Design And Noise Reduction Characteristics Of Composite Acoustic Metamaterials For Air-Conditioning Air Ducts In Urban Trains

In recent years,Chinese urban rail transit has been developing rapidly,which greatly facilitates people’s travel.However,the problem of train noise is becoming more and more serious.With the continuous improvement of people’s living quality and standards,the noise problem has become an important issue that people are concerned about.Train noise is divided into static noise and running noise according to its running state.Among them,the running noise inside the train is the focus of many scholars’research.As for static noise,its impact is becoming increasingly prominent due to lack of sufficient attention.Even the stationary noise of individual vehicles is close to the running noise（the difference is≤2～3d B）,which must be considered.Air-conditioning duct noise is the main noise source in the train when the train is stationary.To control the stationary noise,it is necessary to suppress the air-conditioning duct noise.In this thesis,aiming at the significant problem of the low-frequency noise in the air-conditioning duct of urban trains,based on the coupled analysis theory of thermal viscous acoustics and solid mechanics,and using COMSOL for analysis and research,a composite acoustic metamaterial that absorbs the noise of air-conditioning ducts is designed.The main contents are as follows:（1）When testing the air-conditioning noise of urban trains,it is found that the energy of air-conditioning noise is mainly concentrated in the one-third octave frequency-band with a center frequency of 100～1000 Hz,with obvious mid-low frequency characteristics.Based on the principle of resonant sound absorption,the reason why Helmholtz resonant sound absorber has low-frequency sound absorption performance is analyzed,which provides a theoretical basis for designing a resonant sound absorption structure for absorbing train air-conditioning noise.Aiming at the problem that low-frequency sound waves and sound-absorbing structures are prone to produce thermoacoustic effects,the coupled analysis method of thermal viscous acoustics and solid mechanics is used to explore the sound absorption characteristics of low-frequency sound-absorbing structures,and the calculation of the coupled analysis method is verified by standing wave tube experiments.（2）For the 100～200 Hz air-conditioning noise in trains with the most significant noise energy,an“exponential”resonant sound-absorbing structure is designed.The“exponential”resonant sound absorbing structure has more tunable variables than the traditional sound absorbing structure.The sound absorption performance of an“exponential”resonant sound absorbing structure is analyzed using the thermal viscous acoustics-solid mechanics coupling interface.A single value evaluation index?_Athat comprehensively considers the structural sound absorption coefficient and sound absorption frequency band is proposed.The adjustable sound absorption frequency band of the“exponential-shaped”resonant sound-absorbing structure was explored for the single variable indexδ,tail radius r₀,opening radius R₁,cavity thickness T and cavity radius R₀,and the result shows that“exponential-shaped”,the adjustable resonant frequency range of the sound absorbing structure is between 89～206Hz.The research results provide a reference for the design of broadband sound-absorbing structures.（3）When studying a single“exponential-shaped”resonant sound-absorbing structure,it is found that the structure has good sound-absorbing properties only near the resonant frequency.In order to further study the broadband sound absorption characteristics of the structure,based on the low-frequency sound absorption characteristics of the“exponential”resonant sound absorption structure,a combined structure of multiple“exponential”resonance sound absorption structures in series and parallel is designed.With the increase of the number of series and parallel in the combined structure,the number of sound-absorption frequency bands also increases,thereby improving the sound-absorption performance of the combined structure.The increase in the number of series can shift the sound absorption band of the composite structure to a lower frequency,but it will correspondingly increase the thickness of the composite structure.The parallel structure can absorb the noise in the 100～200 Hz frequency band without increasing the thickness of the structure.At subwavelength dimensions,both series and parallel combination structures can absorb noise below 200 Hz.But compared with the series structure,the parallel structure has a smaller structure thickness.（4）In order to further realize the absorption of medium and low frequency noise in the range of 100～1000 Hz,the“exponential”resonant sound absorption structure and micro-perforation were designed and studied by using the superior medium and low frequency sound absorption performance of the micro-perforated plate sound absorber.The combined structure of the plate sound absorber in series and parallel,the combined structure has both the sound absorption characteristics of the two structures.The combined structure further widens the sound absorption band,and the parallel structure has a smaller thickness than the series structure.The results of the study provide a reference for the design and research of acoustic metamaterials that absorb 100～1000 Hz air-conditioning noise.（5）Due to the limitation of the inner space of the air-conditioning duct of the urban train,based on the parallel combination structure of the“exponential”resonant sound absorption structure and the micro-perforated plate sound absorber,an“exponential”opening Helmholtz type acoustic metamaterial was designed.Then,the absorption of air duct noise in the frequency range of 100～1000 Hz is realized.After adding acoustic metamaterials in the air-conditioning duct,the transmission loss through area is significantly improved,which further enhances the noise-reduction effect of the air-conditioning duct.With the increase in the number of additional acoustic metamaterials in the air-conditioning duct,the transmission loss of the air-conditioning duct also increases.The research in this thesis can provide a theoretical basis and scientific reference for the control of the low-frequency noise in the air-conditioning duct of rail vehicles.