Study On Improving Low Frequency Sound Absorption Performance Of Micro-perforated Composite Structure Using Magnetic Properties

Micro-perforated panel(MPP)absorber is mainly composed of micro-perforated panel and the air-cavity behind it.The MPP absorber has the characteristics of high acoustic resistance and small acoustic mass so that the sound absorbing materials usually filled in the air-cavity can be spared,making the structure simple and environment friendly.But the thickness of the air-cavity is always confined in application,making the acoustic absorptivity in low frequency range not good enough,which limits the wide applications of the MPP absorber.The mechanical impedance plate(MIP)uses mechanical resonance to absorb sound at low frequencies and the structure thickness is thin.The mechanical impedance plate is introduced into the micro-perforated plate cavity to form a micro-perforated plate composite mechanical impedance plate sound-absorbing structure.The combination of mechanical resonance and cavity resonance can meet the high High frequency sound absorption and low frequency sound absorption.However,the mechanical impedance plate has a problem of narrow sound absorption band.First,sum up the knowledge about the working principle of sound absorption of mechanical impedance plates,analyze the acoustic quality factors of mechanical impedance plates,and study the influence of different parameters on the sound absorption band of the structure.The sound absorption structure of the mechanical impedance plate is different from the sound absorption structure such as porous sound absorption material and micro perforated plate.The sound absorption performance of the mechanical impedance plate is affected by the area and the shape parameters of the plate.Through calculation,it is found that the sound quality factor of the structure shows a downward trend with the expansion of the area,which is conducive to widening the bandwidth.At the same time,the influence of the shape of the mechanical impedance plate on the sound absorption performance of the structure is also discussed,and the practical application of the mechanical impedance plate is guided.The results show that the sound absorption performance after the area of the mechanical impedance plate is enlarged in actual use is better than the test results of the standing wave tube experiment.The key to further widening the sound absorption bandwidth is to increase damping.On the basis of the above analysis,the contradiction between the elastic coefficient and damping coefficient of themechanical impedance plate sound absorption structure is pointed out:By changing the structure of viscoelastic materials,reducing the elastic coefficient will inevitably lead to the same proportion of structural damping,which will adversely affect the sound absorption bandwidth and sound absorption coefficient.In order to solve this contradiction,this paper proposes to introduce magnetic materials into the mechanical impedance structure,The research idea is to use the attractive force of the magnetic material to offset part of the elastic force of the viscoelastic material,and the structure of the viscoelastic material does not change,so the damping coefficient of the structure will not decrease.Before analyzing the influence of the sound absorption performance of the magnetic material structure,first introduce the magnetic force calculation process and calculate the magnetic force change curve,and then introduce the magnetic material into the composite structure to establish a mathematical model,The composite sound-absorbing structure is composed of a mechanical impedance plate and a micro-perforated plate after the introduction of magnetic materials.The sound absorption coefficient of the composite structure is calculated by the impedance transfer method,the structural parameters are adjusted reasonably according to the previous theoretical analysis,and the experimental research is carried out.First,the effect of the magnetic force on the elastic coefficient and the damping coefficient of the mechanical impedance part is tested using the half-power bandwidth method,and then used Standing wave tube tests the sound absorption coefficient curve of the composite structure.The experimental results show that :The magnetic attractive force cancels out part of the elastic force of the viscoelastic material,and the elastic coefficient of the structure decreases.At the same time,the damping coefficient of the structure does not decrease in the same proportion as the elastic coefficient,but increases somewhat.The experimental results are basically consistent with the theoretical calculations,indicating that the established calculation model is reasonable;after the magnetic material is introduced into the composite structure,the sound absorption peak of the low frequency part is significantly widened,and the sound absorption coefficient is also improved,which significantly improves the low frequency sound absorption performance.