an active enclosure for the abatement of noise produced by rotating machines l.
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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

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

Andrea Vigliotti

CIRA

overview
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
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

description of the noise source4
Description of the noise source

Rotating machines produce almost periodic noise, having a fundamental frequency and a number of multiples of this frequency

description of noise
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
the acoustic case
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.

system description

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

system description8

a

System description

ideally:

DSP

PA

Hdsp

n1

H2

y3

y2

H1

y1

y4

system description9
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
system description10
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
test set up

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

test set up12
Test set-up

For the testing a dSpace D1104 board has been used

modal characterization
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

preliminary test results
Preliminary test results

Three cases are considered:

  • Purely sinusoidal disturbance
  • Sine sweep
  • Minimization of sinusoidal signal at two different locations
preliminary test results17
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
preliminary test results18
Preliminary test results

Same filter with broad band excitation

preliminary results
Preliminary results

Sine sweep excitation from 100Hz to 500Hz

  • Up to 25 dB attenuation
preliminary results20
Preliminary results

Filter tuned at 334 Hz and 598Hz

preliminary results21
Preliminary results
  • Up to 20 and 10 dB attenuation
preliminary results22
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

preliminary results23
Preliminary results

Noise minimized at two locations

  • Up to 15 and 24 dB attenuation
preliminary results24
Preliminary results

Noise minimized at two locations

open issues and future work
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