introduction: anc · active acoustic absorption general presentation-applications dr. hervélissek...

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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

[email protected]


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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3/22

Introduction: 3D ANC

Contra-productive for complex 3D non-

stationary noises


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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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


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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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

144424443

110

2

10logW

RW

=


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

=

5


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

=


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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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


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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13/22

Principle of active absorption

GOAL:

Active control : modification of Rms

Za(ω)= (Rms (controlled)+ jωMms+(jωCms)-1)/S2=Zc


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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15/22

Application: « active materials »

q

Γq ΓP

“Resonator” = electrodynamic loudspeaker


16/22

Calculations

normalized acoustic admittance (Zc/Za)

as feedback gains increase

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17/22

Calculations

absorption coefficient (α = f°(Za))

as feedback gains increase


18/22

Experimental results

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19/22


measured normalized admittance


20/22


measured absorption coefficients

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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)


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...

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THANK YOU FOR YOUR ATTENTION

[email protected]

introduction: anc · active acoustic absorption general presentation-applications dr. hervélissek...

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