1. December 7, 2011 Acoustic Cloaking: Manipulating Sound with Artificial Materials Bogdan Popa, Lucian Zigoneanu, Steven Cummer Electrical and Computer Engineering Department Duke University Bogdan Popa ECE Dept.

2. TTI/Vanguard December 7, 2011 Significant Demand for Acoustic Technology Long range microphones WW2 technology to achieve directivity ! Bad for low frequency sound Bogdan Popa ECE Dept.

3. TTI/Vanguard December 7, 2011 Significant Demand for Acoustic Technology Long range communication systems / sonic weapons Bulky phased array systems Simple sonic lens would significantly reduce complexity but technology isn’t there yet Bogdan Popa ECE Dept.

4. TTI/Vanguard December 7, 2011 Electromagnetic Lenses Known 3000 Years Ago Bogdan Popa ECE Dept. Nimrud lens, 750 BC

5. TTI/Vanguard December 7, 2011 Sound Isolation Better sound shielding materials are needed Bogdan Popa ECE Dept.

6. TTI/Vanguard December 7, 2011 Hiding Ships Ships invisible to sonar Current solution: keep them out of the water as much as possible Bogdan Popa ECE Dept.

7. TTI/Vanguard December 7, 2011 Hiding Ships Bogdan Popa ECE Dept. Making ships undetectable may not be needed Changing their acoustic signature may be much easier

8. TTI/Vanguard December 7, 2011 Problem: Manipulating Sound is Difficult Nature is not very generous with materials suitable for acoustic applications. Example: Aerial acoustics - need easy to machine materials with properties close to those of air. Need to build artificial materials (metamaterials) Bogdan Popa ECE Dept.

9. TTI/Vanguard December 7, 2011 Metamaterials Metamaterials: artificial structures engineered to mimic the behavior of homogeneous fluids. Bogdan Popa ECE Dept.

10. TTI/Vanguard December 7, 2011 Focusing Lens in Air Bogdan Popa ECE Dept. Zigoneanu et al, Phys. Rev. B, 2011

11. TTI/Vanguard December 7, 2011 Fabrication

12. TTI/Vanguard December 7, 2011 Measurements Bogdan Popa ECE Dept.

13. TTI/Vanguard December 7, 2011 Beam-Bending Lens Bogdan Popa ECE Dept. Popa et al, Phys. Rev. B, 2009

14. TTI/Vanguard December 7, 2011 Active (Smart) Materials Acoustic properties can be electronically controlled by controlling unit cell geometry. Endless possibilities: reconfigurable devices, feedback, etc. Bogdan Popa ECE Dept.

15. TTI/Vanguard December 7, 2011 Transformation Acoustics New design technique that allows unprecedented control over sound propagation [Cummer & Schurig, New J. Phys., 2007] Any geometric transformation of acoustic fields can be realized with complex materials. Bogdan Popa ECE Dept.

16. TTI/Vanguard December 7, 2011 Anisotropic Mass Density Complex materials needed by transformation acoustics devices Anisotropy: sound waves see different mass densities depending on the direction of propagation Bogdan Popa ECE Dept.

17. TTI/Vanguard December 7, 2011 Physically Realizable Anisotropic Effective Mass Anisotropic mass is a strange sounding idea but not difficult to create. Bogdan Popa ECE Dept. Layers of fluids behave as one anisotropic fluid. [Schoenberg & Sen, J. Acoust. Soc. Am., 1983] Rotationally asymmetric solid inclusions produce acoustic anisotropy [Popa & Cummer, Phys. Rev. B, 2009].

18. TTI/Vanguard December 7, 2011 Example: Cloak Goal: bump placed in air on reflecting plane undetectable using sound. [Popa et al, Phys. Rev. Lett., 2011] Bogdan Popa ECE Dept. Material parameters given by transformation acoustics (relative to air):

19. TTI/Vanguard December 7, 2011 Expected Behavior Numerical simulation Bogdan Popa ECE Dept. No object Object Cloaked object

20. TTI/Vanguard December 7, 2011 Fabrication Desired density and modulus  unit cell  bulk material Bogdan Popa ECE Dept.

21. TTI/Vanguard December 7, 2011 Measurements Bogdan Popa ECE Dept. No object (reflecting surface only) Uncloaked Object Cloaked object

22. TTI/Vanguard December 7, 2011 Acoustic Signature Bogdan Popa ECE Dept. Numerical Simulations Measurements

23. TTI/Vanguard December 7, 2011 Sound Shielding Thick wall Sound absorber Bogdan Popa ECE Dept.

24. TTI/Vanguard December 7, 2011 Sound Shielding = Easier Cloaking Curve sound inside absorbing material. Bogdan Popa ECE Dept.

25. TTI/Vanguard December 7, 2011 Challenge: Thinner Devices Bogdan Popa ECE Dept. Volume ratio Cloak : Object 1:11:10 Anisotropy factor (ratio of densities) ~5~150

26. TTI/Vanguard December 7, 2011 Extension to Other Types of Waves Bogdan Popa ECE Dept. Electromagnetic waves [Pendry et al, Science, 2006] Acoustic waves [Cummer & Schurig, New J. Phys., 2007] Elastic waves ?

27. TTI/Vanguard December 7, 2011 Controlling Elastic Waves Bogdan Popa ECE Dept.

28. TTI/Vanguard December 7, 2011 References Schoenberg & Sen, Journal of Acoustical Society of America 73, 61 (1983) Pendry et al, Science 312, 1780 (2006) Cummer & Schurig, New Journal of Physics 9, 45 (2007) Popa & Cummer, Physical Review B 80,174303 (2009) Zigoneanu et al, Physical Review B 84, 024305 (2011) Popa et al, Physical Review Letters 106, 253901 (2011) Bogdan Popa ECE Dept.