1. Dr. Hervé Lissek - Journées d'automne SSA 2006 Active acoustic absorption General presentation - applications Dr. Hervé Lissek Laboratoire d’Electromagnétisme et d’Acoustique EPFL herve.lissek@epfl.ch

2. 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

3. Dr. Hervé Lissek - Journées d'automne SSA 2006 3/22 Introduction: 3D ANC Contra-productive for complex 3D non- stationary noises

4. 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 contrôle1 surface active Z a1 absorption active

5. 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

6. Dr. Hervé Lissek - Journées d'automne SSA 2006 6/22 Building acoustics: room acoustics Reflection coeffiction r function of Z ms Reflection factor R function of Z ms Absorption factor  function of r (R), then of Z ms WiWi W r =RWi  F v

7. Dr. Hervé Lissek - Journées d'automne SSA 2006 7/22 Building acoustics: sound insulation Mechanical impedance of the material: Sound insulation depends on Z ms

8. Dr. Hervé Lissek - Journées d'automne SSA 2006 8/22 Principle of passive damping Acoustic impedance @ interface : Z a (  )= (R ms + j  M ms +(j  C ms ) -1 )/S(R,M,C system,  = pulsation) @ resonance (): Z a (  r )= R ms /S proportional to Z c = .c (characteristic impedance) param of absorption S (absorbing area) M ms (material mass) C ms (material suspension) R ms (losses) Acoustic field S

9. Dr. Hervé Lissek - Journées d'automne SSA 2006 9/22 Principle of passive absorption @ resonance (): Z a (  r )= R ms /S proportional to Z c = .c (characteristic impedance) Note: when Z a =Z c (impedance matching)  transparency : the material is totally absorbent factors of absorption S (absorbing area) M ms (material mass) C ms (material suspension) R ms (losses) Acoustic field S

10. Dr. Hervé Lissek - Journées d'automne SSA 2006 10/22 Specifications of active acoustic absorbers Acoustic resonator  resonance: frequency f r, quality factor Q The aim of active absorption: allow the modification of f r and Q, by way of simple (electric) control  modification of the properties of the absorber: make it more absorbent at resonance (acoustic impedance Z a to match characteristic impedance Z c of the medium) make it absorbent over a wider frequency range possibility to shift the resonance frequency

11. Dr. Hervé Lissek - Journées d'automne SSA 2006 11/22 behaviour of passive absorption f r =30 Hz Z a (  =  r ) ~ ½ Z c factors of absorption S (absorbing area) M ms (material mass) C ms (material suspension) R ms (losses) aim: enhance losses

12. Dr. Hervé Lissek - Journées d'automne SSA 2006 12/22 Ideal behaviour of active absorption Z a (  =  r ) = Z c  TOTAL ABSORPTION ff factors of absorption S (absorbing area) M ms (material mass) C ms (material suspension) variable R ms aim: enhance bandwidth

13. Dr. Hervé Lissek - Journées d'automne SSA 2006 13/22 Principle of active absorption GOAL: Active control : modification of R ms Z a (  )= (R ms (controlled)+ j  M ms +(j  C ms ) -1 )/S 2 =Z c

14. Dr. Hervé Lissek - Journées d'automne SSA 2006 14/22 Principle of active resonance shift GOAL: Active control : modification of f r Z a (  )= (R ms + j  M ms (controlled) +(j  C ms (controlled)) -1 )/S 2

15. Dr. Hervé Lissek - Journées d'automne SSA 2006 15/22 Application: « active materials » q qq PP “Resonator” = electrodynamic loudspeaker

16. Dr. Hervé Lissek - Journées d'automne SSA 2006 16/22 Calculations normalized acoustic admittance (Z c /Z a ) as feedback gains increase

17. Dr. Hervé Lissek - Journées d'automne SSA 2006 17/22 Calculations absorption coefficient (  f°(Z a )) as feedback gains increase

18. Dr. Hervé Lissek - Journées d'automne SSA 2006 18/22 Experimental results

19. Dr. Hervé Lissek - Journées d'automne SSA 2006 19/22 Experimental results measured normalized admittance

20. Dr. Hervé Lissek - Journées d'automne SSA 2006 20/22 Experimental results measured absorption coefficients

21. 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)

22. Dr. Hervé Lissek - Journées d'automne SSA 2006 22/22 Perspectives Optimization 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...

23. Dr. Hervé Lissek - Journées d'automne SSA 2006 THANK YOU FOR YOUR ATTENTION herve.lissek@epfl.ch