Study Of Low Frequency Sound Transmission Loss Of Typical Acoustic Metamaterial

In engineering,low frequency noise problem exists in many areas,such as aircraft,transportation vehicles,shipping,and transformer substation.With the characteristics of spreading far distance,high transmission power and insulation difficulty,low frequency noise causes annoyance to people's common life and harmful influence to the people's health.Thus,there is a high demand on the techniques of low frequency noise attenuation in which sound insulation is an effective approach.The sound transmission loss(STL)of traditional materials is dependent on the area density,which is called the mass-law.Increase of area density can lead to the increase of STL as well as the increase of structure mass which is expected.In recent years,the new concept of acoustic metamaterials(AMM)which provides new idea for low frequency sound insulation was developed.The AMM,which is generally considered to be artificial periodic composites containing periodic subwavelength arrays of resonant units,has been shown to exhibit unusual properties that natural materials do not possess such as negative mass density,negative modulus,and double negative characteristics.The characteristic of negative mass density can break the restriction of the mass-law,which enables the design of lightweight structure with effective sound insulation in low frequency.This paper introduces two kinds of most commonly studied AMMs(AMM plate and AMM membrane)in the noise control area.Systemic research is performed on the calculation,analysis and design of the STL of the two AMMs considering different boundary conditions.The main work and conclusions include:1.The calculation of STL for the finite AMM plate with two boundary(simple support and clamped)based on the modal superposition method,and the calculation of STL for the infinite AMM membrane based on the FEM.Both calculations are verified by comparison with other methods,and the calculation for finite AMM plate is also experimentally validated.2.The study on the property of STL of finite AMM plate with two boundary conditions(simple support and clamped).The research show that in both boundary conditions,the STL increases to a convergent value with the increase of distance between resonators;the peak value of STL and the bands around the peaks increase as the added mass increases;the peak value of STL decreases with the increase of the loss factor.3.The analysis and comparison of STL of AMM plate with three boundary conditions(simple support,clamped and infinite).Through the analysis it's found that by varying the azimuth angle of incident wave the convergent STL curves of three boundary conditions are almost the same,which means boundary condition has little influence on the STL of AMM plate in this situation.By varying the elevation angle of incident wave the convergent STL curves from boundary conditions of simple support and infinite agree to each other,but the STL with clamped boundary is sensitive to the elevation angle of incident wave(about 10 dB increase in STL with 15°increase of the elevation angle).4.The investigation on the STL of infinite AMM membrane,with multi cells.The research finds that: the predicted peak value of STL on infinite AMM membrane is larger than that of finite AMM membrane.Through increasing the mass of frame the STL curve of infinite AMM membrane can eventually agrees with that of finite AMM membrane.In conclusion,this paper developed the methods of calculating STL of AMM plate and AMM membrane with different boundary conditions,and systematically analyzed the influences of different boundary,structure parameters,and different incident angles on the STL of AMM.This research provides theoretical guidance and technique support for the design and application of classic AMM in the sound insulation field.