NVH – What are porous materials and how are they related to automotive applications?

I wanted to do some research so I can better explain what porous materials are and their relevancy to the automotive industry. Luckily, I found this gem of a paper by Hörlin.

What is a Porous Material?

1.2 Modeling of porous materials
A porous material consists of a porous frame structure with an interstitial fluid, e.g. air, filling the voids (the pores). In this context only the fluid in the pores which are communicating are considered as the fluid. (Fluid which are captured in closed cells is considered as part of the frame since it is prohibited to move relative the same.) The main vibro-acoustic features of porous materials are that the frame in a macroscopic sense is often considerably more flexible and less mass dense than the material it is made of and that the fluid in the pores may move relative to the frame, hence interacting dynamically through viscous and inertial couplings. Both the  fluid constituent and the frame constituent are compressible and hence dilatational interactions also occur, even if they in most cases of interest in vibro-acoustics are small. The frame may in acoustic applications be considered as linearly elastic, for small deformations, having internal damping. Hence, as in the case of elastic solid materials, the frame stiffness is in general considered as complex-valued and frequency dependent.

A widely used theory, in linear acoustics, to describe the macroscopical mechanical is known as the Biot theory. [Hierarchical Finite element modeling of Biot's equations for vibro-acoustic modeling of layered poroelastic media]

To compliment Hörlin's description above, I've included a few images of porous materials and trim components.

Microscopic view of porous material. [Source]

 

Trim component with an Acoustic Foam / Porous material [Source]

 

Trim components in a vehicle. [Source]

 

The Relevancy of Porous Materials in the Automotive Industry

1.1 Introduction
As the quest for reduced weight, at equal level of noise, vibration and harshness (NVH) comfort, continues within the vehicle engineering industry, the traditional design and material choices for both vehicle structures and acoustic interior trim components are being revisited. The trend towards light and stiff multi-functional, multi-layer structures creates significant acoustic challenges. Efficient and cost-effective design of these new structures may only be achieved through a strong understanding of the relationships between material properties, combination of materials and their resulting application performance. Clearly then, unified and concurrent engineering approaches, together with a relevant representation of the important physical aspects of multi-layer materials in computer aided engineering (CAE) tools are needed to ensure efficiency and compatibility between different design criteria.

For a private car, for instance, one of the acoustic challenges is to design weight optimal car body multilayer panel structures which reduces the transmission of sound from various noise sources e.g. engine, tyres and aerodynamically induced noise. This thesis is intended as a contribution to a methodology having the potential of meeting this challenge. It is mainly concerned with development of numerical methods to describe vibro-acoustic wave propagation through multiple layered material which includes poroelastic materials. The focus is on hierarchical finite element modeling of weak forms of Biot's equations, forming the basis for needs in new CAE tools. [Hierarchical Finite element modeling of Biot's equations for vibro-acoustic modeling of layered poroelastic media]

How do you analyze poroelastic materials?

According to Mecanum Inc.(source), there are 4 methods. I've ranked them in order of decreasing accuracy:

1. Poroelastic Models (Biot) via FEA codes such as Actran and MSC Nastran
2. Empirical Models
3. Admittance Models
4. Mechanical Models via general FEA codes

Want to expand your knowledge?

• Watch Noise Attenuation: Model Poro-elastic Materials 
• Basics of Acoustical Materials
• Prediction of NVH behaviour of trimmed body components in the frequency range 100–500 Hz
• The NVH CAE Methods Toolbox for sound package development: a quick overview