Sound intensity probe (G.R.A.S. probe used in this article)
Sound intensity calibrator
Pistonphone for the microphone sensitivity check
Test object
In this tutorial, sound intensity will be measured on a computer speaker.
The hardware is setup as follows (see Figure 1, below):
Mount the two microphones into the sound intensity probe.
Plug the microphone channels into the first two channels of the Simcenter SCADAS
Connect the Ethernet data port from the SCADAS to the PC
If using the remote-control buttonson the Sound Intensity Probe, connect the USB from the probe to the PC
Figure 1: Intensity measurement setup consists of a test object, intensity probe, intensity calibrator, SCADAS hardware, and a PC with Simcenter Testlab software.
Once the hardware setup is complete and the SCADAS is connected to the PC, launch the Simcenter Testlab Sound Intensity Testing software.
2. Software Setup:
Launch Simcenter Testlab Sound Intensity Testing (Simcenter Testlab -> Testlab Acoustic -> Sound Intensity Testing). If using token licensing, this takes 37 total (12 for frontend driver and 25 for Intensity Testing).
Open a new project and save it (it is recommended to periodically save the project while working).
In the Channel Setup workbook, fill out the following fields:
OnOff: check the first two channels on. These correspond to the two microphone channels.
Point: give the channels a name.
Input Mode: this will correspond to the microphone type. Two common types of intensity probes include the G.R.A.S 50AI-L (ICP microphones) and the G.R.A.S 50AI-D (polarized microphones).
Figure 2: A sample channel setup for an intensity probe measurement.
Once the microphones have been setup in Channel Setup worksheet, it is time to determine their sensitivity.
3. Microphone Sensitivity Check:
Enter the Calibration worksheet
Calibrate each microphone individually. Follow the instructions in the “Simcenter Testlab Calibration” article under section 2: “Single Microphone”.
Verify that the sensitivity value generated by the calibration is close to the expected value reported on the sensitivity sheet provided with the microphones.
4. Probe Creation:
Now that the sensitivity check is completed on the two individual microphones, the two transducers need to be combined in the software into one intensity probe.
This is done in the “Sound Intensity Setup” worksheet.
The “Probe Definition” setup is in the middle of the “Sound Intensity Setup” worksheet.
To create a new probe (Figure 3):
Ensure the drop-down menu is set to “Sound Intensity PP”
Select “Create New Probe”
Enter a probe name or leave it at the default “Probe 1”
Set the Channel 1 and Channel 2 drop-down menus to match the microphones that were setup in the “Channel Setup” workbook
Fill out the spacer distance (this will also update the frequency span in the “Available Range” field)
Enter a direction. This represents the orientation the probe will be held in during measurement. For example, if using the option where the normal to the intensity mesh surface is +Z, enter the direction as +Z.
After completing these fields, the Status should turn green and read “Probe OK”.
Figure 3: A typical intensity probe setup. Note that there will be a warning indicator at the bottom of the screen until the PRII is measured.
NOTE: It is possible to setup multiple intensity probes in the software. This enables users to measure intensity in multiple directions, or simultaneously measure with multiple spacer widths (thereby covering a wider frequency range). Even an array of intensity probes can be used to measure multiple locations at once.
There will still be an orange warning indicator at the bottom of the screen. This will disappear once the Pressure Residual Intensity Index (PRII) is measured.
5. Create an Acoustic Mesh:
Sound intensity is measured on a surface mesh surrounding the test object. The measurements can be made using either the discrete point method or the scanning method, as seen in the Sound Intensityarticle. This example will use the discrete point method.
In this example, the sound intensity of a speaker is measured (Figure 4). The 3D orange grid below represents the surface on which the intensity will be measured. The center of each mesh square in the grid is where the probe will be placed for the measurement.
Figure 4: Acoustic surface mesh surrounding an audio speaker for intensity test.
To create the acoustic mesh representative of the measurement surface area (Figure 5):
Load the “Geometry” worksheet (Tools -> Add-ins -> Geometry…16 tokens)
In the lower left corner, select “Generate Acoustic Mesh”
The “Acoustic Mesh Generator” window will open
In the upper right, select the “Mesh Mode”. The simplest (and most common) Rectangular mesh will be used in this example.
Fill in the dimensions and start point of the object in the “Absolute Positioning” area
In the “Component” area, fill in the number of subdivisions required in each direction
The way in which the object is scanned is also set here. In the “Node Numbering” area, use the drop-down menus to determine in which order the nodes will be measured.
Press “Create” to generate the mesh.
Figure 5: The Acoustic Mesh Generator window
The resulting mesh looks as follows (Figure 6):
Figure 6: The mesh representing measurement points around the speaker.
To visualize the node names and Euler angles, right click in the geometry, and select Model -> Nodes -> Names / Euler Angles. This may be of aid during the measurement process.
Figure 7: Turn on node names and Euler angles to aid in the visualization of the mesh.
The settings in the “Node Numbering” area determine in which direction the object will be scanned.
In this case, the scan starts in the upper left corner and then goes through a bi-directional down the surface. Therefore, the scanning pattern will look like Figure 8, below.
Figure 8: The measurement sequence as determined in the “Node Numbering” section of the Geometry workbook.
The intensity probe will be held perpendicular to the surface while scanning. The intensity probe will be held at each “orange dot” location. The measurement order will follow the green arrows.
It is possible to add a photo to the mesh to enhance the viewing of the results.
Press “Import Photo to Mesh”
The Import photo to Acoustic Mesh pop-up window opens
Select the mesh component (side) of the geometry to apply the picture to. It is possible to apply a picture to every side. In this case, only the front has a picture applied.
Browse to the photo
It is possible the picture will match the surface perfectly. If not, it is possible to adjust the size, position, and rotation of the photo using the buttons.
Figure 9: Import a photo onto the mesh for improved visualization.
Once the picture is nicely aligned, the measurement parameters can be specified.
6. Measurement Specifications:
Return to the “Sound Intensity Setup” workbook.
Some measurement options will now be set (Figure 10, below).
Duration: Set the duration long enough to capture the sound signature. A ten second duration is usually enough time for each measurement location.
Rate: Here, the number of averages per second is set. Averaging is applied to limit the influence of random noises.
FFT Based Processing: Sound intensities are calculated from instantaneous spectra and averaged afterwards
Time Domain Based Processing: Sound intensities are calculated from the averaged spectra using real-time octave filters.
Both FFT and Time domain processing can be performed in parallel.
FFT based processing and time domain based processing are both enabled.
All three checkboxes are selected which will cause the program to automatically start the measurement at the next point after the defined number of seconds. NOTE: Turn off the auto-increment option if scanning or using a remote control.
Additionally, all of the “Measurement Functions” on the bottom right are checked on to be saved to the project.
Figure 10: The measurement parameters are set in the bottom portion of the “Sound Intensity Setup” workbook.
Now the phase calibration and PRII will be determined.
7. Phase Calibration:
Phase calibration is done to compensate bias errors in measurements with different transducers.
An intensity measurement is performed with two phase matched microphones. In theory, the Frequency Response Function (FRF) between one microphone and the other should have a constant phase of zero degrees over the entire frequency range. Additionally, the amplitude of the FRF should be unity over the range. Realistically though, there may be small deviations. Corrections can be made after measuring the phase deviations.
This is called phase matching or phase calibration.
In this example, a G.R.A.S. 51AB sound intensity calibrator is used. A bi-directional phase calibration will be performed, requiring two separate measurements.
1. Plug the two microphones into the calibrator. Push them until they are well seated. Also connect the source output of the SCADAS frontend hardware to the source input on the calibrator.
Figure 11: The G.R.A.S. calibration has three ports: one for source input, one for Mic A, and one for Mic B.
2. Verify that the two channels are active in channel setup, and that their sensitivities are calibrated.
3. In “Acquisition Setup”, set the Output1 to 0.30V of random pink noise. To select pink noise, go under “More” and press the “Pink Noise” selection as shown in Figure 12. Verify that the signal is being sent to the calibrator by looking at the time traces.
Figure 12: Ensure Pink Noise is selected by going under “More” under “Source Control”.
4. Go to the “Measure” workbook. It is recommended to increase the measurement time to at least 30 seconds. This can be done under the “Tracking” tab.
Figure 13: Increase the measurement duration under the “Tracking” tab in the “Measure” workbook.
5. Enter the “Phase” tab.
Press the M1-M2 radio button
Start the source
Arm the system
Begin the measurement by pressing the play button
Figure 14: This is the first of two measurements for the phase calibration. Ensure the source is on, the arm the system and begin measuring.
6. Once the measurement is finished, the “Phase Correction FRF selection” window pops up. This will show how much the phase differs from zero. The amplitude of the FRF between the mics should be close to unity. Click “Done” to accept the correction FRF. This correction only affects the FFT-Based intensity, not the realtime octave based intensity.
Figure 15: Look at the phase deviation between the two microphones and how it changes versus frequency. Press “Done” to accept the FRF.
7. This is a bi-directional phase calibration. Therefore, another measurement must be completed with the microphone positions swapped on the calibrator.
8. Swap the physical microphone positions on the calibrator as shown in Figure 16.
Figure 16: Swap the physical microphone positions on the calibrator.
9. Press the M2-M1 radio button in the “Phase” area of the “Measure” workbook.
10. Ensure the source is on, and then begin another acquisition.
Figure 17: Swap the physical microphone positions on the calibrator and begin the M2-M1 acquisition.
11. Once both FRFs are measured, the table will be completely populated and the FRFs will be used for phase correction as shown in Figure 18.
Figure 18: Upon completion of the phase measurements, both M1-M2 and M2-M1 should read “measured”.
NOTE: It is possible to load existing FRFs for the phase correction under the “Phase Correction Load…” button.
Once the phase calibration is complete, the Pressure Residual Intensity Index (PRII) will be determined.
8. Limits on Intensity Measurements:
When performing intensity measurements, the maximum and minimum limit intensity values can be calculated. These limits come from the theory combined with information like the phase calibration, etc.
The minimum and maximum sound intensity limits are calculated and displayed alongside the measured sound intensity data to ensure it falls within the limits.
Upper Intensity Limit:
The maximum intensity limit is determined by taking the sound pressure average across the two microphones, on a per octave basis. The measured sound intensity should never exceed this level.
An example of a sound intensity upper limit plot as shown in Figure 19.
Figure 19: The upper sound intensity limit.
Lower Intensity Limit:
Since no microphone pair is perfectly phase matched (even after phase calibration), there will be some small deviations from zero on the measured phase between the two microphones. This deviation is interpreted as intensity along the spacer. This is the noise floor for the intensity measurement.
A quantity called the Pressure Residual Intensity Index (PRII) is an indicator of the noise floor. The PRII is determined by taking the difference between the measured pressure and the measured intensity (in dB) when measuring the same sound pressure with both microphones. In theory, the measured intensity should be zero, but due to phase mismatch it is not. The PRII goes a step further, normalizing for the sound pressure level as well, as shown in Equation 1:
Equation 1: The Pressure Residual Intensity Index (PRII) is the difference of L_p is the measured sound pressure and L_In when measuring the same sound pressure field with both microphones.
The PRII is then used to determine the intensity lower limit, on a per-octave basis.
The intensity lower limit is determined by measuring the sound pressure average across the mics, then subtracting the PRII, then adding a bias error factor. Remember, this is calculated per octave as shown in Equation 2:
Equation 2: Where L_p is the measured sound pressure, and K is the bias error factor.
The bias error factor (K) is a dB value that is determined when selecting the accuracy grade in the Sound Intensity Setup workbook. The bias error factor will be either 7dB or 10dB.
Figure 20: Set the bias error factor (K) in the upper left of the Sound Intensity Setup worksheet.
When PRII is being measured, the same signal is fed to both microphones as mentioned previously. Therefore, the sound pressure reading should be high, while the intensity reading should be zero. Therefore, PRII should be as highest possible value.
When PRII is high, it makes the difference between sound pressure and PRII small, therefore allowing for a minimal lower limit value (Equation X).
Many engineers will measure PRII at the beginning of each test day, to ensure that PRII does not degrade over time.
Below is an example of a sound intensity lower limit (Figure 21):
Figure 21: The lower sound intensity limit.
Ideally, sound intensity measurements will fall between the upper and lower limit, as shown in the graph below (Figure 22):
Figure 22: Check that the measured intensity falls between the upper and lower sound intensity limits.
The next section will detail how to measure PRII so the lower limit can be calculated.
9. Measuring the Pressure Residual Intensity Index (PRII):
To measure the PRII, the G.R.A.S. 51AB sound intensity calibrator is used (without the phase shifter).
1. Plug the two microphones into the calibrator. Push them until they are well seated.
Figure 23: The G.R.A.S. calibration has three ports: one for source input, one for Mic A, and one for Mic B.
2. Verify that the two channels are active in channel setup, and that their sensitivities are calibrated. 3. Ensure that the same output settings are used for the phase calibration as the PRII (in this case, 0.30V, pink noise). 4. Go to the “PRII” tab in the “Measure” workbook. 5. Ensure the source is on. Arm the system and begin the measurement.
6. Once the measurement is completed, the “PRII Selection” window will pop up. Click “Done” to accept the results.
Figure 24: Preview the PRII results. Press “Done” to accept the results. It is also possible to load a previously acquired block.
After completing the hardware setup, channel setup, phase calibration, and PRII, it is time to acquire data! Remove the microphones from the intensity calibrator and reassemble the intensity probe.
10. Acquisition:
It is time to acquire the data. This process should be relatively simple now that the setup steps are complete.
Enter the “Measure” workbook.
The workbook contains the measure panel on the right, the sound intensity graphs on the top (left), and the geometry on the bottom (left) as shown in Figure 25.
Figure 25: The Sound Intensity measurement workbook.
To begin the measurement, press “Arm / Disarm” and then start the measurement by pressing the “Play” button.
The geometry shows the first point that will be measured. The orange color indicates the next point to be measured (Figure 25, above).
Position the probe at the same location as the orange square and ensure it aligned perpendicular to the grid.
Press the play button or use the remote control to begin the measurement.
During the measurement, the upper graphs will populate (Figure 26, below).
On the top left display, there are two narrowband curves:
The red curve is the instantaneous intensity spectrum.
The green curve is the energy averaged intensity spectrum over the measurement duration (10 seconds in this case).
On the octave based graph in the upper middle, the instantaneous intensity is shown with green bars, while the averaged intensity is shown with blue bars. Sometimes you may notice some red bands at lower octaves. This indicates the sound intensity is negative. Therefore, its vector is directed towards the speaker instead of away from it. This is common at very low frequencies, especially outside of the spacer dictated frequency range.
Also, on the graph is the range in which we can trust the measurements. This is composed of a lower limit (deduced from PRII) and the upper limit (derived from SPL).
Remember, the frequency range is also restricted by the spacer length. In this case, the spacer is only appropriate for measuring frequencies from 63-3150Hz even though a larger frequency range is displayed.
Figure 26: The screen will populate with sound intensity information as the measurement is being performed. The upper middle display shows the intensity measurement (green bars) versus the upper limit (magenta line) and lower limit (cyan line). This can be visually checked with the measurement is in progress.
After each measurement is taken, the software will automatically increment to the next measurement and take data.
NOTE: It is possible to control the G.R.A.S. probe with buttons integrated into the probe handle. The button functions include: arming the system, starting the measurement, proceeding to the next measurement, reverting to the previous measurement, ranging, and stopping the measurement. Remember to turn off the auto-increment options in the Sound Intensity Setup worksheet.
Figure 27: The G.R.A.S. 50AI-L Intensity Probe with integrated buttons.
For more information on how to configure the remote buttons, read this article.
Points that have been measured will show up as green on the geometry.
Once all the desired data has been measured, save the project.
