With the rapid development of China’s economy,animal husbandry,as an essential industry in China’s agriculture and rural economy,is gradually changing from traditional decentralized breeding to large-scale,standardized,and mechanized rural scale farms.The increase in machinery and equipment has dramatically improved the farm’s production efficiency and provided a suitable living environment for livestock and poultry.However,the noise generated during its operation may adversely affect the growth and development of livestock and poultry,especially chickens.Among them,the continuous low-frequency noise generated by the draught fan that adjusts the indoor temperature and humidity has the most significant impact and is difficult to eliminate.Therefore,reducing the draught fan noise of chicken farms and improving the growth environment of chickens is a problem that needs to be solved urgently.At present,it is a widely used method to install noise reduction covers at the inlet and outlet of the draught fan.The commonly used noise reduction covers have the disadvantages of a closed structure,large thickness,poor low-frequency effect,and narrow bandwidth,which are not conducive to engineering applications.In recent years,the proposal and development of acoustic metamaterials have supplied new thoughts for designing noise reduction structures.Acoustic metamaterials generally refer to artificial materials with sub-wavelength dimensions in acoustics.They can realize free regulation of the propagation process of sound waves,break the limitations of low-frequency sound absorption under the sub-wavelength dimensions of traditional structures,and have significant application value.Therefore,aiming at the noise problem of draught fan operation in a large-scale breeding environment,this paper carries out the design and research of broadband ventilation and noise reduction structure based on acoustic metamaterials.The main work and research accomplished in this paper are as follows:(1)Measurement and analysis of the acoustic environment and draught fan noise in chicken farms.The noise of the chicken farm is measured with a sound level meter to understand the sound environment of the chicken farm.The draught fan noise data is then collected,and the target value and frequency of noise reduction are determined through spectral analysis.The measurement results show that the draught fan has a significant impact on the sound environment of the chicken farm.The noise near the draught fan is mainly distributed at 80.0~81.8d B(A),which cannot meet the requirements of the specification.It is necessary to reduce noise while considering ventilation by installing corresponding noise reduction structures.The draught fan’s noise is 82.3d B(A),the target noise reduction value is 12.3d B(A),and the noise reduction frequency is 250~800Hz.(2)Vertical broadband ventilation and noise reduction structure design based on chicken farm draught fans.Based on acoustic metamaterials,a micro-perforated plate positive noise reduction cover composed of a vertical ventilation noise reduction structure installed on the front of the indoor draught fan of the chicken farm is designed.The sound absorption performance of the designed vertical ventilation noise reduction structure is preliminarily verified by MATLAB theoretical analysis and COMSOL numerical simulation.The sound absorption mechanism is explored by using the impedance analysis method and reflection coefficient complex frequency surface distribution diagram method,and the influence law of structural parameters on sound absorption performance is summarized.On this basis,the parallel coupling method is used to widen the sound absorption bandwidth of the structure,finally verified by experimental tests.The research results show that the designed broadband structure can achieve broadband high-efficiency ventilation and sound absorption with an average sound absorption coefficient of 0.81 in the range of 290~750Hz.The maximum transmission loss can reach 15.5d B,which can effectively reduce draught fan noise,and the ventilation area is 48.3%.It has a good application prospect in practical application.(3)Tangential broadband ventilation and noise reduction structure design based on the fan of the chicken farm.To not affect the draught fan’s ventilation performance,two kinds of micro-perforated plate tangential noise reduction covers composed of tangential ventilation noise reduction structures are designed based on acoustic metamaterials and installed around the indoor draught fan of the chicken farm.The two tangential ventilation noise reduction structures are studied by COMSOL numerical simulation.The sound absorption performance is analyzed using the graphical method,and the influence of structural parameters on the sound absorption performance is summarized.Then,the sound absorption bandwidth is widened by employing multiple units in series and parallel,which is verified by experimental tests.Finally,based on the designed two broadband structures,the numerical simulations of the noise reduction effect of the micro-perforated plate tangential noise reduction covers are carried out.The research results show that the two designed noise reduction covers can achieve high-efficiency sound absorption at400~1300Hz.The average sound absorption coefficients are 0.84 and 0.82,and the average transmission losses are 10.64 d B and 7.78 d B,respectively,which meet the requirements of the noise reduction target value of the fan.It has a reasonable prospect of engineering application. |