manual in-situ absorption setup

7/27/2019 Manual in-situ Absorption Setup

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Ca l i b r a t i o n r e p o r t PU - P r o b e M o d e l A P a g e 1 o f 1 6

icroflown Technologies B.V.

Manual free field surface impedance setup

Microflown [email protected]

T: +31 316 581 490F: +31 316 581 491


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M i c r o f l o w n T e c h n o l o g i e s B .V .

M a n u a l I n - s i t u a b s o r p t i o n s e t u p P a g e 2 o f 1 6

Table of contents

1. Bill of materials ..................................................................................... 32. Setup of the system ............................................................................... 3

2.1. Hardware ...................................................................................................... 32.2. Schematic setup of the system ......................................................................... 62.3. Program start up ............................................................................................ 7

2.3.1. Running the program ................................................................................. 72.3.2. Edit the settings ....................................................................................... 82.3.3. Other features .......................................................................................... 82.3.4. Calibrate the DAQ ..................................................................................... 9

2.4. Measurements ............................................................................................. 102.4.1. CALIBRATION MEASUREMENT ................................................................... 102.4.2. SAMPLE MEASUREMENT ........................................................................... 112.5. Process the measurements ............................................................................ 122.5.1. CALIBRATION SMOOTHING ...................................................................... 122.5.2. MEASUREMENT SMOOTHING .................................................................... 142.5.3. MODEL ANALYSIS ................................................................................... 15


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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 Signal conditioner Sound card amplifier Cables

o Lemo probe cableo Jack loudspeaker cableo 2 x BNC cables signal conditioner to sound card amplifiero USB sound card cable to computer

2 x 110/230 Volt to 18Volt adaptor + Adaptor cable

2. Setup of the system

2.1. 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 measurementdirection 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 theloudspeaker. Left: Mounting of the PU mini probe. Right: PU match probe


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Figure 4. Front view schematics

Figure 5. Rear view schematics

Sound card amplifier

Power switch

USB. Connect it to thecomputer

Indication led is on when

the sound card amplifier ispowered

Plug the 18 Volt adaptor here

Signal conditioner

Power switch

Switch gain high/low

Switch correction on/off

Indication led is on when the

signal conditioner is powered


Sound card amplifier

BNC pressure input.Connect it to thepressure input of the

signal conditioner

BNC velocity input.Connect it to thevelocity input of the

signal conditioner

BNC speaker.Connect it to the speaker

Record incidation led.When it flashes thesound card is busy

Turn this knob to varythe speaker volume

Signal conditioner

Connection the Lemo cable

Indication leds: correction on/off

BNC velocity output.Connect it to the velocity inputof the sound card amplifier

BNC pressure output.Connect it to the pressure input

of the sound card amplifier

Overload indicationpressure channel

Overload indicationvelocity channel

Indication led: Low/High gain


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5. Connect the Lemo cable that comes from the signal conditioner to the probe. Thentighten the knob.

Figure 6. Connections backside of the surface impedance setup

2.2. Schematic setup of the system

Figure 7. 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

Jack cablefrom amplifier

Connect the Lemocable to the probe

Plug from probe

Tighten the knob


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2.3. Program start up

Switch on the signal conditioner and set it to corrected mode, high gain. Both forimpedance calibration as for the sample measurement

2.3.1. Running the program

Run the Microflown programm SURFACE IMPEDANCE.EXE, for windows XP.

Figure 8. Main window of the impedance program

The photo monitor allows seeing the picture of the sample during the measurement. To take apicture of the sample press the icon take photo with measurement.

Photomonitor

Levelmonitor

ControlsPanel

Analyzer

window

Take photo withmeasurement


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2.3.2. Edit the settings

Open the setting windows (Fig.9).

Edit

Impedance settingsIn the calibration settings insert the corner frequencies for the correction curve as from thecalibration report. From this window it is possible to change the acquisition settings.

Figure 9. Setting Window of the program

2.3.3. Other features

To import file from the older version of the impedance software use the command as showedin fig. 10. File Import from version


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To manually change the input and output volume of the DAQ (Fig.11):Edit Volume control

Figure 11. Settings for input and output Volume Level

2.3.4. Calibrate the DAQ

Open Edit Calibrate input DAQThe new soundcards models have the button for the calibration. Press this button when

starting the calibration procedure.If the soundcard has not the calibration button connect a know input signal in the BNCpressure and velocity input.

Figure 12. Calibration window and new soundcard

Select file calibration valueEnter the right input value for the calibration of the daq (default 0.096Volt Peak).

Input level Output level

New Sound card amplifier

Power switch

USB. Connect it to thecomputer

Indication led is on whenthe sound card amplifier ispowered


Calibration button


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M a n u a l I n - s i t u a b s o r p t i o n s e t u p P a g e 1 0 o f 1 6

2.4. Measurements

Chose the measurement settings from the main window as in figure 8.

- Excitation options: white, sine, sweep noise.- Measurement time (seconds)- Hardware correction: select this option if the conditioner is set in CORRECTION ON

mode.- High gain: select this option if the conditioner is set in HIGH GAIN mode.- Auto input gain control: automatic adaptive setting of the dynamic range.- Auto accept overload: with this option on the measurement is stored even with

overloaded measurement blocks.

2.4.1. CALIBRATION MEASUREMENT

Select Calibrate and press START.The calibration measurement will stop automatically after the time selected. It is storedin the default path (see 3.1.2). Check the signal Level P and U from the green bar. Seealso if the probe or the DAQ is overloaded (Fig.13).

NO TE: Th e o v e r l o a d d i sp l a y f o r t h e p r o b e p e a k o n l y i s w o r k i n g w h e n t h e D AQ

i s c a l ib r a t e d .

The measurements can be performed within a normal room (not anechoic). Make sure

there is not too much background noise during calibration. This will also result in a lesshigh coherence.

Figure 13. Main window surface impedance program

SignalLevelDisplay

Overload

Display

Excitationoption

Auto input

control

Auto acceptoverload

Hardware and gain correction

Measurementtime

Record Button (Start)


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2.4.2. SAMPLE MEASUREMENT

Select Me a s u r e from the Controls panel.

Position the probe close to the sample and press START.The transfer function deviates from the calibration because of the material. Also thecoherence is likely to drop at the frequencies where the sample is reflecting (area ofhigh impedance so the velocity signal will be low). If the signals do not appear properlyin the 2 channel recording panel, check if the sound card is receiving the signals.

Note:

- Display different functions in the graph plot:All the curve displayed in each graph can be changed by selection. When Righ-clickon thediagram the cascade window show the different plotting options:


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- Change format Y and Y axes:A single clickon the X axis switch from lin to log axis plotting. Double clickto change thedynamic range.The option icon Lin and Log changes the Y axis plot (Fig.14).A double click on the Y axis to change the dynamic range.

Figure 14. Main window surface impedance program

- The icon Add data cursor can be used to read the value of the curve in the graph.- The icon take a photo enable taking the picture of the sample before starting the

measurement. The RECORD BUTTON needs to be pressed.

2.5. Process the measurements

2.5.1. CALIBRATION SMOOTHING

Load the calibration file using the button Cal.

Figure 15. Load calibration and apply smoothing

Load file

Smoothing

options

Option for Y axis plotAdd datacursor

Take a photo


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Smoothing OptionsIt is possible to apply different kind of smoothing to cancel reflection.The data is smoothed to filter out unwanted reflections. It is also possible to apply nosmoothing.

Reflections from surfaces other than the sample can affect in situ measurements. Forcalibration the free field response of the sensors is required. During the sample measurement

only the early reflected sound from sample itself should be measured and not the influencefrom walls, floors, ceilings, etc.

Two techniques are used in the software to approach the anechoic response: moving averagein the frequency domain and impulse windowing.

Moving average in the frequency domainThis is a far more simple routine and it produces similar result as the time window technique.

Because the room reflections have a random character, i.e. the reflections are from all possibledirections with all possible phase shifts, the deviation from the anechoic calibration is alsorandom. As long as the deviations are random (and thus the reflections are random), themoving average method will work.

The moving average principle is that a value at a certain frequency point is the average valueof a certain amount of frequency points. The choice for the amount of frequency pointsdepends on the problem and the frequency range. If the influence of parasitic reflections canbe considered strong (e.g. there are high reflective panels near the setup) a wider frequencyband should be used. Two options can be used in the software: l in e a r a n d l o g a r i t h m i c

scaling of the frequency bands.

Depending on the shape of the impedance curve any of the methods can be used. Linearscaling means that the value at every point is mean value of a certain amount of frequencypoints. Logarithmic scaling means that at lower frequencies less fft point are used forcalculation, while at higher frequencies the smoothed values are calculated from a widerfrequency range.

If the impedance is not strongly changing (e.g. the calibration measurement or typical foamsamples) moving average scaling can be applied. The moving average routine should not be

used for samples with strongly varying impedance (e.g. Helmontz of lambda resonatorsamples)

Impulse windowing

The method is based on calculating the impulse response of the measured transfer functionZ=p/u. The room reflections are later in time than the primary response and can be removedmathematically by setting the impulse response to zero by a window. Then the windowedimpulse response is transferred back to the frequency domain. This method works well forhigher frequencies. At lower frequencies the window should be longer and early parasiticreflections can be included. This time window method works well but requires setting thewindow limits.


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The Impulse transfer function method allow to manually filter in the time domain thereflections (Fig.16). This is somewhat time consuming and the routine depends on theoperator.

Figure 16. Impulse response Smoothing

2.5.2. MEASUREMENT SMOOTHING

Load the measurement file using the button Me.

Figure 17. Load measurements and apply smoothing

Follow the same procedure for smoothing as shown in 3.3.1.

Time filtering P

Time filtering U

Load file

Smoothingoptions


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2.5.3. MODEL ANALYSIS

Load the file of the impedance and apply a model for calculation of absorption.

Figure 18. Load measurements and apply smoothing

Select the appropriate Calculation model for absorption.

Impedance model:- Plane wave:

The simplest possible model assumes that the material under test is exposed to a planewave of normal incidence which gives rise to a reflected plane wave. The normalizedspecific impedance at a position at a distance ofh from the material flows from simpleplane wave is given by:

- Mirror source:set the distance sourceprobe and probe-sample.A slightly more complicated model combines the concept of an image source with the

plane wave reflection factor. The plane-wave reflection factor depends only on theimpedance of the material.

- Q term:set the distance sourceprobe and probe-sample.The third approach takes account of the fact that the reflection involves the spherical

reflection coefficient Q unless the source is unrealistically far from the surface.For normal incidence Laplace transform formulation for the calculation ofQ becomes:

Load file Save file

Fileloaded

Impedancemodel

Export file (EXCEL-ASCII)


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It is apparent that the spherical reflection factor Q depends on the unknown impedanceof the material Zin a much more complicated manner than the plane wave reflectionfactor R does; and it can be seen that it also depends on h and hs.The impedance is given by:

It is possible to save and load the impedance measurement files. Different curves in the same

graph can be plotted. Use the save and load Icon as in figure 18.

The export option (in ascii or excel) in available by using the export icons (in fig.18). All themeasurements loaded and displayed in the graph are exported.

manual in-situ absorption setup

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