Design And Experimental Research Of Subwavelength Acoustic Absorbers

With the rapid development of air transport industry and the improvement of people’s living standards,the requirements of airworthiness standards for civil aircraft noise are getting higher and higher,which means that new requirements are put forward for noise control technology.It means that new noise control technologies are required.Aero-engine noise is one of the main noise sources of aircraft.The aircraft noise can be reduced significantly,if the aero-engine noise is effectively reduced.On the other hand,in order to improve the comfort of passengers in the cabin of civil aircraft,sound insulation and sound absorption materials are needed to reduce the noise level in the cabin.In order to further reduce aero-engine noise and cabin noise and adapt to the new airworthiness standard of aviation noise,this thesis puts forward a new noise control method from three aspects: ventilated acoustic absorbers,sub-wavelength sound absorption material and tunable acoustic absorbers.The numerical and experimental sound absorption and insulation performances of the proposed absorbers are presented in this thesis.Firstly,a ventilated spiral waveguide is designed as an acoustic absorber based on HQ(Herschel-Quincke)tube theory.The noise reduction frequency band of the designed muffler model is predicted by using the interference theory model,and the theoretical model is modified according to the experimental results.Finally,the solid model is manufactured by 3D modeling and 3D printing technology.The experimental results show that the noise reduction of the target frequency band can be realized effectively.Secondly,a theory for the design of the half-wavelength tube is proposed,which is simpler and has better low-frequency acoustic performance than HQ tube.The effects of half-wavelength tube diameter,tube length,number and branch distance on the whole half-wavelength tube system are analyzed by numerical calculation method.Based on the theoretical model,a wide-band half-wavelength tube system is designed to achieve a wide-band noise suppression effect of no less than 20 d B in the 350-1350 Hz band.Considering the viscous heat loss,the half-wavelength tube theory is further optimized,and the optimized theory is more in line with the reality.In order to verify the halfwavelength tube theory,a standard impedance tube experimental platform is designed according to the international standard(ISO 10534-2: 1998).The results show that the theory is in good agreement with the experiment.In this thesis,taking the half-wave tube as the basic unit,the sub-wavelength sound absorption structure is designed,and based on this sound absorption structure,a full ventilation noise reduction solution is proposed,and its noise suppression performance is explored by numerical calculation and experimental methods.The results show that both the designed sound absorption structure and the fully ventilated muffler have good noise reduction effect in 330-460 Hz.Finally,in order to deal with the complex noise environment,this paper further designs two kinds of frequency adjustable mufflers: variable opening area HQ tube and adjustable quality sound absorber.The HQ tube with variable opening area is designed to adjust the natural frequency of the muffler by adjusting the opening area of the HQ tube.The relationship between opening area and acoustic impedance is deduced,and the sound transmission losses of different cross sections are calculated by acoustic simulation software,and verified by experiments.The results show that the frequency modulation and noise reduction of 187-296 Hz can be realized.Another kind of frequency adjustable muffler is the adjustable sound quality absorber.Based on the folded half-wavelength tube as the back cavity and the rotor with different aperture,the sound quality adjustable absorber is designed.The experimental results show that the frequency modulation bandwidth of the designed adjustable sound quality muffler can reach more than 137 Hz,and the sound absorption coefficient in the tunable frequency band is more than 0.75.the experimental results show that the frequency modulation bandwidth of the designed adjustable sound quality muffler is more than 0.75.the experimental results show that the frequency modulation bandwidth of the designed adjustable sound quality muffler is more than 0.75.The experimental results show that the frequency modulation bandwidth of the designed adjustable sound quality muffler is more than 0.75.The experimental results show that the frequency modulation bandwidth of the designed adjustable sound quality muffler is more than0.75.The experimental results show that the frequency modulation bandwidth of the designed tunable sound quality muffler is more than 0.75 and the sound absorption coefficient is more than 0.75 in the tunable frequency band.The proposed subwavelength sound absorbing material design,ventilated muffler and tunable acoustic absorber provide a new reference for aircraft noise control and muffler design.