AcouS STIFF calculation model

There are two types of basic components in AcouS STIFF®:

First basic component : the plate

This element is solid, homogeneous, isotropic, rectangular is shape, its thickness is small compared to its lengths and widths and also before the wavelengths of play, especially in solids. Subsequently, this only works in Plate bending : no shear wave, surface compression...is taken into account.

It also represents a barrier which is supposed to be completely airtight; the speed of a particle of air on the Plate is equal to the speed of the plate at the point. Which distinguishes it fundamentally from the other basic component, the Porous.

Important parameters :

• length, width (m)
• thickness (mm)
• density (kg/m3)
• Young's modulus (in N/m2) : is the stiffness in tension or compression of the material forming the plate. This is the dynamic Module of Young, which may differ slightly from the static module.
• internal loss factor (dimensionless) : combining the magnitude of the intrinsic material losses and those due to mounting. The material database provides such value, which can only be approximate and has been developed through practice.

Second basic component : the porous

It consists of a solid element or an aggregate of solid elements with blank spaces that could be saturated by a fluid. It presents itself as the assembly of two phases :

- a solid phase representing the skeleton or structure of the environment consists of :

• fiber in the case of mineral wool (rock, glass) or of textile fabrics,
• a matrix in the case of foam,
• grains in the case of sand, stacking logs...

- a fluid phase, for example a gas saturating the voids of the skeleton, in this case it will be air.

All porous materials characterized by such behavior in the acoustic software AcouS STIFF® are permeable to air, for example, foams are open pores. Indeed, the acoustic behavior of closed-cell foam (e.g. polystyrene) is of the type Plate. This is the fundamental difference between the two basic components.

Important parameters :

• thickness (mm)
• resistivity to the flow of air (or in Pascal s/m2 or rayls/m): measurable physical parameters, intrinsic characteristic of a porous material. It is to some related to the density which it incorporates among other information. It calculates the coefficient of grain flow (currently informative parameters) from the density of porous material, in order to provide feedback to users accustomed to using this coefficient.
• Young's modulus (in N/m2): comparable to the compressibility of air saturating the structure; isothermal compressibility at low frequencies because there is heat exchange between air and the structure during adiabatic acoustic vribrations, from a certain frequency located sufficiently high in the audible range not to be taken into account.