7/27/2019 Manual Surface Impedance

1/11

Calibration report PU-Probe Model A Page 1 of 11

icroflown Technologies B.V.

Manual free field surface impedance setup

Author:

Microflown TechnologiesDhr. E. Tijsinfo@microflown.com

tijs@microflown.comT: +31 316 581 490F: +31 316 581 491

7/27/2019 Manual Surface Impedance

2/11

7/27/2019 Manual Surface Impedance

3/11

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

7/27/2019 Manual Surface Impedance

4/11

Microflown Technologies B.V.

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

7/27/2019 Manual Surface Impedance

5/11

Microflown Technologies B.V.

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

7/27/2019 Manual Surface Impedance

6/11

Microflown Technologies B.V.

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

7/27/2019 Manual Surface Impedance

7/11

Microflown Technologies B.V.

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

7/27/2019 Manual Surface Impedance

8/11

Microflown Technologies B.V.

Calibration report PU-Probe Model A Page 8 of 11

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

7/27/2019 Manual Surface Impedance

9/11

Microflown Technologies B.V.

Calibration report PU-Probe Model A Page 9 of 11

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.

7/27/2019 Manual Surface Impedance

10/11

Microflown Technologies B.V.

Calibration report PU-Probe Model A Page 10 of 11

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

7/27/2019 Manual Surface Impedance

11/11

Microflown Technologies B.V.

Calibration report PU-Probe Model A Page 11 of 11

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.