Broadband Ventilated Acoustic Structure With Micro-perforates For Noise Reduction

The generation and development of the acoustical metamaterials in recent years provides new design ideas and methods for noise reduction.With acoustical properties that go beyond the limit of natural materials,the advent of acoustical metamaterials enables various fascinating wave-steering phenomena,such as negative refraction,acoustic focusing,acoustic cloaking,abnormal acoustic insulation and so on,by adjusting the parameters of the acoustical metamaterials.Acoustical metamaterials can break through the limitations of the law of acoustic quality,and it can control acoustic waves of long wavelength with subwavelength scales,which is beneficial to shrink the volume of the structure and makes the structure more compact and practical.Acoustical metamaterials has great potential applications in noise reduction at low frequency.A broadband ventilated acoustic structure with micro-perforates for noise reduction was proposed in this paper on the basis of the acoustical metamaterials and the characteristics of the aerodynamic noise generated by the fan.It can achieve absorption over a wide band at low frequency with good ventilation performance.This paper mainly includes the followings:In Chapter ?,the background and significance of the research was introduced first,and then the development of the acoustical metamaterials in the field of noise reduction were briefly outlined.In Chapter ?,the mechanism of the fan noise and its sound source were introduced first.Then we used PROE,FLUENT,ACTRAN to model and simulate the acoustic radiation model of the fans in a cabinet.The simulations were on the basis of the aerodynamic theories.When simulated,the fan were set under different operating conditions.It could be found that the aerodynamic noise generated by the fan are mainly composed of a series of discrete noise and broadband noise by analyzing the simulated results.In addition,the discrete noise plays the dominant role in the aerodynamic noise generated by the fans.When the speed of the fan increased,the frequency and sound pressure corresponding to the peak point of the discrete noise increased,too.The discrete noise generated by the fan were mainly concentrated between 500Hz and 1200Hz according to the simulated results.In Chapter III,the evaluated parameters of the noise was introduced first and then the noise field generated by the fans in the cabinet at different positions under different working conditions were measured and analyzed.The results of the measurements showed that the spectrum of the noise generated by the fan in the cabinet will change with the increase of the operating conditions of the fan,and the high-frequency components would gradually appear.The discrete noise were mainly concentrated between 500Hz and 1500Hz according to the measurements,showing well agreement with the simulation results.In addition,by analyzing the directivity map of the noise field generated by the fan,it could be seen that the noise exhibits a certain directional distribution in space,and the directionality of the noise under different operating conditions were basically the same.In Chapter IV,the research progress of acoustic absorber was first briefly reviewed.Then,a broadband ventilated acoustic structure with micro-perforates for noise reduction was proposed.The mechanism of the proposed absorber was theoretically analyzed in detail by using the acoustic impedance circuit,and the effects of geometric parameters of the acoustic absorber on the eigenfrequencies and the sound absorption bandwidth of the structure were studied using COMSOL software.Both theoretical analysis and numerical simulation results show that a wider bandwidth of sound absorption can be achieved at a specific low frequency band by adjusting the structural parameters of the proposed structure.In addition,an experiment was also conducted using a four-microphone method in the anechoic chamber.Good agreement was observed between the simulations and experiments,with both showing the high absorption coefficient and good ventilation performance simultaneously provided by our designed structure.The structure proposed in this paper has a compact structure,wide absorption bandwidth,and good ventilation performance.It not only can be used to reduce fan noise in duct,but also can be used in existing small-scale integrated devices.It has great potential applications in noise reduction at low frequency.In Chapter V,we summarized the main content of this paper,and looked forward to the future work.