice constant, the length of the air channel and the oscillator material on the first band gap are investigated by finite element method and equivalent model method. The study shows that the increase of the length of air channel and lattice constant will reduce the lower limit of the first band gap when other structural parameters remain unchanged. Moreover, the increase of the density of the oscillator material can effectively reduce the upper and lower limits of the second band gap, which further reveals the essence of the formation of the band gap of the structure and verifies the accuracy of the equivalent model. This study provides theoretical support for low frequency noise control and broadens the design of low-frequency phononic crystals.

To solve the problem of low-frequency noise in the environment, a two-dimensional Helmholtz-type phononic crystal with a labyrinth tube was designed in the paper. First, the low-frequency band structure was calculated by the finite element method (FEM) and transfer matrix method (TMM). Second, the bandgap formation was analyzed by using an acoustic pressure field, and the “spring-oscillator” equivalent model of the structure was established. Finally, the influences of structural parameters on the first bandgap were investigated. Results show that there are four bandgaps in the frequency range of 0–300 Hz, and the lower limit of the first bandgap can be as low as 12.15 Hz, which improves the low-frequency sound insulation ability of phononic crystals of the same size. The calculation results of the two methods (FEM and TMM) are basically consistent. Research on the influencing factors of the bandgap shows that the increase in the length of the tube will reduce the upper and lower limits of the bandgap and narrow the bandgap width. With the increase of the lattice constant, the upper limit of the bandgap decreases, while the lower limit of the bandgap remains unchanged. The design provides a new method to solve the problem of low-frequency noise reduction.