chapter 14 acoustic materials jean-louis...

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 1

Chapter 14Acoustic materialsJean-Louis Migeot

1. Acoustic materials

2. Sandwich panels

3. Biot theory

4. Some measurement techniques

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 2

Chapter 14Acoustic materialsJean-Louis Migeot

1. Acoustic materials

2. Sandwich panels

3. Biot theory

4. Some measurement techniques

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 3

Path: Structure-borne Noise & Air-borne noise

Ratio of structure-borne noise to air-borne noise of a typical car

( Source: SAE International )

Air-borne Structure-borne

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 4

Ubiquity of acoustic trim

© Autoneum

Damping

Absorption

Insulation

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 5

Absorber, Insulator & Damper

➢ Absorber:

typically porous material

performance increases with material thickness

poor insulator

➢ Insulator

impervious (non-porous) material

attenuation increases with mass of the material

poor absorber

➢ Damper

visco-elastic material characterized by a complex Young’s modulus (loss factor)

example: metal-polymer-metal sandwich, butyl, elastomer, etc.

no direct acoustic effects

➢ The three behaviors can be combined in composite sandwich panels to provide a good mix of absorption, insulation and damping

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 6

Foam and felts

Viscous dissipation by friction on the skeleton

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 7

Absorption: limits of open porous layers

➢ f = 100 Hzl/4 = 0.85 m

➢ f= 1,000 Hzl = 0.09 m

➢ f=5,000 Hzl = 0.02 m

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 8

Sound absorption in a car

( Source: SAE International )

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 9

Damping: constrained vs. unconstrained layer

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 10

Damping and absorption: complementary effects

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 11

Damping and absorption: complementary effects

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 12

Chapter 14Acoustic materialsJean-Louis Migeot

1. Acoustic materials

2. Sandwich panels

3. Biot theory

4. Some measurement techniques

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 13

Sandwich panels

Heavy Layer

Porous Layer

Damping Layer

Frame

Metal Layer

© Autoneum

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 14

Vibration Insulation

➢ The multi-layer acts as a double mass-spring system where the mass is made of the sheet metal and heavy layer and the spring stiffness is determined by the skeleton stiffness, the resistivity of the porous material and the compressibility of the air inside the pores

➢ Below the cut-offfrequency, vibrations are amplified

➢ Above this frequencyvibrations are reduced

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 15

Resonance Shift

Mass Effect Stiffness Effect Compressibility

Heavy Layer (r)

Porous Layer (E and Q)

Metal Layer

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 16

Differential stiffness and damping

➢ Bending wave celerity in the top and bottom layer are much different …

➢ … and damping in the heavy layer and the porous material further reduces the vibration amplitudes on the treated side.

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 17

Impact on radiation efficiency

➢ The noise radiation by the panel without treatment is much higher than this of the treated panel because of the reduced vibration level, the increased vibration damping and the lower radiation efficiciency of the heavy layer due to the shorter wavelength of bending waves

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 18

Flow resistance

+ + + + +

- - - -

- - - - -

+ + + +

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 19

Energy dissipation

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 20

Surface impedance

➢ The surface impedance of a composite depends on all its internal parameters and its structure can be optimized to meet specific requirements.

Effect of screen resistivity

Screen

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 21

Chapter 14Acoustic materialsJean-Louis Migeot

1. Acoustic materials

2. Sandwich panels

3. Biot theory

4. Some measurement techniques

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 22

Maurice Anthony Biot (1905-1985)

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 23

Poro-Elastic Model

➢ A porous material contains two phases (skeleton + air in the pores). Biot’s theory describes the interactions between the 2 phases.

➢ Hypothesis:

l is much greater than the details of the porous material

displacements are small (linear elasticity)

continuous air phase (closed pores are part of the skeleton)

elastic skeleton

no viscous effects linked to fluid trapped in closed pores

no thermo-mechanical coupling effects involved

➢ The theory sets up

an overall equilibrium equation for the mixture

the generalized Darcy law for the fluid phase

➢ Constitutive relations are written for the mixture (total stresses) and for the fluid (pressure)

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 24

Variables

➢ The following variables define the state of the system

p pressure in the fluid phase

u displacement of the frame

U displacement of the fluid

U-u relative displacement of fluid with respect to frame

➢ And the following measurable material properties are used

rs density of the frame

rF density of the fluid

W porosity

R Resistivity

a Tortuosity

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 26

Porosity W

A small volume change is imposed. The porosity can be deduced from the pressure increase.

Autoneum’s Porpos Porosity

Measurement System

W = =+

= -V

V

V

V V

V

V

f

t

f

s f

s

t

1

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 28

Static Resistivity R

➢ The pressure drop between both sides of the sample directly gives the resistivity

Sample thickness h

Pressure drop Dpv

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 29

Resistivity

Autoneum’s AFR Airflow

Resistance Measurement System

Autoneum’s CARE+ portable airflow

resistance measurement system

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 30

Tortuosity a

➢ At high frequency, the sound speed is only determined by the tortuosity

Picture courtesy of Walter Lauriks - KU Leuven

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 31

Tortuosity a

➢ The difference of electric resistivity of a conducting fluid and the sample saturated with that fluid provides a measurement of the tortuosity

Picture courtesy of Walter Lauriks - KU Leuven

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 33

Dynamic Moduli

➢ The transfer function between the displacement at the top and bottom of the sample give E and G from which n is found

Picture courtesy of Walter Lauriks - KU Leuven

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 34

Biot a Parameter

➢ Measurement on a sample in a Kundt tubes allows to identify the adequate value of the acoupling coefficient

Autoneum’s Elwis

Measurement System

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 42

Key Takeaways

➢ Three functions:

Damping

Absorption

Insulation

➢ Absorption: limits of open layers

➢ Damping: constrained vs. unconstrained layers

➢ Sandwich panels

➢ Biot theory and Biot parameters

➢ Some test set-up: RTC3, alpha-cabin, …

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 43

Chapter 14Acoustic materialsJean-Louis Migeot

1. Acoustic materials

2. Sandwich panels

3. Biot theory

4. Some measurement techniques

© Jean-Louis Migeot – MSC Software – Free Field Technologies – Université Libre de Bruxelles – Conservatoire Royal de Musique de Liège – IJK Numerics 44

Airborne vs. structure-borne transmission