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.
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.