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Dr. Hervé Lissek - Journées d'automne SSA 2006
Active acoustic absorption General presentation - applications
Dr. Hervé Lissek
Laboratoire d’Electromagnétisme et d’Acoustique
EPFL
Dr. Hervé Lissek - Journées d'automne SSA 2006
2/22
Introduction: ANC
“Usual” active noise control (ANC):
aims at interfering a primary sound field (the
unwanted noise) with a synthesized secondary
sound field
Efficient for harmonic sound in 1-D propagation,
even more if stationary
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Dr. Hervé Lissek - Journées d'automne SSA 2006
3/22
Introduction: 3D ANC
Contra-productive for complex 3D non-
stationary noises
Dr. Hervé Lissek - Journées d'automne SSA 2006
4/22
Introduction: Active absorber
Aim of this research:
design a broadband acoustic absorber
Principle:
active impedance control of a resonator
contrôle1contrôle1
surface activesurface active
ZZa1a1
absorption absorption
activeactive
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Dr. Hervé Lissek - Journées d'automne SSA 2006
5/22
Introduction: Active absorber
Aim of this research:
design a broadband acoustic absorber
Principle:
active impedance control of a resonator
Assessed applications:
room acoustics
active noise attenuation (harmonic noise sources)
low frequency control, modal control
Dr. Hervé Lissek - Journées d'automne SSA 2006
6/22
Building acoustics: room acoustics
Reflection coeffiction r function of Zms
Reflection factor R function of Zms
Absorption factor α function of r (R), then of Zms
{
material mechanical properties
1( )ms
losses mass
complian
ms
ce
ms
ms
R MCiv
Z iF
ω ωω
= = + +123
123
144424443
Wi
Wr=RWi
θ
F v
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Dr. Hervé Lissek - Journées d'automne SSA 2006
7/22
Building acoustics: sound insulation
Mechanical impedance of the material:
Sound insulation depends on Zms
{
material properties
1( )ms
losses mass
co
ms m
mplia
s
s
nce
m
R MZ ii
F
v Cω ω
ω= = + +
123123
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110
2
10logW
RW
=
Dr. Hervé Lissek - Journées d'automne SSA 2006
8/22
Principle of passive damping
Acoustic impedance @ interface :
Za(ω)= (Rms+ jωMms+(jωCms)-1)/S (R,M,C system, ω = pulsation)
@ resonance ( ):
Za(ωr)= Rms /S proportional to Zc = ρ.c (characteristic impedance)
param of absorption
S (absorbing area)
Mms (material mass)
Cms (material
suspension)
Rms (losses)
Acoustic field
S
1
2 .s
r
m msM
fCπ
=
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Dr. Hervé Lissek - Journées d'automne SSA 2006
9/22
Principle of passive absorption
@ resonance ( ):
Za(ωr)= Rms /S proportional to Zc = ρ.c (characteristic impedance)
Note: when Za=Zc (impedance matching)
� transparency : the material is totally absorbent
factors of absorption
S (absorbing area)
Mms (material mass)
Cms (material
suspension)
Rms (losses)
Acoustic field
S
1
2 .s
r
m msM
fCπ
=
Dr. Hervé Lissek - Journées d'automne SSA 2006
10/22
Specifications of active acoustic
absorbersAcoustic resonator
� resonance: frequency fr , quality factor Q
The aim of active absorption:
allow the modification of fr and Q, by way of simple (electric) control
� modification of the properties of the absorber:
• make it more absorbent at resonance (acoustic impedance Za to match characteristic impedance Zc of the medium)
• make it absorbent over a wider frequency range
• possibility to shift the resonance frequency
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Dr. Hervé Lissek - Journées d'automne SSA 2006
11/22
behaviour of passive absorption
fr=30 Hz
Za(ωωωω=ωωωωr) ~ ½ Zc factors of absorption
S (absorbing area)
Mms (material mass)
Cms (material
suspension)
Rms (losses)
aim: enhance losses
Dr. Hervé Lissek - Journées d'automne SSA 2006
12/22
Ideal behaviour of active absorption
Za(ωωωω=ωωωωr) = Zc���� TOTAL ABSORPTION
∆∆∆∆f
factors of absorption
S (absorbing area)
Mms (material mass)
Cms (material
suspension)
variable Rms
aim: enhance bandwidth
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Dr. Hervé Lissek - Journées d'automne SSA 2006
13/22
Principle of active absorption
GOAL:
Active control : modification of Rms
Za(ω)= (Rms (controlled)+ jωMms+(jωCms)-1)/S2=Zc
Dr. Hervé Lissek - Journées d'automne SSA 2006
14/22
Principle of active resonance shift
GOAL:
Active control : modification of fr
Za(ω)= (Rms+ jωMms(controlled) +(jωCms(controlled))-1)/S2
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Dr. Hervé Lissek - Journées d'automne SSA 2006
15/22
Application: « active materials »
q
Γq ΓP
“Resonator” = electrodynamic loudspeaker
Dr. Hervé Lissek - Journées d'automne SSA 2006
16/22
Calculations
normalized acoustic admittance (Zc/Za)
as feedback gains increase
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Dr. Hervé Lissek - Journées d'automne SSA 2006
17/22
Calculations
absorption coefficient (α = f°(Za))
as feedback gains increase
Dr. Hervé Lissek - Journées d'automne SSA 2006
18/22
Experimental results
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Dr. Hervé Lissek - Journées d'automne SSA 2006
19/22
Experimental results
measured normalized admittance
Dr. Hervé Lissek - Journées d'automne SSA 2006
20/22
Experimental results
measured absorption coefficients
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Dr. Hervé Lissek - Journées d'automne SSA 2006
21/22
Conclusion
Validation of the concept of active materials:
�calculations promise good performances, even
without specific transducer (usual loudspeaker)
�experimental results show good tendency
�not much expensive in regards with the acoustic
gains:
�cheap actuators (usual loudspeakers)
�cheap sensors (back-electret microphones)
�simple electric control (discrete components)
Dr. Hervé Lissek - Journées d'automne SSA 2006
22/22
PerspectivesOptimization in progress :
� dedicated transducers
� enhanced control
� assess energy transfer
Potential transfer of technology: industries concerned with non-stationary broadband or narrow band noises
�optimize the concept within applied frameworks
� aircraft engines (SNECMA)
� electric industry
� railway industry
� buildings
� etc...