Study On Low-frequency Broadband Sound Absorption Performance Of Subwavelength Composite Structure

Whether in the air or underwater,the wavelength of low-frequency noise is long,and the problems caused by the control of traditional materials,such as large volume,large weight and high cost,resulting in failure to meet the needs of practical applications.In recent years,metamaterials/structures have received widespread attention and rapid development due to their good low-frequency sound absorption properties.Metamaterials are artificial micro-structured materials whose properties are not only related to the base material,but also depend on the material unit structure and its arrangement,and have exotic properties that traditional materials do not possess.Compared with traditional materials,metamaterials have smaller structural size and higher energy dissipation rate,which provide new ideas for low-frequency vibration and noise control.This paper investigates the low-frequency broadband acoustic absorption in air and underwater based on the subwavelength composite structure,mainly using theoretical analysis,numerical calculation and experimental verification.The details are as follows:1)The sub-wavelength composite sound absorption structure based on porous material and FP(Fabry-Pérot)channel is proposed,which strikes a balance between low frequency and broadband sound absorption,with the average absorption coefficient reaching 90%.The pressure-thermal viscosity sound finite element numerical analysis model of the composite sound absorption structure is established,and the experimental results of the impedance tube verify the good sound absorption properties.Based on the finite element analysis method,the effects of some structural parameters on the sound absorption performance of composite sound absorption structure are discussed.Moreover,the sound absorption properties are compared with other composite structures.The results show that the composite sound absorption structure has a certain application value in the field of noise control.2)An equivalent theoretical impedance of a square-hole perforated panel is calculated.Moreover,the sound absorption structure is designed using perforated panels and folded channels.Then the designed structure is investigated theoretically and numerically and the obtained results are verified through the experiment.The performed analysis shows that the sound dissipation mainly originates from the friction effect in the narrow area and the viscoheat conversion in the folded channel.The analysis the zero-pole of the reflection coefficient shows that the distance between the zero point and the real axis affects the absorption peak,and the distance between the zero point and the pole affects the absorption bandwidth.Analyzing the relative characteristic impedance shows that when the value of real and imaginary parts approach 1 and 0,respectively,the acoustic dissipation improves.In the designed subwavelength structure with a thickness of 53 mm,an average absorption coefficient reaches0.88 in the frequency range 269-541 Hz.This scheme is expected to be applied to control lowfrequency noise.3)Inspired by aero-acoustic metamaterials,a perfect underwater acoustic meta-structure is designed theoretically(derived from the transfer function method)and numerically,with good low-frequency absorption performance(α>0.8),broadband absorption performance(semiabsorbent bandwidth up to 83%)and strong resistance to deformation.The results of theoretical predictions and numerical calculations of acoustic performance are in good agreement.It is demonstrated that sound energy is mainly dissipated by the shear wave loss in the rubber coating due to waveform conversion at the acoustic coupling boundary.A parametric study on the sound absorption performance of the structures confirms strong tunability,indicating a possible lowfrequency broadband absorber design.An underwater meta-structure with low-frequency(<1000 Hz)and broadband(221–972 Hz)acoustic absorption is realized by optimizing the main structural parameters of the absorber.Moreover,at a relatively regulated underwater lowfrequency wavelength,the spatial folded structure enables a deep subwavelength dimension(λ/106).Due to its many advantages,the proposed meta-structure has wide potential applications in underwater noise control.