Measurement of the sound absorption coefficient in situ

Transcription

1 Measurement of the sound absorption coefficient in situ Dipl.-Ing. Erwin Kuipers Dr.-Ing. Ysbrand Wijnant Prof. Dr.-Ing. André de Boer October 18 th 2012 SGA-SSA Dübendorf

2 CONTENTS Introduction Research purpose The LPW-method Area-averaging Practical examples Conclusions Outlook 2

3 INTRODUCTION All current absorption measurement methods (laboratory or in situ) require a model of the overall sound field to determine the incoming acoustic power This is only possible when: The sound field may be approximated by an analytical solution (typically spherical waves in a semi-free field or plane waves in a tube) Implications a) behavior of the absorbing surface must be known b) behavior of the environment must be known 3

4 RESEARCH OBJECTIVE Purpose Measurements for non-locally reacting surfaces Plate absorbers, diffusers. Sound absorption coefficient QRD diffuser After: Wu, Cox, and Lam (2000) 4

5 LPW-METHOD Idea behind the Local Plane Wave-method: Describe the sound field in front of a surface locally as the sum of two, oppositely directed, plane waves Reflected waves Incident waves Surface 5

6 LPW-METHOD How to determine the amplitudes? PP-probe PU-probe Image sources: B&K and Microflown 6

7 y [mm] AREA-AVERAGING Effective sound absorption coefficient curves for 121 different points on a square area of a perforated panel at normal incidence 300 Measurement grid mm mm x [mm] 7

8 AREA-AVERAGING Ergebnis [-] f [Hz] 8

9 APPLICATION CASE 1 Measurement of the effective area-averaged sound absorption coefficient of a door opening to a small, well-damped room Position sensor Source 9

10 [m] [m/s] y [m] PRACTICAL CASE 1 Scanning the door opening in 60 s Scanning curve x [m] Scanning distance Scanning velocity t [s] t [s] 10

11 PRACTICAL CASE 1 Results in 1/3-octaves (2 source positions, 3 repetitions) 0.9 Surface weighted absorption coefficient Source pos 1 - Meas. 1 Source pos 1 - Meas. 2 Source pos 1 - Meas. 3 Source pos 2 - Meas. 1 Source pos 2 - Meas. 2 Source pos 2 - Meas f [Hz] 11

12 PRACTICAL CASE 1 5 [m] Spatial distribution of the sound absorption coefficient 0 y [m] [m/s] x [m] Scanning distance Scanning velocity Scanning curve overall overall

13 PRACTICAL CASE 2 Measurement of the distribution of the sound absorption coefficient of a car seat a) Overall effective sound absorption coefficient ( Hz) overall overall

14 PRACTICAL CASE 2 b) Effective sound absorption coefficient in 1/3-octaves 14

15 CONCLUSIONS The LPW-method yields accurate results for normal or near-normal incidence Valid for laboratory as well as in situ measurements Opens possibility to measure surfaces of which the sound absorption coefficient can not be determined with conventional methods 15

16 OUTLOOK Extensions of the method to better cope with oblique incidence Application to non-locally-reacting surfaces and practical cases 16

17 Research partially supported by: n H Akustik & Design AG Web: erwin.kuipers.nl@gmail.com CAE Software & Systems GmbH 17

18 REFERENCES [1] Y.H. Wijnant, E.R. Kuipers and A. de Boer, Development and application of a new method for the in-situ measurement of sound absorption, Proc. of ISMA31, Leuven, Belgium, (2010). [2] Y.H. Wijnant, Patent (pending) Werkwijze en inrichting voor het bepalen van de akoestische absorptie- en transmissie-coefficient, in het bijzonder op een gekozen positie in de ruimte. NL , University of Twente, Enschede (2010). [3] E.R. Kuipers, Y.H. Wijnant and A. de Boer, Theory and application of a new method for the in-situ measurement of sound absorption, DAGA 2011, Düsseldorf, Germany (2011). [4] E.R. Kuipers, Y.H. Wijnant and A. de Boer, In situ Messung des Schallabsorptionsgrades mit der LPW-methode: Analyse für schrägen Einfall, DAGA2012, Darmstadt, Germany (2012). [5] E.R.Kuipers, Y.H.Wijnant, and A.de Boer, A numerical study of a method for measuring the effective in situ sound absorption coefficient, J.Acoust.Soc.Am, 132(3), EL236-EL242 (2012). (free download at 18

19 APPENDIX 1 - THEORY Area-averaged sound absorption coefficient: 19

20 APPENDIX 1 - THEORY Active sound power (dissipated in the panel) Incident sound power? 20

21 APPENDIX 1 - THEORY Local Plane Wave assumption (LPW) 2 plane waves in direction 21

22 APPENDIX 1 - THEORY Expressions for sound pressure und particle velocity 22

23 APPENDIX 1 - THEORY Active sound intensity in direction Incident sound intensity in direction 23

24 APPENDIX 2 SIMULATION OF A PARTICULAR CASE Configuration Point source Taken from: E.R.Kuipers, Y.H.Wijnant, and A.de Boer, A numerical study of a method for measuring the effective in situ sound absorption coefficient, J.Acoust.Soc.Am, 132(3), EL236-EL242 (2012). (free download at 24

25 APPENDIX 2 SIMULATION OF A PARTICULAR CASE Incident acoustic intensity 25

26 APPENDIX 2 SIMULATION OF A PARTICULAR CASE Incident acoustic power Exact LPW 26

27 APPENDIX 2 SIMULATION OF A PARTICULAR CASE Effective area-averaged sound absorption coefficient Surface area: 0.25x0.25 m Specific acoustic surface impedance: 27