Sound Insulation Mechanisms And Design Method Of Multilayer Composite Metastructures

The developments of vehicles such as aircrafts,rockets,aircraft carriers,trains constantly pursue energy-saving,high-speed,heavy-load.Vibration and noise problems are becoming more and more prominent,affecting the comfort,safety and reliability of vehicles.Acoustic quality of vehicle cabin is one of the key performance indicators reflecting the technical level of a vehicle.The problem of noise control for vehicle cabins has attracted extensive attention in industry and academia.Sound insulation is one of the most widely used technologies in noise control of vehicle cabins.The key point to realize sound insulation is to design and use acoustic structures and materials having a high sound transmission loss(STL).However,traditional sound insulation structures and materials are restricted by the mass law,and thus they cannot effectively block noise transmission in the low-frequency range if they are lightweight and thin.As a result,they cannot meet the urgent needs of cabin noise control for the new generation of large vehicles.The design of high performance low-frequency sound insulation structures/materials has become a common key technical problem to be solved urgently in the field of noise control.In recent years,the development of acoustic metamaterials has provided a new idea for the design of low-frequency sound insulation structures/materials.Acoustic metamaterials are a new type of composite materials consisting of periodically specially designed artificial microstructures embedded in a matrix material.They have many extraordinary physical properties.Metastructures are developed by analogy to acoustic metamaterials.They are a kind of new composite structures constructed by mechanical structures with periodically attached or embedded microstructural units.They have supernormal physical properties similar to acoustic metamaterials.It has been found that some specially designed metastructures can break the mass law of sound insulation and achieve high STL at low frequencies.However,under the weight and thickness constrains,existing sound insulation metastructures generally have some shortcomings.For instance,they typically possess only a narrow band of sound insulation in the low-frequency range,and they cannot simultaneously achieve a great sound insulation in the medium-high frequency range,which limit their application in practical engineering.In this thesis,aiming at developing high performance sound insulation structures for vehicles,several types of multilayer composite metastructures are proposed and studied.In order to achieve broadband low-frequency sound insulation performance under the weight and thickness constrains,a systematic investigation of the modelling,analysis and design of the multilayer composite matastructures is carried out.The main work and findings of this thesis are as follows:1.An efficient calculation method for sound insulation characteristics of multilayer composite metastructures is developed.Based on the dynamic equivalent model of a monolayer metastructure and acoustic medium,and combined with the acoustic transfer matrix method,a semi-analytical method for calculating sound insulation characteristics of multilayer composite metastructures under the conditions of normal incident,oblique incident and diffuse field is proposed.Compared with the traditional finite element method,the calculation efficiency is greatly improved.It provides a fast and efficient calculation and analysis tool for the design of multilayer composite metarstructures.2.The low-frequency sound insulation mechanisms and behavior of multilayer composite metastructures are revealed.Three kinds of typical multilayer composite metastructures,which contain block microstructures,strip microstructures or sound absorbing metamaterials,are proposed to overcome the limitation of low-frequency narrow-band sound insulation performance of traditional monolayer metastructures.By analyzing the structure vibration field and sound energy flow at the STL peak and dip frequencies,the low-frequency sound isolation mechanisms of multilayer composite metamaterials are clarified.The regulation rules of structure and material parameters on sound insulation characteristics are systematically analyzed,and the key design parameters affecting the wideband sound insulation performance are obtained.3.The low-frequency and broadband sound insulation design method of multilayer composite metastructures is systematically studied.New design methods are proposed to achieve low frequency and broadband sound insulation by multilayer composite metamaterials.The methods are developed based several ideas including: i)adjusting the key design parameters to achieve optimal STL performance;ii)stacking double metastructure layers with different tuned STL peak/dip frequencies;iii)properly allocating surface mass to multilayer metastructures;iv)integrating acoustic absorption metamaterials.By using the new design methods,lightweight and thin multilayer composite metastructures are designed to realize ultra-broadband low-frequency sound insulation performance.Large size samples of multilayer composite metastructures are fabricated.The performance of low-frequency and broadband sound insulation is verified by measured STL.In summary,this thesis has carried out a systematic and in-depth theoretical and experimental study on the innovative construction of multilayer composite metastructures,the calculation of sound insulation characteristics,the interpretation of sound insulation mechanisms and the design method of low-frequency and broadband sound insulation for the urgent needs of sound insulation structures for vehicle cabin noise reduction.The findings of this thesis provide systematic theoretical and methodological guidance for the design and application of new metastructures with broadband sound insulation capability,and provide new technical supports for solving the problem of cabin noise control of the new generation of vehicles.