Manual Surface Impedance
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Transcript of Manual Surface Impedance
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Calibration report PU-Probe Model A Page 1 of 11
icroflown Technologies B.V.
Manual free field surface impedance setup
Author:
Microflown TechnologiesDhr. E. [email protected]
[email protected]: +31 316 581 490F: +31 316 581 491
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Microflown Technologies B.V.
Calibration report PU-Probe Model A Page 3 of 11
1. Bill of materials Surface Impedance software Grid with elastics fastened and spherical loudspeaker mounted. Specified probe (PU mini and/or PU match) with mounting screw(s). Handle Mini tripod to mount under the handle Lemo probe cable Jack loudspeaker cable Signal conditioner & sound card amplifier Cables signal conditioner to sound card amplifier USB sound card cable to computer 2 x 110/230 Volt to 18Volt adaptor 2 x Adaptor cable
2. Setup of the system2.1. Hardware1. Carefully remove the protective grid from the probe. Especially the PU match probe can
be fragile. Mount the probe on the grid. The probe should be orientated as indicated inFigure 1. The blue arrow indicated the measurement direction. The measurement
direction should point to the loudspeaker.2. Then tighten the screw (red arrow, Figure 1).
Figure 1. The sensitive direction is indicated with the blue arrow and should point towards the
loudspeaker. Left: Mounting of the PU mini probe. Right: PU match probe
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Calibration report PU-Probe Model A Page 4 of 11
3. Then put the protective grid on the probe. Make sure the pillars of the grid are not infront of the sensor, see Figure 2. They can obstruct and alter the measurement
Figure 2. The pillars of the protective grid should not be in front of the sensor
4. Connect all electronics
Figure 3. Connections signal conditioner and sound card amplifier
Center ofthe sensor
18V powersupply
18V powersupply
Correction
(set on)
Gain setting(set to high gain)
Power switchsignal conditioner
USB cable tocomputer
Power switchamplifier
Sound card / channel 1
Rear view
Front view
Sound cardamplifier
Signalconditioner
Signalconditioner
Sound card
amplifier
Sound card / channel 2
Loudspeaker volume
BNC to loudspeaker
Particle velocityoutput
Sound pressureoutput
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Calibration report PU-Probe Model A Page 5 of 11
5. Connect the Lemo cable that comes from the signal conditioner to the probe. Thentighten the knob.
Figure 4. Connections backside of the surface impedance setup
6. Switch on the signal conditioner and set it to corrected mode, high gain. Both forimpedance calibration as for the sample measurement.
Figure 5. The signal conditioner should be set on high gain, corrected mode
2.2. Schematic setup of the system
Figure 6. Schematic layout of the setup
Distance sphere-probe (hs h) : Distance from the front of the loudspeaker to the center ofthe probe
Distance probe-sample (h) : Distance from the center of the probe to the material
hs - h
hs
h
Sphericalloudspeaker
Probe
Sample
Right switch up:high gain
Left switch down:corrected mode
Orange led
Jack cablefrom amplifier
Connect the Lemocable to the probe
Plug from probe
Tighten the knob
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Calibration report PU-Probe Model A Page 6 of 11
3. Surface Impedance Program
3.1. Program start up
3.1.1. Sound card settings7. Connect the sound card the computer8. Please make sure the following sound card settings apply at all times (Figure 7)
a. Double click the volume icon or select the sound and audio devicesproperties from the Control Panel
b. For both the Playback and Recording the USB sound card should be selected.c. Playback > With the Speaker slider the speaker volume can be adjustedd. Playback > Mute the Line outpute. Recording / Capture > Select the Line input and position the slider between
the two lower stripes (the precise level is not needed)
Figure 7. Left: Recording sound card properties. Right: Playback properties
Set speakerto maximum
Select the USBAudio sound card
Mute theLine output
Select theLine input
Volumeicon
Select the USBAudio sound card
Recording
Playback
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Calibration report PU-Probe Model A Page 7 of 11
3.1.2. Running the program
9. Open Matlab and select the folder where the program is located10.Run SURFACE IMPEDANCE PROGRAM.M (right mouse button > run)
Figure 8. Matlab opening window. Run SURFACE_IMPEDANCE_PROGRAM.M
3.2. Measurements11.Click the Measure button, see Figure 9. The 2 channel recording window will open.
Figure 9. Main window surface impedance program
Measure
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3.2.1. Settings
12.Under Options >> Settings the measurement parameters can be changed.
Figure 10. Settings window of the 2 channel recording panel
3.2.2. Measure
Figure 11. 2 channel recording measurement panel
13.Select Measurement or Calibration first. The measurement can automatically besaved to a subfolder in the Measurement or Calibration folder. This subfolder will
automatically contain the date.14.Press Start to start measuring.
Measurementtime (seconds)
Select USB audio ifnot selected
Ambienttemperature
Sample rate
Number of fft
points
Save themeasurement
Select Calibrationor Measurement
Transfer functionmagnitude u/p
Auto spectrum:Pressure (ch1)Velocity (ch2)
Transfer functionphase u/p
Coherence
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15.Calibration. The measurements can be performed within a normal room (not anechoic).But please make sure there is not too much background noise during calibration. Thiswill also result in a less high coherence. Make sure there are no drastic peaks in boththe magnitude and the phase of the transfer function like in Figure 11. The phase flips
of course from 180 to -180, but this is okay.
16.Sample measurement. The transfer function deviates from the calibration because ofthe material. Also the coherence is likely to drop at the frequencies where the sample isreflecting (area of high impedance so the velocity signal will be low).
17.Press Save. A window will open. The measurement can be saved to a .mat file in thespecified directory. Avoid names containing an underscore (_), a comma separator(,) or a spacebar ( ).
If for somewhat reason the signals do not appear properly in the 2 channel recording panel,check if the sound card is receiving the signals. In that case please re-initialize the sound cardas described under 3.1.1 Sound card settings
3.3. Process the measurements
3.3.1. Load files18.Load the calibration file
Figure 12. Load calibration / measurements
19.Load the measurement file. All windows will automatically refresh. When a differentmeasurement is loaded it is not necessary to reload the calibration file.
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3.3.2. Screen content
Figure 13. Screenshot of the program with an example measurement loaded
Transfer function of sound pressure (ch. 1) divided by particle velocity (ch. 2)The raw data is smoothed to filter out unwanted reflections. Please look at the data carefully tosee if the smoothed line follows the raw data sufficiently. If this is not the case the number ofaverages can be influenced by changing the value of the averaging coefficient # averagemeasured sample. A value of zero means the data is not smoothed. For most samples this
parameter can have a value of 1 or higher. But some samples have a very drastic behavior
(like the quarter lambda sample). To follow the measured data correctly the averagingcoefficient should be lower.
Upper left graph : The magnitude of the raw data with all reflections and thesmoothed response. Both the calibration measurement as thesample measurement is displayed.
Upper right graph : The phase of the raw data and the smoothed response. Calibrationmeasurement and sample measurement.
Impedance Middle left graph : The magnitude response of impedance divided byc(density of
air and speed of sound) Middle right graph : The phase response of impedance
Change the axisproperties of the selectedgraph and click Apply
Fill in the distancesused in the
Averaging coefficient
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Absorption / Reflection coefficient Lower left graph : The magnitude response of reflection and the absorption
coefficient. Both lines can be selected or unselected with thecheckmarks below the graph
Figure 14. The checkbox should be selected to display the absorption or reflection curve
Lower right graph : Phase of the reflection coefficient
3.3.3. Separate plots
The curves from the main window and some additional curves can be plotted.
Figure 15. The curves can be plotted in a separate window.
3.3.4. Workspace
After loading a calibration and measurement file, automatically the following values will be
exported to the workspace.
Figure 16. Data will automatically be exported to the Matlab workspace
1. Name Measurement file
2. Name Calibration file3. Column 1: Coherence
pressure-speaker
Column 2: Coherence
velocity-speaker4. Matrix with all data
5. Names of the columns ofTotalMatrix
6. All measurement settings
1.2.3.4.
5.6.