V031716 1 | P a g e

Supplementary User Manual for BSWA Impedance Tube Measurement Systems

V031716 2 | P a g e

Contents Software Installation .............................................................................................. 3

Absorption Measurements -- ASTM Method ......................................................... 4

Hardware Set-Up .............................................................................................. 4

Sound card Settings .......................................................................................... 5

Microphone Calibration .................................................................................... 6

Absorption Testing Process ............................................................................... 7

Channel Calibration ..................................................................................... 9

Making a Measurement ............................................................................. 12

Analyzing Data ........................................................................................... 15

Transmission Loss Measurements – ASTM Method ............................................. 17

Hardware Set-Up ............................................................................................ 17

Sound card Settings ........................................................................................ 18

Microphone Calibration .................................................................................. 19

Transmission Loss Testing Process .................................................................. 20

Channel Calibration ................................................................................... 23

Making a Measurement ............................................................................. 28

Analyzing Data ........................................................................................... 34

V031716 3 | P a g e

Software Installation Prior to making any measurement, ensure proper installation of VA-LAB2 software (when using MC3522 sound card) or VA-LAB4 (when using MC3242 sound card).

VA-Lab software is available in the following versions:

VA-LAB2 – makes the use of a 2-channel data acquisition card.

VA-LAB4 – makes the use of a 4-channel data acquisition card. This version is necessary for

impedance tube systems made for Transmission Loss testing. It is not required for Absorption

testing.

License

A license file “valab.lic” will need to be copied into the VA-LAB install folder (Program Files/VA-LABx) in

order for VA-LAB to run.

You may use a temporary valab.lic file if one was provided. Otherwise, follow the instruction below:

After opening VA-LAB, a window will appear displaying a machine code. Email this machine code to your

local distributor of your BSWA Impedance tube system for a permanent license. Clicking on Help>Menu

in the software will show the license information.

Sound Card Driver

For most systems, it is not necessary to install a driver for a 2-channel sound card such as the MC3522. For a 4-channel sound card, such as the MC3242, a National Instruments driver will need to be installed. Make sure VA-LAB software is installed prior to installing the National Instruments driver.

Follow these directions only if you are using MC3242 sound card:

After finishing the installation of the National Instruments driver, connect the MC3242 sound card to the PC via a USB cable. Navigate to Start/All Programs/National Instruments/Measurement & Automation Explorer or NI MAX.

The sound card will appear under “My System>Devices and Interfaces.” Make sure the name of the sound card is “Dev1”as in the image below. Otherwise, right click on the name to rename the device.

The NI hardware that VA-Lab software can support is “NI USB 9233” and “NI USB 9234”.

V031716 4 | P a g e

Absorption Measurements -- ASTM Method This following section is written for BSWA Impedance Tube Systems conforming to the ASTM E 1050

Standard Test Method for Impedance and Absorption of Acoustical Materials using a tube, Two

Microphones and A Digital Frequency Analysis.

Hardware Set-Up Figure 1 shows hardware configurations using an MC3522 two channel card with a built in amplifier.

Figure 2 shows hardware configurations using an MC3242 four channel card along with an external PA50 amplifier.

Figure 1 – Absorption Measurement Set-Up using an MC3522 Sound Card

Figure 2 – Absorption Measurement Set-Up using an MC3242 Sound Card

MC3522 USB AMP

OUTPUT

GAIN INPUTS

USB

1 2

INPUTS

OUTPUTS

MC3242

PA50

1 2 1 2 3 4

USB

Wide microphone spacing on 100mm and 60mm tubes only

Wide microphone spacing on 100mm and 60mm tubes only

V031716 5 | P a g e

The following instructions have been written using the VA-LAB4 software but can also be used for VA-LAB2 software for absorption measurements.

Sound card Settings The following steps show how to select the proper sound card in VA-LAB.

1. After opening VA-LAB4, click on the “Setting” button in the main screen to open the Measurement Setup window.

2. In the Measurement Setup window, select the appropriate soundcard for your system and click OK.

Measurement Window in VA-LAB4 Measurement Window in VA-LAB2

V031716 6 | P a g e

Microphone Calibration The following steps show how to calibrate microphones used for absorption testing.

1. Ensure each microphone is connected to a channel on the data acquisition card using a BNC-to-SMB cable.

2. Click on the “Calibration” button in the main screen of VA-LAB to open the Calibration window. 3. In the calibration window, make sure the Frequency and Amplitude input matches the

Frequency and Amplitude of the calibrator used to calibrate microphones. If you are using BSWA CA115 calibrator, the Amplitude should be set to 114dB and Frequency to 1000Hz.

4. The sensitivity values used are shown in the right frame. Choose the channel to be calibrated in this frame. The corresponding channel will be highlighted with red color. Insert the microphone into the calibrator and wait 10 seconds for the pressure inside to equalize.

5. Turn on the calibrator. (If the index value is equal to the actual value of the output of calibrator, it is not necessary to calibrate this channel again.)

6. Click on Calibrate to calibrate the selected microphone. 7. Repeat steps 4-6 for each microphone. 8. Click Save and Close to exit the Calibration window.

V031716 7 | P a g e

Absorption Testing Process In the main screen of VA-LAB, click on the Transfer Function Method (ASTM) to open the Impedance Tube Test window.

1. In the Impedance Tube Test window, click on Edit to select the Impedance Tube Setting window. 2. Choose Absorption Measurement from the top drop down menu.

Tube Selection

In the TUBE Choose drop down list, select the type of tube to be used for testing. The distance between two microphones, the distance from the sample to the nearest microphone, the tube diameter and effective frequency range are automatically set for each tube in the drop down list.

For each tube, the absorption results will only be calculated for the effective frequency range. To get results across a wider frequency spectrum, other tubes with different inner diameters available from BSWA should be used.

Note: In tube Choose drop down list, the 100mm tube and the 100mm Wide Spacing Large Tube refer to a single 100mm tube system provided by BSWA. The difference between the two tests is the distance between two microphone locations. The Wide Spacing configuration requires that Channel 1 microphone is positioned in the insert closest to the loudspeaker. This position extends the distance between the two microphones which allows for measurements in low frequencies. Figure 3 shows the set-up for a measurement using a Wide-Spacing Tube from the Tube Choose drop down list. This wide-spaced microphone location is only available with the 100mm and 60mm diameter tubes. The microphone locations can be used when doing wide-spacing measurements. Microphone locations not being used during a measurement should be capped off with provided inserts to prevent any noise leakage from the inside of the tube.

V031716 8 | P a g e

Figure 3. Microphone locations used for Wide-Spacing measurements.

Whether you are have chosen a 100mm tube, 100mm Wide Spacing Large Tube or any other tube in the Tube Choose drop down list, note that the reference image in the Impedance Tube Test Setting Window does not change. As mentioned in the last note, when a 100mm or 60mm Wide Spacing Tube is selected, the location of Channel 1’s position requires to be changed closer to the loudspeaker for low frequency testing. In such a case, DO NOT refer to the software window’s image as it does not include the position of this location. Figure 4 shows what the reference image should look like when any 100mm or 60mm diameter tube is chosen from the Tube Choose list.

Figure 4.

Thickness of the sample (m) should be filled with the actual value.

INPUTS

OUTPUTS

MC3242

PA50

1 2 1 2 3 4

V031716 9 | P a g e

Figure 5

ENVIRONMENT: Fill the atmospheric pressure, temperature and relative humidity to calculate the air density, sound velocity and characteristic impedance of air. The default value of atmospheric pressure, temperature and humidity is 101325Pa, 20℃ and 50% respectively.

Once all the parameters in the Impedance Tube Settings windows have been specified, clicking OK will exit the window and go back to Impedance Tube Test window.

Channel Calibration Calibration is to correct the measured transfer function data for mismatch in both the amplitude and phase responses of the measurement channels. It must be done before the test of specimen. Attach the source tube together with a sample holder containing a highly absorptive material to prevent strong acoustic reflections and to obtain the most accurate correction factor possible.

Signal-to-Noise Ratio

1. In the Impedance Tube Test window, click Channel Calibration to enter the interface of calibration.

2. In the Channel Calibration Window, select the Signal-to-Noise Ratio Tab. The Signal-to-Noise Ratio is useful to adjust the output of the sound source in order to generate

Sample

V031716 10 | P a g e

sufficient signal at both microphone locations such that the measured signal in each test frequency band is least 10 dB greater than the background noise. The 10dB difference is the default value in accordance to the ASTM standard but can be adjusted to a value of choice.

3. Insert the microphones into the microphone locations as shown in Figure 6.

Figure 6

If you are using a tube with a Wide Spacing configuration, insert channel 1 microphone to the outer inserts of the tubes, closest to the loudspeaker as shown in Figure 7.

Figure 7

CH. 1 CH. 2

CH. 1 CH. 2

V031716 11 | P a g e

4. Begin the Signal-To-Noise test by clicking Start. During the initial several seconds, the system will not generate sound during its measurement of the background noise. The system will then automatically proceed with generating noise. Adjust the amplifier’s gain or the computer’s volume control until the indicator in the software turns green.

5. Once the indicator is lit green, the signal-to-noise ratio is acceptable. Clicking Stop will stop and complete the Signal-To-Noise test. Proceed to the Channel Calibration tab.

Channel Calibration

1. In the Channel Calibration Window, select the Channel Calibration Tab. 2. If the Auto Stop button is checked, the measurement duration (in seconds) can be defined.

Define a measurement duration of at least 30 seconds. 3. Click on the Step 1 tab. 4. Place the microphones in the locations as shown in the diagram of the Channel Calibration Step

1 tab and click on Start to begin measurement. When the shown curve begins to smooth, click Stop. The calibration data will be saved automatically. Once the measurement is complete, click on the Step 2 tab.

5. Place the microphones in the locations as shown in the diagram of the Channel Calibration Step 2 tab and click on Start to begin measurement again. Once the measurement is finished, the calibration process is complete.

The order of the two steps is random, but it must be ensured that the microphones’ positions are accordance with the picture in chosen tab. If you are using a tube with a Wide Spacing configuration, insert the microphones as shown in Figure 7.

Note: The calibration data is stored automatically and the software will show “calibrated” as long as the calibrated file of current tube exists. If the microphone connecting the test channels are changed the system is off for a long time or a different tube size is selected, the calibration has to be completed again before beginning new measurement.

V031716 12 | P a g e

Making a Measurement • Confirm channel calibration has been completed prior to proceeding with sample

measurements. • Ensure that the microphone in Channel 1 is near the loudspeaker and the microphone in

channel 2 is near the sample. • Any unused microphone port should be capped off. • Remove any absorptive material used for calibrating the channels from the sample holder. • Place the specimen under test inside the holder. The side of the sample facing the

loudspeaker should be flush with the opening of the sample holder. Ensure there is no space between the specimen and the backing plate except when simulating a suspended ceiling tile. The back plate of the holder should be adjusted so it is in contact with the rear surface of the sample. The test sample shall fit snugly in the holder though it should not be compressed excessively nor fitted so tightly that it bulges. It is recommended to fill in the interspaces by using Vaseline or petroleum jelly between the sample and the tube. The test sample can be held firmly, if necessary, by adhesive tape or grease. For example, samples such as carpet material should be firmly attached to the back plate using double-sided adhesive tape to avoid vibration and unwanted air gaps.

1. Press Start measurement to begin the measurement. Wait until the curve begins to smooth out to stop the measurement by clicking the Stop measurement button. Ensure a measurement is made for at least 30 seconds.

2. Click Show Result to check the results in the Current Result tab. The Current Result tab displays various results. The second column shows the measured absorption coefficient values for each frequency. The spectrum type can be selected in the drop down list in right side of the window: The spectrum has the following options: 1/1 OCTAVE, 1/3 OCTAVE, 1/6 OCTAVE, 1/9 OCTAVE, 1/12 OCTAVE, and Original Curve. The graph in the bottom is the curve of the selected column. Selecting a column in the table will change the content of the graph.

V031716 13 | P a g e

The metric represented by each column are shown in the Table 1:

Freq. Hz. α |R| Rr Ri r/¦Ñc x/¦Ñc g¦Ñc b¦Ñc

Frequency Absorption Coefficient

Amplitude of Reflection Coefficinet

Reflection Coefficient Real

Reflection Coefficient Imaginary

Resistance Reactance Conductance Susceptance

Table 1

3. Click Add to List to move the current result into the Result List tab. 4. It is recommended that one sample should be tested at least 3 times to get an average result.

V031716 14 | P a g e

5. Return to the measurement page, reposition the sample and repeat steps 1-3 for additional

measurements as needed.

6. All the measured results will be displayed in the Result List tab. Clicking on the Average button

will calculate the average values for all data in the Result List.

7. Save the average result by choosing the data type in the Export Operation drop down list (if in the Current Result tab, this operation will save the Current Result only) Selecting the Normal Incidence Sound Absorption Coefficient (*.txt) will save the absorption data.

V031716 15 | P a g e

Analyzing Data For a system comprising of a 100mm diameter and a 30mm diameter impedance tube, three different measurements using both of the tubes will be required for full spectrum results. This requires making and saving the original curve of the Sound Absorption Coefficient (*.txt) measurements using a 100mm Wide Spacing Large Tube, 100mm Large Tube and 30mm Tube from the Tube Choose drop down menu (see section on Tube selection)

1. Click on Data Combination button within the main Impedance Tube Test (ASTM) window. 2. Click Load to choose the result needed to combine different measurements. Load the saved

Sound Absorption Coefficient (*.txt) results of each tube measurement into the designated file location. The data which will be combined should match with the title in front of Load button otherwise the combined result will be inaccurate.

3. Click Combine to combine the selected curves. You can click Smooth to reduce the roughness of the curve.

4. The table results may be changed between 1/3, 1/6, 1/9, 1/12 Octave band frequencies from the drop down menu located on the top-right side of the combination window.

5. You may export the combined results using the Export Operation dropdown list located below the results table.

V031716 16 | P a g e

6. Clicking Report will bring up an Impedance Tube Result Report window where various parameters can be defined to be included in the final report.

7. Clicking on the Export in the bottom of the Impedance Tube Result Report window will generate a final report into Excel.

8. Clicking Data Comparison will open an Impedance Tube Test – Data Comparison window for comparing different saved measurement results. The different testing data can be loaded as shown in the image. Up to 10 results can be compared in one time.

V031716 17 | P a g e

Transmission Loss Measurements – ASTM Method The following section is written for BSWA Impedance Tube Systems conforming to the ASTM E 2611-09

Standard Test Method for Measurement of Normal Incidence Sound Transmission of Acoustical

Materials Based on the Transfer Matrix Method. This procedure involves the use of additional hardware

such as the 4-channel MC3242 Data Acquisition card, 4 microphones, PA50 amplifier and extension

tubes.

Hardware Set-Up

Figure 1 shows the hardware setup for Transmission Loss Testing.

INPUTS

OUTPUTS

MC3242

PA50

1 2 1 2 3 4

Wide microphone spacing location on 100mm and 60mm tubes only

V031716 18 | P a g e

Sound card Settings The following steps show how to select the proper sound card in VA-LAB.

1. After opening VA-LAB4, click on the “Setting” button in the main screen to open the Measurement Setup window.

2. In the Measurement Setup window, select the appropriate soundcard for your system and click OK.

V031716 19 | P a g e

Microphone Calibration The following steps show how to calibrate microphones used for Transmission Loss testing.

1. Ensure each microphone is connected to a channel on the data acquisition card using a BNC-to-SMB cable.

2. Click on the “Calibration” button in the main screen of VA-LAB to open the Calibration window. 3. In the calibration window, make sure the Frequency and Amplitude input matches the

Frequency and Amplitude of the calibrator used to calibrate microphones. If you are using BSWA CA115 calibrator, the Amplitude should be set to 114dB and Frequency to 1000Hz.

4. The sensitivity values used are shown in the right frame. Choose the channel to be calibrated in this frame. The corresponding channel will be highlighted with red color. Insert the microphone into calibrator and wait 10 seconds for the pressure inside to equalize.

5. Turn on the calibrator. (If the index value is equal to the actual value of the output of calibrator, it is not necessary to calibrate this channel again.)

6. Click on Calibrate to calibrate the selected microphone. 7. Repeat steps 4-6 for each microphone. 8. Click Save and Close to exit the Calibration window.

V031716 20 | P a g e

Transmission Loss Testing Process

1. In the main screen of VA-LAB, click on the Transfer Function Method (ASTM) to open the Impedance Tube Test window.

2. In the Impedance Tube Test window, click on Edit to select the Impedance Tube Setting window.

3. Choose Transmission Measurement from the top drop down menu.

Tube Selection

In the Tube Choose drop down list, select the type of tube to be used for testing. The microphone spacing, the distance from the sample to the nearest microphone, the tube diameter and effective frequency range are automatically set for each tube in the drop down list.

This information becomes user-definable by selecting ‘User-Defined’ from the Tube Choose drop-down list. This will allow the effective frequency range to be extended outside of the predefined range as shown below.

V031716 21 | P a g e

Note: For each tube, the transmission loss results will only be calculated for the effective frequency range. The working frequency must be in the range of the theoretic range otherwise, data calculated outside the working frequency range will be inaccurate. To get results across a wider frequency spectrum, other tubes with different inner diameters available from BSWA should be used.

In Tube Choose drop down list, the 100mm tube and the 100mm Wide Spacing Large Tube refer to a single 100mm tube system provided by BSWA. The difference between the two tests is the distance between two microphones locations. The Wide Spacing configuration requires that Channel 1 microphone is positioned in the insert closest to the loudspeaker and Channel 4 microphone is positioned in the insert near the extension tube’s back opening. This position extends the distance between the two microphones which allows for measurements for low frequencies. Figure 2 shows the set-up for a measurement using a Wide-Spacing Tube from the Tube Choose drop down list. This wide-spaced microphone location is only available with the 100mm and 60mm diameter tubes. The microphone locations can be used when doing a Wide-Spacing measurements using a 4-channel MC3242 data acquisition card. Microphone locations not being used during a channel calibration and measurement should always be capped off with provided inserts to prevent any noise leakage from the inside of the tube.

Figure 2: Microphone locations used for Wide-Spacing measurements.

Whether you have chosen a 100mm tube, 100mm Wide Spacing Large Tube or any other tube in the Tube Choose drop down list, note that the reference image in the Impedance Tube Test Setting Window does not change. As mentioned above, when a 100mm or 60mm Wide Spacing Tube is selected, the location of Channel 1’s and Channel 4’s microphones position requires to be changed closer for low frequency testing. In such a case, DO NOT refer to the window’s image as it does not include the position of this location which may cause confusion for the user. Figure 3 shows what the reference image should look like when any 100mm or 60mm diameter tube is chosen form the Tube Choose list.

INPUTS

OUTPUTS

MC3242

PA50

1 2 1 2 3 4

V031716 22 | P a g e

Figure 3

Thickness of the sample (m) should be filled with the approximate thickness of the sample.

ENVIRONMENT: Fill the atmospheric pressure, temperature and relative humidity to calculate the air density, sound velocity and characteristic impedance of air. The default value of atmospheric pressure, temperature and humidity is 101325Pa, 20℃ and 50% respectively.

Once all the parameters in the Impedance Tube Settings windows have been specified, clicking OK will exit the window and go back to Impedance Tube Test window.

V031716 23 | P a g e

Channel Calibration Calibration is to correct the measured transfer function data for mismatch in both the amplitude and phase responses of the measurement channels. It must be done before the test of a specimen.

For channel calibration, attach the source tube, sample tube and extension tube as shown in Figure 5. Place a highly absorptive material in the back of the tube. Do not attach the end cover on the extension tube.

Signal-to-Noise Ratio

6. In the Impedance Tube Test window, click Channel Calibration to enter the interface of calibration.

7. In the Channel Calibration Window, select the Signal-to-Noise Ratio Tab.

The Signal-to-Noise Ratio is useful to adjust the output of the sound source in order to generate sufficient signal at both microphone locations such that the measured signal in each test frequency band is least 10 dB greater than the background noise.

Source Tube Sample Tube

Extension Tube

Highly Absorptive Material

V031716 24 | P a g e

8. Insert the microphones into the microphone locations as shown in Figure 4.

Figure 4

If you are using a tube with a Wide Spacing configuration, insert channel 1 and 4 microphones to the outer inserts of the tubes as shown in Figure 5. Ensure any unused ports are capped off.

Figure 5

Ch. 1

Ch. 2

Ch. 3

Ch. 4

Ch. 1

Ch. 2

Ch. 3

Ch. 4

V031716 25 | P a g e

9. Press Start to begin the Signal-to-Noise Ratio test. The initial part of the test will measure the background noise. During this short period, no noise will be generated inside the tube. The loudspeaker will begin to generate noise inside the tube several second later. If the signal to noise ratio in each frequency band of each microphone is at least 10dB, the indicator in the Channel Calibration window will turn green. If the indicator is red, adjust the gain of the PA50 amplifier or the computer’s volume control until it turns green.

10. Once the indicator is lit green, the signal-to-noise ratio is acceptable and clicking Stop will stop finish the test.

Channel Calibration

Since the transfer is a complex ratio of the acoustic pressure responses, any mismatch in the amplitude or phase response of the two microphone systems will affect the accuracy of the transfer function measurement. The following sequence of the measurement and computations provides a means for correcting the measured transfer data in both measurement channels.

1. Click on the Channel Calibration tab of the Channel Calibration window. 2. Remove the end cover of the extension tube and insert cone foam as shown in Figure 6. Do not

attach the end cover back on the tube while the foam is in the tube.

Figure 6

3. A total of 6 transfer functions will need to be calculated to complete the channel calibration. With all of the microphones inserted in their respective locations as shown in Figure 8, follow calibration steps 1 and 2 for each microphone pair beginning with H21, H31 and H41. An image will appear in the window with the proper position of the microphones for each step.

NOTE: The image of the impedance tube in the Channel Calibration window is for illustrative purposes only and does not display the presence of the sample holder; however, the sample holder needs to be in place during the entire channel calibration. See Figure 7

4. The following steps describe the channel calibration procedure in detail:

a. Select the H21 tab and the Step 1 tab. Place the microphones into the tube as shown the image of the window and begin the measurement by clicking on Start. End the

Ch. 1

Ch. 2

Ch. 3

Ch. 4

V031716 26 | P a g e

measurement after 30 seconds. When the ‘Auto Stop’ checkbox is selected, automatic measurement duration can be defined.

b. Switch Channel 1’s microphone position with Channel 2’s microphone positions and repeat the measurement.

c. Select the H31 tab and the Step 1 tab. Place the microphones into the tube as shown the image of the window and begin the measurement by clicking on Start. End the measurement after 30 seconds.

d. Switch Channel 1’s microphone position with Channel 3’s microphone positions and repeat the measurement.

e. Select the H41 tab and the Step 1 tab. Place the microphones into the tube as shown the image of the window and begin the measurement by clicking on Start. End the measurement after 30 seconds.

f. Switch Channel 1’s microphone position with Channel 4’s microphone positions and repeat the measurement.

The transfer function data for each measurement can be saved by selecting the ‘Save transfer function data’ checkbox.

Note: The calibration data is stored automatically and the software will show “calibrated” as long as the calibrated file of current tube exists. If the microphone connecting the test channels are changed, the system is off for a long time or a different tube size is selected, the calibration has to be completed again before beginning new measurement.

V031716 27 | P a g e

Figure 7

Empty sample holder needs to be in place during channel calibration

V031716 28 | P a g e

Making a Measurement • Confirm channel calibration has been completed prior to proceeding with sample

measurements. • Insert the microphones into the microphone holders as shown in the diagram of the Sample Test

window (Figure 8).

Figure 8

• Any unused microphone ports on the tubes should be capped off.

Sample Placement 1. Make sure the sample is inserted inside the sample holder between the source and extension

tube. The sample should be inserted from the sample holder’s top opening and aligned so that the surface of the sample to be tested faces the speaker directly on the closest edge of the top opening as shown in Figure 9 and Figure 10 below. The correct sample placement is important as it will correspond to the correct distance from sample to the nearest microphone, L1, as well as L2, as defined in the impedance tube setting window after selecting a tube from the Tube Choose drop down list.

For The L1 and L2 distances will need to be adjusted if the sample is placed in a different location inside the sample holder. The L1 and L2 distances become user-definable after selecting ‘User-Defined’ from the Tube Choose drop down list (see Tube Selection paragraph of section V. -Transmission Loss Testing Process)

Figure 9

Source Tube Extension Tube

Sample Holder

Sample

V031716 29 | P a g e

Figure 10

2. Make sure the sample holder opening is sealed as shown in Figure 11 below using the provided

hardware.

Figure 11

Source Tube

Sample Holder

Extension Tube

Sample surface

V031716 30 | P a g e

Starting a Measurement

1. From the main Impedance Tube Test window, click on Sample Test. 2. The sample measurement procedure is a two-step process. The two steps are designated by

tabs labeled Step 1 and Step 2. a. Click on Step 1 tab and insert a highly absorptive material inside the end of the

extension tube (foam cone if supplied). Do not attach the end cover on the extension tube while the absorptive material is in place.

b. Click on Start Measurement to begin the test. End the measurement after at least 30 seconds. When the ‘Auto Stop’ checkbox is selected, automatic measurement duration can be defined.

c. Click on Step 2 tab and remove the absorptive material from back of the extension tube. The Step 2 of the measurement procedure can be made with an open or closed termination. It is recommended that an open termination is used during the 2nd Step.

d. Click on Start Measurement to begin the test. End the measurement after at least 30 seconds.

Note: The transfer function data for each measurement can be saved by selecting the ‘Save transfer function data’ checkbox.

V031716 31 | P a g e

3. Click Show Result to check the results in the Current Result tab (Figure 12).

Figure 12 The 3rd column of the Current Result labeled TLn will show the transmission loss results in dB for each frequency. The spectrum type can be selected in the drop down list in right side of the window. The spectrum has the following options: 1/1 OCTAVE, 1/3 OCTAVE, 1/6 OCTAVE, 1/9 OCTAVE, 1/12 OCTAVE, and Original Curve. The graph in the bottom is the curve of the selected column. Selecting a column in the table will change the content of the graph. The metric represented by each column are shown in the Table 1:

Freq. Hz. tr ti TLn Rr Ri A k’r k’i zr zi

Frequency Transmission Coefficient Real

Transmission Coefficient Imaginary

Transmission Loss (dB)

Reflection Coefficient Real

Reflection Coefficient Imaginary

Absorption Coefficient

Propagation Wave Number Real

Propagation Wave Number Imaginary

Characteristic Impedance Real

Characteristic Impedance Imaginary

Table 1

V031716 32 | P a g e

The Export Operation drop down allows the displayed table values to be exported and saved to a .txt file. The data type allowed for export includes:

• Transmission Coefficient (real and imaginary / tr and ti ) • Normal Incidence Transmission Loss (TLn) • Reflection Coefficient (real and imaginary / Rr and Ri) • Absorption Coefficient (A) • Propagation Wave Number in Material (real and imaginary / k’r and k’i) • Characteristic Impedance in Material (real and imaginary / zr and zi) • Table Data – all of table displayed table data

Note: Only the data for the selected spectrum frequencies will be saved.

4. Clicking Add to List moves the current result into the Result List tab. The Result List tab (Figure

13) is useful when multiple measurements are needed to be averaged. The results of the Result List tab can also be exported via the Export Operation drop down menu. Clicking on the Average Result button will calculate the average of all the measurements in the Result List tab. The average results can then be viewed in the Average Result tab.

Figure 13

5. The number of averages can be viewed in the bottom left corner of the screen. Save the average result by choosing the data type in the Export Operation drop down list of the Average Result tab. Selecting the Normal Incidence Transmission Loss (*.txt) will save the transmission loss data only (Figure 14).

V031716 33 | P a g e

Figure 14

6. Once all the measurement data is saved, clicking on Clear List of the Result List tab will clear all the measurements placed into the Result List. This step is important when making a measurement using a new sample.

V031716 34 | P a g e

Analyzing Data For a system comprising of a 100mm diameter and a 30mm diameter impedance tube, three different measurements using both of the tubes will be required for full spectrum results. This requires making and saving the original curve of the Normal Incidence Transmission Loss (*.txt) using a 100mm Wide Spacing Large Tube, 100mm Large Tube and 30mm Tube from the Tube Choose drop down menu (see section on Tube selection)

1. Click on Data Combination button within the main Impedance Tube Test (ASTM) window.

2. Click Load to choose the result needed to combine different measurements. Load the saved Normal Incidence Transmission Loss (*.txt) results of each tube measurement into the designated file location. The data which will be combined should match with the title in front of Load button otherwise the combined result will be inaccurate. (Figure 15)

Figure 15

V031716 35 | P a g e

3. Click Combine to combine the selected curves. You can click Smooth to reduce the roughness of the curve.

4. The table results may be changed between 1/3, 1/6, 1/9, 1/12 Octave band frequencies from the drop down menu located on the top-right side of the combination window.

5. You may export the combined results using the Export Operation dropdown list located below the results table.

6. Clicking on the Export in the bottom of the Impedance Tube Result Report window will generate a final report into MS Excel.

For additional information on the VA-LAB software and the Impedance Tube System, refer to the VA-LAB user manual.