Simcenter Testing Solutions Measuring Sound Absorption with an Impedance Tube

Direct YouTube link: https://youtu.be/9VTC2LIOir8


This article details determining the Sound Absorption of a material using Simcenter Testlab and an impedance tube.

Index:

1. Equipment Needed
2. Setting Up the Impedance Tube
      2.1 Tube Assembly
      2.2 Inserting Microphones
      2.3 Full Setup and Wiring
3. Simcenter Testlab Impedance Software
      3.1 Getting Started
      3.2 Channel Setup
      3.3 Microphone Calibration
      3.4 Source Setup
      3.5 Tube Dimensions and Phase Calibration
      3.6 Sample Preparation
      3.7 Sample Positioning in Tube
      3.8 Measure Absorption in Tube

Impedance tubes that can be used with these instructions include: 

• These instructions are illustrated with the Mecanum Impedance Tube, part number F-MECMCICA-G sold by Siemens.
• The instructions also include settings for the Spectronics Impedance Tube, part number F-SPEMCIUS-G, also sold by Siemens.
• ​​​​​Custom tubes can also be used as shown in Figure 1. The user is responsible for entering the appropriate settings (tube dimensions, etc.) in the software. 

Figure 1: Custom tube setup in Simcenter Testlab Absorption workbook.

Other equipment needed is covered in the next section.

1. Equipment Needed

The following equipment is needed for sound absorption testing (Figure 2):
 

Figure 2: Required equipment for sound absorption testing.

• Computer with Simcenter Testlab software
• SCADAS hardware with built-in source
• Impedance tube with sections that include:
  • Speaker with microphone holder tube
  • Rigid termination tube
  • Anechoic termination tube
• Amplifier
• Microphones
  • Mecanum Tube (MECMCICA-G): Two quarter inch microphones (GRAS 46BD or PCB 378C10) 
  • Spectronics Tube (SPEMCIUS-G): Two half inch microphones (GRAS, PCB, …)
• Cables
• Ethernet cable: Connect SCADAS to data acquisition computer
• Amplifier cables: From SCADAS to amplifier, from amplifier to tube
• Microphone cables: From microphones to SCADAS
• Test Articles
  • Material Sample(s)

2. Setting Up the Impedance Tube

Before performing any measurements, the tube must be assembled and the microphones must be inserted.

2.1 Tube Assembly

Begin by taking the end with the loudspeaker and the anechoic termination out of the case. Connect them together as shown in Figure 3. 

Figure 3: Connect loudspeaker end and anechoic termination together.

Use the black clips to secure the two pieces together as shown in Figure 4.

Figure 4: Black clips are used to secure the termination end and microphone holder together.

The anechoic termination contains absorbing material that is suitable for phase calibration.  Phase calibration will be performed before doing the actual measurements.

Then the entire tube is assembled as shown in Figure 5.
 

Figure 5: Complete assembled impedance tube for absorption testing with anechoic termination for phase calibration.

Next the microphones can be inserted into the tube.

2. 2 Inserting Microphones

The microphones must be inserted at the proper height into the tube.  There will be two microphones between the sample and the speaker.

Depending on the tube design, there may be three microphone holders on each side of the sample.  Only two microphone holders will be used for an absorption test:

• Biggest Microphone Spacing: By using the microphone holders closest to the speaker and farthest from the speaker, the useable frequency range is 50 Hz to 2400 Hz.  This is 65 mm distance apart on the Mecanum tube.
• Smallest Microphone Spacing: By using the two microphone holders furthest from the speaker, the useable frequency range is 119 Hz to 5700 Hz.  This is 29 mm distance on the Mecanum tube.
• Unused Microphone Holder: The unused microphone holder will be sealed off with a white dummy microphone stick. 

To start, insert the microphone through the black collet as shown in Figure 6.  Place the microphone all the way down into the tube until it cannot go further.

Figure 6: After inserting the microphone through the black collet holder, put the microphone into the tube until it stops.

After the microphone will not travel further, tighten the collet around the microphone as shown in Figure 7.

Figure 7: After pushing the microphone into the tube, thread the black collet around the microphone to secure it.

Thread the black collet to until it holds the microphone firmly in place. Note that the collet will still have exposed threads after securing the microphone.

A guide for how the microphone should be sitting in the holder is shown in Figure 8.

Figure 8: Microphone holder not used should be secured with white dummy plug (left), microphone not pushed until stopped at incorrect height (middle), and microphone installed properly (right).

In any impedance tube, the microphones ends are intended to be flush with the top of the inner tube walls as shown in Figure 9.

Figure 9: In impedance tubes, the microphone end should be flush with the tube edge.

Two microphones should be installed in total for an absorption test.

2.3 Full Setup and Wiring

The entire system should be configured as shown in Figure 10.

Figure 10: Connections between different components of an absorption setup.  For the initial phase calibration, the anechoic termination should be used (pictured).

Some cabling guidance (Figure 11):

• The two microphones should be plugged into the first two channels of the Simcenter SCADAS.  The first microphone is closest to the speaker end of the tube.
• The DAC output of the SCADAS has an adaptor to BNC.  The BNC is converted to RCA to plug into the back of the amplifier.
• The amplifier output is sent to the speaker input.

Figure 11: Cable connections of amplifier, speaker input.

Initially, the gain should be turned down on the amplifier for the tube (Figure 12).

Figure 12: Turn the gain as low as possible initially on the amplifier (turn counter clockwise as far as possible).

Be sure to turn the SCADAS hardware on.

3. Simcenter Testlab Impedance Software

Once the hardware is setup, the software can be started and set up.

3.1 Getting Started

Choose “Start -> Programs -> Simcenter Testlab -> Testlab Acoustic -> Sound Absorption Testing using Impedance Tube” as shown in Figure 13.

Figure 13: Start the icon labelled “Sound Absorption Testing using impedance tube”.

Using Simcenter Testlab tokens, a total of 48 tokens (and a Desktop license) are needed.  The frontend driver requires 12 tokens, and the absorption software requires 36. 

After the software starts, choose “File -> Save As…” to store into a project file.

3.2 Channel Setup

In the Channel Setup, there should be two microphones active as shown in Figure 14.  The first microphone (labelled MicA) should correspond to the microphone closest to the speaker end of the tube.

Figure 14: Channel Setup of Sound Absorption software.

Moving away from the speaker, the microphones should be MicA then MicB, in that order. MicA should be marked as the reference.

The input mode of the microphones should be ICP for the microphones previously specified in the equipment list at the beginning of the article.  It is also possible to use 200V polarization microphones and other configurations.

The correct sensitivity values for the microphones should be entered in the Actual Sensitivity field.  It is good practice to calibrate the microphones as explained in the next section.

3.3 Microphone Calibration

Insert one of the two microphones into the calibrator as shown in Figure 15.
 

Figure 15: Calibrating microphones.

Then go to the “Calibration” worksheet as shown in Figure 16. Enter the calibration frequency and amplitude into the sheet.  In this case:

• Units = Pa
• Frequency = 1000 Hz
• Level = 94 dB(Rms)

Figure 16: Set the calibration units, frequency, and level in the upper middle.  Under the “Advanced…” button on the right, turn “Timeout” off.

Turn the calibrator on so it generates the calibration tone.  Press the “Check” and “Start” button in the lower left of the screen. The software will automatically detect and start calibrating the microphone as shown in Figure 17.

Figure 17: Software indicating the calibration of first microphone is in progress.

Switch microphones in the calibrator. Because the “Timeout” option was turned off, it will automatically find the second microphone and calibrate as shown in Figure 18.

Figure 18: Software indicating the calibration of second microphone is in progress.

The software will display a “Finished” message in green.  Press “Accept” to use the new calibration values.

For more information about calibration, see the Knowledge base article “Simcenter Testlab: Calibration”.

3.4 Source Setup

Go to the Scope worksheet. Make sure that under “Tools -> Add-ins” the Source Control add-in is turned on.  Change the source settings on the right side of the worksheet as shown in Figure 19.

Figure 19: Source settings are located on the right side of the Scope workbook.

Make sure the source signal type is Random, and the voltage is higher than zero.  For example, it can be set to 0.5 volts. 

After pressing the “Start Source” button on the middle right side of the screen, adjust the amplifier gain until the interior noise levels of the tube reach 100 dB RMS on the frequency spectrums.

If the channels and sources are properly setup, the next step is Tube selection and Phase Calibration.

3.5 Tube Dimensions and Phase Calibration

In the setup tab, tube dimensions can be entered and phase calibration performed.

First select the tube in the middle left. The dimensions of three different tubes are available by default: Mecanum Low Frequency, Mecanum High Frequency, and Spectronics:

• The Mecanum Low Frequency is used when two microphones are used with a spacing (s) of 65 mm. The x1 dimension is 135 mm, and the diameter (d) is 34.9 mm.
• The Mecanum High Frequency is used when two microphones are used with a spacing (s) of 29 mm. The x1 dimension is 99 mm, and the diameter (d) is 34.9 mm.
• The Spectronics tube spacing (s) is 29.21 mm. The x1 dimension is 80 mm, and the diameter (d) is 34.86 mm.

Dimensions for other tubes can also be entered.

Next, phase discrepancies in the measurement chain (wires, microphones, data acquisition hardware) are measured. This removes any “noise” from the absorption measurement caused by the hardware rather than the test article.

The microphones will be measured in different positions in the presence of the generated broadband noise to determine the phase errors. Frequency response functions (H) will be measured between the microphones. The phase calibration should be done with an empty tube and anechoic termination. The anechoic termination of the tube is filled with sound absorbing material.

If the tube being used is not in the list, dimensions can be entered and a new one created (Figure 20). 

Figure 20: Setup section of “Absorption in Tube” worksheet.

First click on the “Measure H12” button to measure the microphones in their base positions as shown in Figure 21.

Figure 21: Press the “Measure H12” button (upper right).  Message is lower left will indicate measurement is finished.

After the measurement is done, it is displayed and the status area in the lower left will say “H12 measurement done”.  H is short for the Frequency Response Function (or FRF).

Then switch the two microphones and press the “Measure H21” button as shown in Figure 22.

Figure 22: Press the “Measure H21” button after swapping MicB and MicA.

Important: After the H21 measurement is done, the microphones must be returned to their original position!

Press the “Check Setup” button in the lower left.  If everything is OK, the material sample can be loaded into the tube and measurement performed.

3.6 Sample Preparation
 

The fit of the material sample to the tube walls is critical.  The sample should have been cut to exactly the tube diameter (Figure 23):

• Too Small - There should not be a gap between the sample and the wall.
• Too Big – The sample should not be so large that it is deformed or compressed when sitting in the tube. 

Figure 23: Samples must be cut to exact dimensions of tube to ensure proper measurements.

Some tubes are delivered with material cutters to ensure the sample is cut to the exact diameter of the tube as shown in Figure 24.

Figure 24: Material samples and cutter.

Waterjet cutting is commonly used to prepare samples with the correct dimensions.

3.7 Sample Positioning in Tube

Remove the anechoic termination tube, which is only used for phase calibration.  The actual absorption measurement of the sample is performed with the rigid termination tube. A rigid termination is used so that all sound is either absorbed or reflected by the sample.

The rigid termination has plunger that moves a small, circular disk within the tube back and forth. This disk provides the rigid boundary condition for testing the sample.  By moving the disk on the plunger, different length samples can be tested while ensuring they are backed up to a rigid termination.

Pull back the plunger on the rigid termination and insert the sample as shown in Figure 25.

Figure 25: Insert the sample into the rigid termination end.

The sample must be positioned at the correct position lengthwise within the tube. The surface of the sample should be flush at the connection of tube termination and the tube center. This is done by pushing the sample into the rigid end of the tube, then moving the plunger until the sample is flush at the connection as shown in Figure 26.
 

Figure 26: Animation showing the proper positioning of the sample in the tube. Sample is pushed into the rigid termination end, then the plunger is used to make it flush to the interface.

When finished, the tube should look like Figure 27.  Note that the plunger may be at a different position depending on the sample thickness.

Figure 27: Assembled impedance tube with rigid termination ready for absorption measurement.

3.8 Measure Absorption in Tube

Click on the Measure tab in the upper left of the “Absorption in Tube” workbook.  Enter the sample name/type in the upper right.  Then push the measure button.  The results of a measurement are shown in Figure 28.

Figure 28: After pressing the Measure tab in the upper left, entering the sample name and pressing Measure, the absorption results should be shown.

Three plots are shown in the software after the measurement:

• Top: FRF between MicA and MicB
• Middle: Coherence of FRF between MicA and Mic B.  Should be as close to one as possible.
• Bottom: Absorption coefficient of the material sample versus frequency.  Values range from zero (no absorption) to one (all sound absorbed).

For more information about absorption and how to interpret results, see the Knowledge base article “Sound Absorption”.

It is also possible to take multiple measurements of samples and create an average (Figure 29).

Figure 29: After measuring several samples, click on the average button in the lower left.
 

Several samples of the same material can be tested to understand the variation between samples.

Measurements include:
1. Normal incident absorption coefficient
2. Normal incident sound pressure reflection factor
3. Normal surface specific acoustic impedance ratio
4. Normal surface specific acoustic admittance ratio



Questions?   Email peter.schaldenbrand@siemens.com or contact Siemens Support Center.

Related Links

• Index of Testing Knowledge Articles
• Sound Absorption
• Sound Transmission Loss
• Measuring Sound Transmission Loss using an Impedance Tube
• History of Acoustics
• Microphone Basics
• Sound Pressure
• What is a decibel?
• What is A-weighting?
• Octaves and Human Hearing
• What is Sound Power?
• Decibel Math
• What is the Acoustic Quantity called Q?
• Noise level certification, how to select the right standard?
• Sound pressure, sound power, and sound intensity: What is the difference?
• Sound fields: Free and diffuse field, near and far field
• Loudness and Sones
• Auditory Masking
• Tone-to-Noise Ratio and Prominence Ratio
• Fluctuation Strength and Roughness
• Critical Bands in Human Hearing
• Kurtosis
• Sound Quality Jury Testing
• Fundamentals of Sound Seminar - Part I
• Fundamentals of Sound Seminar - Part II
• Acoustic Testing YouTube Playlist