In finite size micro-perforated plate structure, the cross-sectional area size of back cavity will affect the resonant frequency of structure. Based on transfer matrix and the characteristics of acoustic propagation in variable cross-section channel, the sound absorption characteristics of the double-layer micro-perforated plate structure with variable cross-section back cavity are studied and analyzed, and a theoretical analysis model of the variable cross-section back cavity micro-perforated plate structure is established. By comparing the theoretical model with the finite element model, the effect of abrupt changes in the cross-sectional area of the back cavity on the noise reduction performance is obtained. As for the double-layer micro-perforated plate in this paper, the bigger the cross-sectional area of back cavity of inner micro-perforated plate, the lower the frequency of first peak absorption coefficient of structure will be and the higher the frequency corresponding to second absorption coefficient peak of structure. Utilizing this feature, a combined micro-perforated plate structure is designed, which has back cavities with different inner cross-sectional areas, and ultimately broadening the structural sound absorption band. Additionally, through using 3D printing technology to produce samples and conducting experimental tests in the impedance tube. Experiments show that the structure can achieve an absorption coefficient of more than 0.8 within the frequency range of 500–1650 Hz, which further improving the noise reduction performance of the MPP structure. The feasibility of variable-sectional back cavity structure for the design of low-frequency and broadband noise reduction absorber is verified.

The Helmholtz resonant structure with rectangular extended neck is designed to solve low-frequency broadband sound absorption problem in this work. Theoretical and finite element absorption models are established and be used for low-frequency acoustic design. What makes it interesting is that all parameters of the rectangular extended neck Helmholtz resonator structure can be adjusted to shift the working frequency. Based on the regularity of the structural parameters, four coupling structures with different neck depths, neck opening areas, cavity cross-sectional areas, and cavity depths are designed respectively, each of which exhibited multiple sound absorption coefficient peaks to enhance the low-frequency absorption capacity of the structure. To further analyze the effectiveness of coupling structure, the broadband acoustic absorption mechanism of the coupled structure is analyzed based on particle vibration velocity distribution. It is found that cells with different acoustic impedance contributed differently to the sound absorption, and cells with longer necks provided better noise reduction for low-frequency. The experiment is verified in the impedance tube, result shows that the coupling structure with 9 cells and a cavity depth of only 4 cm achieved an average sound absorption coefficient of above 0.8 at 210–340 Hz, which verified the accuracy of the theoretical model. Overall, the Helmholtz resonant cavity acoustic structure with rectangular extension neck designed in this work has a simple structure with low-frequency broadband acoustic absorption performance. This provides a new approach for designing low-frequency broadband acoustic structure.

Acoustic liners are the most effective solution to attenuate the noise of ducts with flows but still suffer from narrow sound absorption bandwidth and heavy bulk. In this work, we present a new type of acoustic meta-liner structure, which is composed of perforated plates and coiled-up Fabry–Pérot (FP) channels with carefully designed equivalent length. By tuning the geometric parameters of the perforated plate and the optimal combination of the coiled-up FP channels, perfect impedance matching is achieved in a broadband range. The strong dissipation of sound energy could also be observed under different speeds of grazing flow and high incident sound intensity. It is analytically, numerically, and experimentally demonstrated that for the structure, over 90% sound absorption is achieved in the broadband range from 500 to 3000 Hz in the absence of flow and the condition of grazing flow with a speed of 30–98 m/s, coupled with a 90–130 dB incident sound pressure level. Moreover, the thickness of the proposed simply structured meta-liner is 44 mm, which is only 8/125th of the wavelength corresponding to 500 Hz. As a deep sub-wavelength liner, it exhibits potential application prospects in the field of fluid–solid coupled machinery such as aero-engine systems and ventilation duct systems.

Acoustic liners are the most commonly used dissipative muffler for aero-engines but remain a challenge to design a broadband acoustic liner under the grazing flow. In this work, a novel neck-embedded meta-liner is proposed for the broadband sound-absorbing under the grazing flow with a wide speed range. The meta-liner is composed of many neck-embedded Helmholtz Resonators, and the idea of bending the subsequent cavity is introduced to improve low-frequency sound absorption performance. The acoustic impedance model of the meta-liner with neck-embedded slit is first detailly established by analyzing the acoustic characteristics of the equivalent elliptical pipe and using the matrix transfer method. Then by designing the local coupling between channels, efficient broadband impedance modulation and broadband matching with air impedance under grazing flow conditions are realized. The proposed meta-liner exhibits the excellent sound attenuation capacity in the frequency domain of 500–3000 Hz under the grazing flow speeds from 0 m s⁻¹ to 98 m s⁻¹ and the sound absorption performance is greatly boosted compared with the traditional Double- Degree-Of-Freedom liner. Features mentioned above have been verified numerically and in a series of comparative experiments. Besides, the thickness of the meta-liner is only 50 mm. Our work provides a possible design reference for the new generation of acoustic liners and has potential applications in noise reduction engineering.