| Due to the characteristics of long wavelength,strong penetrability and non-attenuation of low-frequency noise,it is difficult to solve the low-frequency noise pollution in the development of modern engineering carrier equipment.Traditional sound-absorbing materials,such as porous sound-absorbing materials,can achieve effective absorption only when the thickness is greater than one quarter of the sound wave wavelength.To achieve low-frequency noise absorption,a larger thickness is required,which compresses the internal space of the equipment cabin,and is difficult to meet the needs of practical engineering applications.Therefore,how to control large-wavelength sound waves with small-scale materials and structures is one of the most important basic scientific issues in noise suppression.Microperforated panels absorber have received considerable attention and development in the field of noise suppression due to their excellent sound absorption characteristics,adjustable sound absorption bandwidth,high durability,and high structural strength.The traditional microperforated panel absorber is suitable for low and medium frequency sound absorption.If its resonance frequency moves to a lower frequency,the depth of the back cavity of the microperforated panel absorber needs to be deepened,which also limits its application in engineering practice.The helical-channel sound absorption structure formed based on the space-coiling principle can make the sound wave obtain a longer acoustic propagation path under the same structure thickness,and has excellent low frequency and sub-wavelength scale sound absorption characteristics,but it can only achieve perfect sound absorption at a single frequency,and still has large design improvement space.Based on this,this paper proposes the monohelical-channel microperforated panel structure based on the sound absorption characteristics of the microperforated panel absorber and the introduction of helical-channel back cavity design.Through the combination of theoretical calculation,numerical simulation and experimental test,the sound absorption characteristics are systematically studied,and the physical mechanism behind it is analyzed.And based on this,through the principle of multi-resonance coupling,the multihelical-channel microperforated panel structure is further proposed,which has excellent low-frequency,broadband and sub-wavelength sound absorption characteristics.The main research contents and innovations of this paper are as follows:(1)The monohelical-channel microperforated panel structure is designed by combining the helical channel with the microperforated panel.Based on the microperforated panel theory and the equivalent volume method,the theoretical model of acoustic impedance and sound absorption was established,and then the numerical simulation calculation model of its sound absorption performance was established through the COMSOL finite element simulation software.And its acoustic impedance tube test sample was manufactured by 3D printing.The sound absorption frequency band with the sound absorption coefficient greater than 0.8 is defined as the effective sound absorption frequency band to characterize the sound absorption characteristics.The results show that the three methods of theoretical calculation,numerical simulation,and experimental testing have excellent consistency in predicting the sound absorption performance of monohelical-channel microperforated panel structure,verifying the accuracy of theoretical calculation and numerical simulation.The effective sound absorption frequency band obtained by the three methods is about 160-170 Hz,while the overall thickness of the structure is only 65 mm,and the ratio of the sound wave wavelength at 160 Hz is 1/33,which shows that the monohelical-channel microperforated panel structure has the characteristics of deep sub-wavelength scale sound absorption,that is,it has the ability to absorb sound when the sound wavelength is larger than the structure scale.(2)The sound absorption mechanism of the monohelical-channel microperforated panel structure is revealed from theoretical analysis and numerical simulation.Through theoretical analysis,it is found that the structure resonates when the normalized acoustic impedance ratio is 0,and its normalized acoustic impedance ratio is close to 1.At this time,the maximum sound absorption coefficient of the structure is obtained.Combined with numerical simulation,it is found that the air particle velocity and thermal viscosity loss energy in the micropore at the resonance frequency are the largest.The validated numerical simulation model is used to analyze and study the influence of various geometric parameters on the sound absorption performance of the monohelical-channel microperforated panel structure,and the sensitivity analysis of the influence of each parameter on the sound absorption performance is carried out.The results showed that the number and radius of helical channels have a greater sensitivity to the peak frequency of sound absorption of the monohelical-channel microperforated panel structure.The width and height of helical channels,as well as the aperture,thickness,and perforation rate of microperforated panel,have a greater sensitivity to the peak frequency of sound absorption coefficient of the monohelical-channel microperforated panel structure.(3)In order to broaden the bandwidth of structure,the multihelical-channel microperforated panel structure is designed by coupling the sound absorption units of multiple monohelical-channel microperforated panel absorber in parallel.The sound absorption characteristics and physical mechanism of multihelical-channel microperforated panel structure were studied by combining theoretical calculation,numerical simulation and experimental test.The results show that its effective sound absorption frequency range is280-505 Hz,and there are 6 sound absorption peaks with sound absorption coefficient reaching quasi-perfect sound absorption,while its structural thickness(106mm)is only1/11-1/6 of the working frequency wavelength.Then the influence of geometric structure parameters on the sound absorption performance of multihelical-channel microperforated panel structure is analyzed and studied.It is found that changing a certain parameter alone cannot take into account the low frequency and broadband band of the multihelical-channel microperforated panel structure.(4)The genetic algorithm is used to optimize the structural parameters of multihelical-channel microperforated panel structure.Compared with the control group before optimization,the effective sound absorption bandwidth of the optimized structure in the low frequency range increased by 40.4%(213 Hz → 299 Hz),and the initial effective sound absorption frequency shifted to the low frequency range(280 Hz→201 Hz),with the structure wavelength ratio almost unchanged.Comparing the optimized multihelical-channel microperforated panel structure with other structures in the subwavelength-scale sound absorption field,it is found that it has certain advantages in low-frequency and broadband sound absorption. |
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