On The Modeling Of Sound Transmission Properties Of Sound Insulation Structures With Apertures Or Slits

Predicting sound transmission properties (STP) of sound insulation structures (SIS) is an important part of low noise design of a product and is also a basis for noise control engineering. The existence of apertures or slits (hereinafter referred to as apertures) usually couldn’t be avoided due to structure, manufacturing process and sealing effect, such as the split between the door and the body of a vehicle, the apertures in the sound insulation wall of vehicle engine, the acoustic liner of an aero engine inlet etc. Studying STP of those structures, determining the relationship between STP and dimensions of apertures, deriving the formulas for STP prediction will be of great importance in those areas.The dissertation is based on the national defense973subproject and focuses on the study of apertures in SIS. The prediction of STP of many kinds of apertures in normal incidence, oblique incidence and diffuse acoustic field is well studied. Formulas for determining sound properties of complex apertures, large size rectangular and circular apertures in full frequency band are derived. And on these basis, the approach for predicting STP of apertures with mean flow is also proposed. The main research work and innovative results of this dissertation are as follows:1) On the modeling of STP of complex apertures. A general method for calculating sound transmission coefficient (STC) and sound transmission loss (STL) is proposed and formulas for this method are also derived. Formulas are validated through comparing the calculation with typical examples, such as straight apertures, conical apertures, apertures with abrupt construction etc. The impact of aperture dimensions on STL is analyzed. The study provides an analytical calculation method for SIS with complex apertures. The application scope is also extended by this approach.2) On the modeling of STP of large size apertures in full frequency band. Calculation frequency usually above plane wave cutoff because of large aperture dimensions and formulas based on plane wave theory will no longer applicable. A full frequency band calculation method for determining STC and STL for large size rectangular and circular apertures is proposed. The method also gives a significant speed advantage. Formulas are validated by comparing the calculation results with existing analytical methods and acoustic FEM. The impact of dimensions of apertures on STL is analyzed. The veracity and superiority of this method are validated though engineering project. This method provides a basis for SIS design.3) On the calculation of radiation impedance at the orifice of apertures in SIS. STP is closely related to the radiation impedance at the orifice of an aperture. Radiation impedance lies on the surface vibration modal of radiator. A method for calculating radiation impedance which considering vibration modal of radiator is proposed. Four integrals is down to double integrals through variable substitution and singular value problem is solved by polar transformation. Then the complexity is reduced. The method provides significant support for STP calculation of aperture in SIS.4) On the application study of STP calculation of SIS with apertures considering the structural sound transmission (SST). The leaks of aperture is calculated by the methods proposed in this dissertation and the structural sound transmission coefficient (SSTC) is determined by existing method in literature. Total STC of SIS with a rectangular aperture is calculated by patch weighted sound transmission coefficient method (PWSTCM). This study provides an approach for calculating STC for SIS with aperture and also provides a quantitative indicator for study the impact of the presence of an aperture in SIS. The study is validated by acoustic finite element method (FEM).5) On the modeling STP of apertures with mean flow. Formulas for calculating STP of apertures with mean flow is derived based on the formulas derived in this dissertation and mean flow wave equation. The formulas are validated through comparing the results with acoustic FEM. The impact of mean flow is studied and the study provides a reference for aero engine acoustic liner design.