© 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
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