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An active enclosure for the abatement of noise produced by rotating machines

An active enclosure for the abatement of noise produced by rotating machines Andrea Vigliotti CIRA Overview Description of the noise source The active case System description Test set-up Preliminary results Open issues and future work Description of the noise source

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An active enclosure for the abatement of noise produced by rotating machines

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  1. An active enclosure for the abatement of noise produced by rotating machines Andrea Vigliotti CIRA

  2. Overview • Description of the noise source • The active case • System description • Test set-up • Preliminary results • Open issues and future work

  3. Description of the noise source Rotating machines, such as electrical motors, pumps, turbines, compressors, are found in a large number of systems and represent often a major source of noise and disturbance. The abatement of such sources can be obtained by the design of specific enclosure but these solutions are often very heavy or unacceptably heavy. Because of the nature of the noise field produced by rotary machines good results in terms of noise reduction and lightness can be obtained by the use of active noise control techniques

  4. Description of the noise source Rotating machines produce almost periodic noise, having a fundamental frequency and a number of multiples of this frequency

  5. Description of noise Main characteristic are: • locally narrow bandwidth • a discrete number of frequencies are involved and can be separately treated • predictable • rotary machines often operates at fixed regimes

  6. The acoustic case • A light case with active walls can be designed to mitigate the disturbance produced by the rotating unit The advantages offered in this case is that it is possible to install the transducer on the same surface which is radiating the noise. Thus it is possible to act on the source itself.

  7. n2 n1 n3 System description A feed-forward control system • Noise can be fed to a digital signal processor and correlated to an accelerometer measurement taken on the unit Advantage is that it is possible to sense accurately the disturbing signal and design a purely feed forward control system. DSP PA

  8. a System description ideally: DSP PA Hdsp n1 H2 y3 y2 H1 y1 y4

  9. System description Known issues for the general case • H1 and H2 are difficult to obtain in an analytical form • Hdsp migth not be a casual system In the present application • H1, H2, Hdsp need only to be evaluated at the frequencies of interest • being the noise periodic we can predict the future

  10. System description • Each frequency component of the disturbing signal can be individually filtered and sent to the actuator and the enclosure wall, and each enclosure panel can be driven individually

  11. n2 n1 Test set-up Waveform generataor DSP PA • dSpace 1104 DSP board • Agilent 3230 Waveform generator • Nat. Inst. 6535 Daq card • Laptop and desktop pcs • Microphones • Piezoelectric patches • Power amplifiers • Matlab /simulink Data acquisition and processing

  12. Test set-up For the testing a dSpace D1104 board has been used

  13. Test set-up

  14. Test set-up

  15. Modal characterization H1 The frequency response functions have been experimentally evaluated between the loudspeaker and the microphones and between the actuators and the microphones H2

  16. Preliminary test results Three cases are considered: • Purely sinusoidal disturbance • Sine sweep • Minimization of sinusoidal signal at two different locations

  17. Preliminary test results Sinusoidal disturbance Freq: 334 Hz Attenuation: ~20 dB • in sinusoidal excitation very good results because of high radiating efficiency of the panels • things get worst in large bandwidth excitation because of the uncontrolled behaviour around the resonance

  18. Preliminary test results Same filter with broad band excitation

  19. Preliminary results Sine sweep excitation from 100Hz to 500Hz • Up to 25 dB attenuation

  20. Preliminary results Filter tuned at 334 Hz and 598Hz

  21. Preliminary results • Up to 20 and 10 dB attenuation

  22. Preliminary results Exact solution might not exist Optimum has to be searched and trade off need to be taken DSP PA Hdsp DSP PA n1 Hdsp n1 y3 y2 a y1 y4

  23. Preliminary results Noise minimized at two locations • Up to 15 and 24 dB attenuation

  24. Preliminary results Noise minimized at two locations

  25. Open issues and future work • Poor panels efficiency as actuator away from its natural frequencies • Noise can easily be minimized at single points difficulties arises when dealing with large volumes • Phase of disturbing signal critical • Stiffer and lighter composite box

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